Hypothetical charge of an atom if all its bonds to different atoms were fully ionic
In
chemistry, the oxidation state, or oxidation number, is the hypothetical
charge of an atom if all of its
bonds to other atoms were fully
ionic. It describes the degree of
oxidation (loss of
electrons) of an
atom in a
chemical compound. Conceptually, the oxidation state may be positive, negative or zero. While fully ionic bonds are not found in nature, many bonds exhibit strong ionicity, making oxidation state a useful predictor of charge.
The oxidation state of an atom does not represent the "real" charge on that atom, or any other actual atomic property. This is particularly true of high oxidation states, where the
ionization energy required to produce a multiply positive ion is far greater than the energies available in chemical reactions. Additionally, the oxidation states of atoms in a given compound may vary depending on
the choice of
electronegativity scale used in their calculation. Thus, the oxidation state of an atom in a compound is purely a formalism. It is nevertheless important in understanding the nomenclature conventions of
inorganic compounds. Also, several observations regarding chemical reactions may be explained at a basic level in terms of oxidation states.
Oxidation states are typically represented by
integers which may be positive, zero, or negative. In some cases, the average oxidation state of an element is a fraction, such as 8/3 for
iron in
magnetiteFe3O4 (
see below). The highest known oxidation state is reported to be +9, displayed by
iridium in the
tetroxoiridium(IX) cation (IrO+4).[1] It is predicted that even a +10 oxidation state may be achieved by
platinum in tetroxoplatinum(X), PtO2+4.[2] The lowest oxidation state is −5, as for
boron in Al3BC[3] and
gallium in
pentamagnesium digallide (Mg5Ga2).
In inorganic nomenclature, the oxidation state is represented by a
Roman numeral placed after the element name inside parentheses or as a superscript after the element symbol, e.g.
Iron(III) oxide.
The term oxidation was first used by
Antoine Lavoisier to signify the reaction of a substance with
oxygen. Much later, it was realized that the substance, upon being oxidized, loses electrons, and the meaning was extended to include other
reactions in which electrons are lost, regardless of whether oxygen was involved.
The increase in the oxidation state of an atom, through a chemical reaction, is known as oxidation; a decrease in oxidation state is known as a
reduction. Such reactions involve the formal transfer of electrons: a net gain in electrons being a reduction, and a net loss of electrons being oxidation. For pure elements, the oxidation state is zero.
IUPAC definition
IUPAC has published a "Comprehensive definition of the term oxidation state (IUPAC Recommendations 2016)".[4] It is a distillation of an
IUPAC technical report "Toward a comprehensive definition of oxidation state" from 2014.[5] The current IUPAC Gold Book definition of oxidation state is:
Oxidation state of an atom is the charge of this atom after ionic approximation of its heteronuclear bonds...
and the term oxidation number is nearly synonymous.[7]
The underlying principle is that the ionic charge is "the oxidation state of an atom, after ionic approximation of its bonds",[8] where ionic approximation means, hypothesizing that all bonds are ionic. Several criteria were considered for the ionic approximation:
Extrapolation of the bond's polarity;
from the electronegativity difference,
from the dipole moment, and
from quantum‐chemical calculations of charges.
Assignment of electrons according to the atom's contribution to the bonding
Molecular orbital (MO)[8][9]/ the electron's allegiance in a
LCAO–MO model.[10]
In a bond between two different elements, the bond's electrons are assigned to its main atomic contributor/higher electronegativity; in a bond between two atoms of the same element, the electrons are divided equally. This is because most electronegativity scales depend on the atom's bonding state, which makes the assignment of the oxidation state a somewhat circular argument. For example, some scales may turn out unusual oxidation states, such as −6 for
platinum in PtH2−4, for
Pauling and
Mulliken scales.[11] The dipole moments would, sometimes, also turn out abnormal oxidation numbers, such as in
CO and
NO, which
are oriented with their positive end towards oxygen. Therefore, this leaves the atom's contribution to the
bonding MO, the atomic-orbital energy, and from quantum-chemical calculations of charges, as the only viable criteria with cogent values for ionic approximation. However, for a simple estimate for the ionic approximation, we can use
Allen electronegativities,[8] as only that electronegativity scale is truly independent of the oxidation state, as it relates to the average valence‐electron energy of the free atom:
Introductory chemistry uses postulates: the oxidation state for an element in a chemical formula is calculated from the overall charge and postulated oxidation states for all the other atoms.
where OS stands for oxidation state. This approach yields correct oxidation states in oxides and hydroxides of any single element, and in acids such as
sulfuric acid (H2SO4) or
dichromic acid (H2Cr2O7). Its coverage can be extended either by a list of exceptions or by assigning priority to the postulates. The latter works for
hydrogen peroxide (H2O2) where the priority of rule 1 leaves both oxygens with oxidation state −1.
Additional postulates and their ranking may expand the range of compounds to fit a textbook's scope. As an example, one postulatory algorithm from many possible; in a sequence of decreasing priority:
An element in a free form has OS = 0.
In a compound or ion, the sum of the oxidation states equals the total charge of the compound or ion.
Fluorine in compounds has OS = −1; this extends to
chlorine and
bromine only when not bonded to a lighter halogen, oxygen or nitrogen.
Group 1 and
group 2 metals in compounds have OS = +1 and +2, respectively.
Hydrogen has OS = +1 but adopts −1 when bonded as a
hydride to metals or metalloids.
Oxygen in compounds has OS = −2 but only when not bonded to oxygen (e.g. in peroxides) or fluorine.
This set of postulates covers oxidation states of fluorides, chlorides, bromides, oxides, hydroxides, and hydrides of any single element. It covers all
oxoacids of any central atom (and all their fluoro-, chloro-, and bromo-relatives), as well as
salts of such acids with group 1 and 2 metals. It also covers
iodides,
sulfides, and similar simple salts of these metals.
where each "—" represents an electron pair (either shared between two atoms or solely on one atom), and "OS" is the oxidation state as a numerical variable.
After the electrons have been assigned according to the vertical red lines on the formula, the total number of valence electrons that now "belong" to each atom is subtracted from the number N of valence electrons of the neutral atom (such as 5 for nitrogen in
group 15) to yield that atom's oxidation state.
This example shows the importance of describing the bonding. Its summary formula, HNO3, corresponds to two
structural isomers; the
peroxynitrous acid in the above figure and the more stable
nitric acid. With the formula HNO3, the
simple approach without bonding considerations yields −2 for all three oxygens and +5 for nitrogen, which is correct for nitric acid. For the peroxynitrous acid, however, the two oxygens in the O–O bond each has OS = −1 and the nitrogen has OS = +3, which requires a structure to understand.
Analogously for
transition-metal compounds; CrO(O2)2 on the left has a total of 36 valence electrons (18 pairs to be distributed), and
hexacarbonylchromium (Cr(CO)6) on the right has 66 valence electrons (33 pairs):
A key step is drawing the Lewis structure of the molecule (neutral, cationic, anionic): atom symbols are arranged so that pairs of atoms can be joined by single two-electron bonds as in the molecule (a sort of "skeletal" structure), and the remaining valence electrons are distributed such that sp atoms obtain an
octet (duet for hydrogen) with a priority that increases in proportion with electronegativity. In some cases, this leads to alternative formulae that differ in bond orders (the full set of which is called the
resonance formulas). Consider the
sulfate anion (SO2−4) with 32 valence electrons; 24 from oxygens, 6 from sulfur, 2 of the anion charge obtained from the implied cation). The
bond orders to the terminal oxygens do not affect the oxidation state so long as the oxygens have octets. Already the skeletal structure, top left, yields the correct oxidation states, as does the Lewis structure, top right (one of the resonance formulas):
The bond-order formula at the bottom is closest to the reality of four equivalent oxygens each having a total bond order of 2. That total includes the bond of order 1/2 to the implied cation and follows the 8 – N rule[5] requiring that the main-group atom's bond order equals 8 – N valence electrons of the neutral atom, enforced with a priority that proportionately increases with electronegativity.
This algorithm works equally for molecular cations composed of several atoms. An example is the
ammonium cation of 8 valence electrons (5 from nitrogen, 4 from hydrogens, minus 1 electron for the cation's positive charge):
Drawing Lewis structures with electron pairs as dashes emphasizes the essential equivalence of bond pairs and lone pairs when counting electrons and moving bonds onto atoms. Structures drawn with electron dot pairs are of course identical in every way:
The algorithm's caveat
The algorithm contains a caveat, which concerns rare cases of
transition-metalcomplexes with a type of
ligand that is reversibly bonded as a
Lewis acid (as an acceptor of the electron pair from the transition metal); termed a "Z-type" ligand in Green's
covalent bond classification method. The caveat originates from the simplifying use of electronegativity instead of the
MO-based electron allegiance to decide the ionic sign.[4] One early example is the O2S−RhCl(CO)(
PPh3)2 complex[12] with
sulfur dioxide (SO2) as the reversibly-bonded acceptor ligand (released upon heating). The Rh−S bond is therefore extrapolated ionic against Allen electronegativities of
rhodium and sulfur, yielding oxidation state +1 for rhodium:
Algorithm of summing bond orders
This algorithm works on Lewis structures and bond graphs of extended (non-molecular) solids:
Oxidation state is obtained by summing the heteronuclear-bond orders at the atom as positive if that atom is the electropositive partner in a particular bond and as negative if not, and the atom’s formal charge (if any) is added to that sum.
Applied to a Lewis structure
An example of a Lewis structure with no formal charge,
illustrates that, in this algorithm, homonuclear bonds are simply ignored (the bond orders are in blue).
Carbon monoxide exemplifies a Lewis structure with
formal charges:
To obtain the oxidation states, the formal charges are summed with the bond-order value taken positively at the carbon and negatively at the oxygen.
Applied to molecular ions, this algorithm considers the actual location of the formal (ionic) charge, as drawn in the Lewis structure. As an example, summing bond orders in the
ammonium cation yields −4 at the nitrogen of formal charge +1, with the two numbers adding to the oxidation state of −3:
The sum of oxidation states in the ion equals its charge (as it equals zero for a neutral molecule).
Also in anions, the formal (ionic) charges have to be considered when nonzero. For sulfate this is exemplified with the skeletal or Lewis structures (top), compared with the bond-order formula of all oxygens equivalent and fulfilling the octet and 8 − N rules (bottom):
Applied to bond graph
A
bond graph in
solid-state chemistry is a chemical formula of an extended structure, in which direct bonding connectivities are shown. An example is the AuORb3perovskite, the unit cell of which is drawn on the left and the bond graph (with added numerical values) on the right:
We see that the oxygen atom bonds to the six nearest
rubidium cations, each of which has 4 bonds to the
auride anion. The bond graph summarizes these connectivities. The bond orders (also called
bond valences) sum up to oxidation states according to the attached sign of the bond's ionic approximation (there are no formal charges in bond graphs).
Determination of oxidation states from a bond graph can be illustrated on
ilmenite, FeTiO3. We may ask whether the mineral contains Fe2+ and Ti4+, or Fe3+ and Ti3+. Its crystal structure has each metal atom bonded to six oxygens and each of the equivalent oxygens to two
irons and two
titaniums, as in the bond graph below. Experimental data show that three metal-oxygen bonds in the octahedron are short and three are long (the metals are off-center). The bond orders (valences), obtained from the bond lengths by the
bond valence method, sum up to 2.01 at Fe and 3.99 at Ti; which can be rounded off to oxidation states +2 and +4, respectively:
Balancing redox
Oxidation states can be useful for balancing chemical equations for oxidation-reduction (or
redox) reactions, because the changes in the oxidized atoms have to be balanced by the changes in the reduced atoms. For example, in the reaction of
acetaldehyde with
Tollens' reagent to form
acetic acid (shown below), the
carbonyl carbon atom changes its oxidation state from +1 to +3 (loses two electrons). This oxidation is balanced by reducing two Ag+ cations to Ag0 (gaining two electrons in total).
An inorganic example is the Bettendorf reaction using
tin dichloride (SnCl2) to prove the presence of
arsenite ions in a concentrated
HCl extract. When arsenic(III) is present, a brown coloration appears forming a dark precipitate of
arsenic, according to the following simplified reaction:
Here three
tin atoms are oxidized from oxidation state +2 to +4, yielding six electrons that reduce two arsenic atoms from oxidation state +3 to 0. The simple one-line balancing goes as follows: the two redox couples are written down as they react;
One tin is oxidized from oxidation state +2 to +4, a two-electron step, hence 2 is written in front of the two arsenic partners. One arsenic is reduced from +3 to 0, a three-electron step, hence 3 goes in front of the two tin partners. An alternative three-line procedure is to write separately the
half-reactions for oxidation and reduction, each balanced with electrons, and then to sum them up such that the electrons cross out. In general, these redox balances (the one-line balance or each half-reaction) need to be checked for the ionic and electron charge sums on both sides of the equation being indeed equal. If they are not equal, suitable ions are added to balance the charges and the non-redox elemental balance.
Appearances
Nominal oxidation states
A nominal oxidation state is a general term with two different definitions:
Systematic oxidation state is chosen from close alternatives as a pedagogical description. An example is the oxidation state of phosphorus in
H3PO3 (structurally
diprotic HPO(OH)2) taken nominally as +3, while
Allen electronegativities of
phosphorus and
hydrogen suggest +5 by a narrow margin that makes the two alternatives almost equivalent:
Both alternative oxidation numbers for phosphorus make chemical sense, depending on which chemical property or reaction is emphasized. By contrast, a calculated alternative, such as the average (+4) does not.
Ambiguous oxidation states
Lewis formulae are rule-based approximations of chemical reality, as are
Allen electronegativities. Still, oxidation states may seem ambiguous when their determination is not straightforward. If only an experiment can determine the oxidation state, the rule-based determination is ambiguous (insufficient). There are also truly
dichotomous values that are decided arbitrarily.
Oxidation-state determination from resonance formulas
Seemingly ambiguous oxidation states are derived from a set of
resonance formulas of equal weights for a molecule having heteronuclear bonds where the atom connectivity does not correspond to the number of two-electron bonds dictated by the
8 − N rule. An example is
S2N2 where four resonance formulas featuring one S=N double bond have oxidation states +2 and +4 for the two sulfur atoms, which average to +3 because the two sulfur atoms are equivalent in this square-shaped molecule.
A physical measurement is needed to determine oxidation state
when a
non-innocentligand is present, of hidden or unexpected redox properties that could otherwise be assigned to the central atom. An example is the
nickeldithiolate complex, Ni(S 2C 2H 2)2− 2.[5]: 1056–1057
when the redox ambiguity of a central atom and ligand yields dichotomous oxidation states of close stability, thermally induced
tautomerism may result, as exemplified by
manganesecatecholate, Mn(C6H4O2)3.[5]: 1057–1058 Assignment of such oxidation states requires spectroscopic,[13] magnetic or structural data.
when the bond order has to be ascertained along with an isolated tandem of a heteronuclear and a homonuclear bond. An example is
thiosulfateS 2O2− 3 having two possible oxidation states (bond orders are in blue and formal charges in green):
The S–S distance measurement in
thiosulfate is needed to reveal that this bond order is very close to 1, as in the formula on the left.
Ambiguous/arbitrary oxidation states
when the electronegativity difference between two bonded atoms is very small (as in
H3PO3). Two almost equivalent pairs of oxidation states, arbitrarily chosen, are obtained for these atoms.
when an electronegative
p-block atom forms solely homonuclear bonds, the number of which differs from the number of two-electron bonds suggested by
rules. Examples are homonuclear finite chains like
N− 3 (the central nitrogen connects two atoms with four two-electron bonds while only three two-electron bonds[14] are required by
8 − N rule) or
I− 3 (the central iodine connects two atoms with two two-electron bonds while only one two-electron bond fulfills the 8 − N rule). A sensible approach is to distribute the ionic charge over the two outer atoms.[5] Such a placement of charges in a
polysulfideS2− n (where all inner sulfurs form two bonds, fulfilling the 8 − N rule) follows already from its Lewis structure.[5]
when the isolated tandem of a heteronuclear and a homonuclear bond leads to a bonding compromise in between two Lewis structures of limiting bond orders. An example is
N2O:
The typical oxidation state of nitrogen in N2O is +1, which also obtains for both nitrogens by a molecular orbital approach.[15] The formal charges on the right comply with electronegativities, which implies an added ionic bonding contribution. Indeed, the estimated N−N and N−O bond orders are 2.76 and 1.9, respectively,[5] approaching the formula of integer bond orders that would include the ionic contribution explicitly as a bond (in green):
Conversely, formal charges against electronegativities in a Lewis structure decrease the bond order of the corresponding bond. An example is
carbon monoxide with a bond-order estimate of 2.6.[16]
Fractional oxidation states
Fractional oxidation states are often used to represent the average oxidation state of several atoms of the same element in a structure. For example, the formula of
magnetite is Fe 3O 4, implying an average oxidation state for iron of +8/3.[17]: 81–82 However, this average value may not be representative if the atoms are not equivalent. In a Fe 3O 4 crystal below 120 K (−153 °C), two-thirds of the cations are Fe3+ and one-third are Fe2+ , and the formula may be more clearly represented as FeO·Fe 2O 3.[18]
Likewise,
propane, C 3H 8, has been described as having a carbon oxidation state of −8/3.[19] Again, this is an average value since the structure of the molecule is H 3C−CH 2−CH 3, with the first and third carbon atoms each having an oxidation state of −3 and the central one −2.
An example with true fractional oxidation states for equivalent atoms is potassium
superoxide, KO 2. The diatomic superoxide ion O− 2 has an overall charge of −1, so each of its two equivalent oxygen atoms is assigned an oxidation state of −1/2. This ion can be described as a
resonance hybrid of two Lewis structures, where each oxygen has an oxidation state of 0 in one structure and −1 in the other.
For the
cyclopentadienyl anionC 5H− 5, the oxidation state of C is −1 + −1/5 = −6/5. The −1 occurs because each carbon is bonded to one hydrogen atom (a less electronegative element), and the −1/5 because the total ionic charge of −1 is divided among five equivalent carbons. Again this can be described as a resonance hybrid of five equivalent structures, each having four carbons with oxidation state −1 and one with −2.
Examples of fractional oxidation states for carbon
Finally, fractional oxidation numbers are not used in the chemical nomenclature.[20]: 66 For example the red lead
Pb 3O 4 is represented as lead(II,IV) oxide, showing the oxidation states of the two nonequivalent
lead atoms.
Many compounds with
luster and
electrical conductivity maintain a simple
stoichiometric formula, such as the golden
TiO, blue-black
RuO2 or coppery
ReO3, all of obvious oxidation state. Ultimately, assigning the free metallic electrons to one of the bonded atoms is not comprehensive and can yield unusual oxidation states. Examples are the LiPb and Cu 3Au ordered
alloys, the composition and structure of which are largely determined by
atomic size and
packing factors. Should oxidation state be needed for redox balancing, it is best set to 0 for all atoms of such an alloy.
List of oxidation states of the elements
This is a list of known oxidation states of the
chemical elements, excluding
nonintegral values. The most common states appear in bold. The table is based on that of Greenwood and Earnshaw,[21] with additions noted. Every element exists in oxidation state 0 when it is the pure non-ionized element in any phase, whether monatomic or polyatomic
allotrope. The column for oxidation state 0 only shows elements known to exist in oxidation state 0 in compounds.
A figure with a similar format was used by
Irving Langmuir in 1919 in one of the early papers about the
octet rule.[179] The periodicity of the oxidation states was one of the pieces of evidence that led Langmuir to adopt the rule.
Use in nomenclature
The oxidation state in compound naming for
transition metals and
lanthanides and
actinides is placed either as a right superscript to the element symbol in a chemical formula, such as FeIII or in parentheses after the name of the element in chemical names, such as iron(III). For example, Fe 2(SO 4) 3 is named
iron(III) sulfate and its formula can be shown as FeIII 2(SO 4) 3. This is because a
sulfate ion has a charge of −2, so each iron atom takes a charge of +3.
History of the oxidation state concept
Early days
Oxidation itself was first studied by
Antoine Lavoisier, who defined it as the result of reactions with
oxygen (hence the name).[180][181] The term has since been generalized to imply a formal loss of electrons. Oxidation states, called oxidation grades by
Friedrich Wöhler in 1835,[182] were one of the intellectual stepping stones that
Dmitri Mendeleev used to derive the
periodic table.
William B. Jensen[183] gives an overview of the history up to 1938.
Use in nomenclature
When it was realized that some metals form two different binary compounds with the same nonmetal, the two compounds were often distinguished by using the ending -ic for the higher metal oxidation state and the ending -ous for the lower. For example, FeCl3 is
ferric chloride and FeCl2 is
ferrous chloride. This system is not very satisfactory (although sometimes still used) because different metals have different oxidation states which have to be learned: ferric and ferrous are +3 and +2 respectively, but cupric and cuprous are +2 and +1, and stannic and stannous are +4 and +2. Also, there was no allowance for metals with more than two oxidation states, such as
vanadium with oxidation states +2, +3, +4, and +5.[17]: 84
This system has been largely replaced by one suggested by
Alfred Stock in 1919[184] and adopted[185] by
IUPAC in 1940. Thus, FeCl2 was written as
iron(II) chloride rather than ferrous chloride. The Roman numeral II at the central atom came to be called the "
Stock number" (now an obsolete term), and its value was obtained as a charge at the central atom after removing its ligands along with the
electron pairs they shared with it.[20]: 147
Development towards the current concept
The term "oxidation state" in English chemical literature was popularized by
Wendell Mitchell Latimer in his 1938 book about electrochemical potentials.[186] He used it for the value (synonymous with the German term Wertigkeit) previously termed "valence", "polar valence" or "polar number"[187] in English, or "oxidation stage" or indeed[188][189] the "state of oxidation". Since 1938, the term "oxidation state" has been connected with
electrochemical potentials and electrons exchanged in
redox couples participating in redox reactions. By 1948, IUPAC used the 1940 nomenclature rules with the term "oxidation state",[190][191] instead of the original[185]valency. In 1948
Linus Pauling proposed that oxidation number could be determined by extrapolating bonds to being completely ionic in the direction of
electronegativity.[192] A full acceptance of this suggestion was complicated by the fact that the
Pauling electronegativities as such depend on the oxidation state and that they may lead to unusual values of oxidation states for some transition metals. In 1990 IUPAC resorted to a postulatory (rule-based) method to determine the oxidation state.[193] This was complemented by the synonymous term oxidation number as a descendant of the Stock number introduced in 1940 into the nomenclature. However, the terminology using "
ligands"[20]: 147 gave the impression that oxidation number might be something specific to
coordination complexes. This situation and the lack of a real single definition generated numerous debates about the meaning of oxidation state, suggestions about methods to obtain it and definitions of it. To resolve the issue, an IUPAC project (2008-040-1-200) was started in 2008 on the "Comprehensive Definition of Oxidation State", and was concluded by two reports[5][4] and by the revised entries "Oxidation State"[6] and "Oxidation Number"[7] in the
IUPAC Gold Book. The outcomes were a single definition of oxidation state and two algorithms to calculate it in molecular and extended-solid compounds, guided by
Allen electronegativities that are independent of oxidation state.
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^Disodium helide, (Na+)2He(e-)2, has been synthesized at high pressure, see Dong, Xiao; Oganov, Artem R.; Goncharov, Alexander F.; Stavrou, Elissaios; Lobanov, Sergey; Saleh, Gabriele; Qian, Guang-Rui; Zhu, Qiang; Gatti, Carlo; Deringer, Volker L.; Dronskowski, Richard; Zhou, Xiang-Feng; Prakapenka, Vitali B.; Konôpková, Zuzana; Popov, Ivan A.; Boldyrev, Alexander I.; Wang, Hui-Tian (6 February 2017). "A stable compound of helium and sodium at high pressure". Nature Chemistry. 9 (5): 440–445.
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^
abcdeNa(−1), K(−1), Rb(−1), and Cs(−1) are known in
alkalides; the table by Greenwood and Earnshaw shows −1 only for Na and also erroneously for Li; no lithides are described.
^Tetrazoles contain a pair of double-bonded nitrogen atoms with oxidation state 0 in the ring. A Synthesis of the parent 1H-tetrazole, CH2N4 (two atoms N(0)) is given in Henry, Ronald A.; Finnegan, William G. (1954). "An Improved Procedure for the Deamination of 5-Aminotetrazole". J. Am. Chem. Soc. 76 (1): 290–291.
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^Gold heptafluoride is calculated to be the pentafluoride with a molecular F2 ligand. Himmel, Daniel; Riedel, Sebastian (2007). "After 20 Years, Theoretical Evidence That 'AuF7' Is Actually AuF5•F2". Inorganic Chemistry. 46 (13): 5338–5342.
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^Ne(0) has been observed in Cr(CO)5Ne. Perutz, Robin N.; Turner, James J. (August 1975). "Photochemistry of the Group 6 hexacarbonyls in low-temperature matrices. III. Interaction of the pentacarbonyls with noble gases and other matrices". Journal of the American Chemical Society. 97 (17): 4791–4800.
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^The compound
NaCl has been shown in experiments to exists in several unusual
stoichiometries under high pressure, including Na3Cl in which contains a layer of sodium(0) atoms; see Zhang, W.; Oganov, A. R.; Goncharov, A. F.; Zhu, Q.; Boulfelfel, S. E.; Lyakhov, A. O.; Stavrou, E.; Somayazulu, M.; Prakapenka, V. B.; Konôpková, Z. (2013). "Unexpected Stable Stoichiometries of Sodium Chlorides". Science. 342 (6165): 1502–1505.
arXiv:1310.7674.
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^Mg(0) has been synthesized in a compound containing a Na2Mg22+ cluster coordinated to a bulky organic ligand; see Rösch, B.; Gentner, T. X.; Eyselein, J.; Langer, J.; Elsen, H.; Li, W.; Harder, S. (2021). "Strongly reducing magnesium(0) complexes". Nature. 592 (7856): 717–721.
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^Al(−2) has been observed in Sr14[Al42[Ge]3, see Wemdorff, Marco; Röhr, Caroline (2007). "Sr14[Al42[Ge]3: Eine Zintl-Phase mit isolierten [Ge]4–- und [Al48–-Anionen / Sr14[Al42[Ge]3: A Zintl Phase with Isolated [Ge]4–- and [Al48– Anions". Zeitschrift für Naturforschung B (in German). 62 (10): 1227.
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^P(0) has been observed, see Wang, Yuzhong; Xie, Yaoming; Wei, Pingrong; King, R. Bruce; Schaefer, Iii; Schleyer, Paul v. R.; Robinson, Gregory H. (2008). "Carbene-Stabilized Diphosphorus". Journal of the American Chemical Society. 130 (45): 14970–1.
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10.1021/ja807828t.
PMID18937460.
^Chlorine(0) is present as dichlorine in {SnCl62--Cl2}x and {PbCl62--Cl2}x polymeric anion complexes see Usoltsev, Andrey N.; Korobeynikov, Nikita A.; Kolesov, Boris A.; Novikov, Alexander S.; Samsonenko, Denis G.; Fedin, Vladimir P.; Sokolov, Maxim N.; Adonin, Sergey A. (February 24, 2021). "Rule, Not Exclusion: Formation of Dichlorine-Containing Supramolecular Complexes with Chlorometalates(IV)". Inorg. Chem. 60 (6): 4171–4177.
doi:
10.1021/acs.inorgchem.1c00436.
PMID33626273.
S2CID232047538.
^The equilibrium Cl2O6⇌2ClO3 is mentioned by Greenwood and Earnshaw, but it has been refuted, see Lopez, Maria; Juan E. Sicre (1990). "Physicochemical properties of chlorine oxides. 1. Composition, ultraviolet spectrum, and kinetics of the thermolysis of gaseous dichlorine hexoxide". J. Phys. Chem. 94 (9): 3860–3863.
doi:
10.1021/j100372a094., and Cl2O6 is actually chlorine(V,VII) oxide. However, ClO3 has been observed, see Grothe, Hinrich; Willner, Helge (1994). "Chlorine Trioxide: Spectroscopic Properties, Molecular Structure, and Photochemical Behavior". Angew. Chem. Int. Ed. 33 (14): 1482–1484.
doi:
10.1002/anie.199414821.
^Ca(I) has been observed; see Krieck, Sven; Görls, Helmar; Westerhausen, Matthias (2010). "Mechanistic Elucidation of the Formation of the Inverse Ca(I) Sandwich Complex [(thf)3Ca(μ-C6H3-1,3,5-Ph3)Ca(thf)3] and Stability of Aryl-Substituted Phenylcalcium Complexes". Journal of the American Chemical Society. 132 (35): 12492–501.
doi:
10.1021/ja105534w.
PMID20718434.
^
abcOctacarbonyl complexes isolated of Ca, Sr, Ba have been observed in a neon matrix, but it remains unclear whether these are metal(0) complexes because calculations disagree whether the metal is covalently or ionically bonded to the ligands; see Wu, X.; Zhao, L.; Jin, J.; Pan, S.; Li, W.; Jin, X.; Wang, G.; Zhou, M.; Frenking, G. (2018). "Observation of alkaline earth complexes M(CO)8 (M = Ca, Sr, or Ba) that mimic transition metals". Science. 361 (6405): 912–916.
Bibcode:
2018Sci...361..912W.
doi:
10.1126/science.aau0839.
PMID30166489.
S2CID52131470
^Sc(0) has been observed; see F. Geoffrey N. Cloke; Karl Khan & Robin N. Perutz (1991). "η-Arene complexes of scandium(0) and scandium(II)". J. Chem. Soc., Chem. Commun. (19): 1372–1373.
doi:
10.1039/C39910001372.
^Sc(I) has been observed; see Polly L. Arnold; F. Geoffrey; N. Cloke; Peter B. Hitchcock & John F. Nixon (1996). "The First Example of a Formal Scandium(I) Complex: Synthesis and Molecular Structure of a 22-Electron Scandium Triple Decker Incorporating the Novel 1,3,5-Triphosphabenzene Ring". J. Am. Chem. Soc. 118 (32): 7630–7631.
doi:
10.1021/ja961253o.
^Sc(II) has been observed; see Woen, David H.; Chen, Guo P.; Ziller, Joseph W.; Boyle, Timothy J.; Furche, Filipp; Evans, William J. (January 2017). "Solution Synthesis, Structure, and CO Reduction Reactivity of a Scandium(II) Complex". Angewandte Chemie International Edition. 56 (8): 2050–2053.
doi:
10.1002/anie.201611758.
PMID28097771.
^Ti(I) has been observed in [Ti(η6-1,3,5-C6H3iPr3)2][BAr4] (Ar = C6H5, p-C6H4F, 3,5-C6H3(CF3)2); see Calderazzo, Fausto; Ferri, Isabella; Pampaloni, Guido; Englert, Ulli; Green, Malcolm L. H. (1997). "Synthesis of [Ti(η6-1,3,5-C6H3iPr3)2][BAr4] (Ar = C6H5, p-C6H4F, 3,5-C6H3(CF3)2), the First Titanium(I) Derivatives". Organometallics. 16 (14): 3100–3101.
doi:
10.1021/om970155o.
^Ti(−1) has been reported in [Ti(
bipy)3−, but was later shown to be Ti(+3); see Bowman, A. C.; England, J.; Sprouls, S.; Weihemüller, T.; Wieghardt, K. (2013). "Electronic structures of homoleptic [tris(2,2'-bipyridine)M]n complexes of the early transition metals (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta; n = 1+, 0, 1-, 2-, 3-): an experimental and density functional theoretical study". Inorganic Chemistry. 52 (4): 2242–2256.
doi:
10.1021/ic302799s.
PMID23387926. However, Ti(−1) occurs in [Ti(η-C6H6− and [Ti(η-C6H5CH3)]−, see Bandy, J. A.; Berry, A.; Green, M. L. H.; Perutz, R. N.; Prout, K.; Verpeautz, J.-N. (1984). "Synthesis of anionic sandwich compounds: [Ti(η-C6H5R)2– and the crystal structure of [K(18-crown-6)(µ-H)Mo(η-C5H5)2]". Inorganic Chemistry. 52 (4): 729–731.
doi:
10.1039/C39840000729.
^Jilek, Robert E.; Tripepi, Giovanna; Urnezius, Eugenijus; Brennessel, William W.; Young, Victor G. Jr.; Ellis, John E. (2007). "Zerovalent titanium–sulfur complexes. Novel dithiocarbamato derivatives of Ti(CO)6: [Ti(CO)4(S2CNR2)]−". Chem. Commun. (25): 2639–2641.
doi:
10.1039/B700808B.
PMID17579764.
^Mn(–2) has been described erroneously by Greenwood as [MnPc]2–; for a correct explanation, see Gcineka Mbambisa; Prudence Tau; Edith Antunes; Tebello Nyokong (2007). "Synthesis and electrochemical properties of purple manganese(III) and red titanium(IV) phthalocyanine complexes octa-substituted at non-peripheral positions with pentylthio groups". Polyhedron. 26 (18): 5355–5364.
doi:
10.1016/j.poly.2007.08.007.
^Fe(VII) has been observed in [FeO4−; see Lu, Jun-Bo; Jian, Jiwen; Huang, Wei; Lin, Hailu; Zhou, Mingfei (2016). "Experimental and theoretical identification of the Fe(VII) oxidation state in FeO4−". Physical Chemistry Chemical Physics. 18 (45): 31125–31131.
Bibcode:
2016PCCP...1831125L.
doi:
10.1039/C6CP06753K.
PMID27812577.
^Fe(VIII) has been reported; see Yurii D. Perfiliev; Virender K. Sharma (2008). "Higher Oxidation States of Iron in Solid State: Synthesis and Their Mössbauer Characterization – Ferrates – ACS Symposium Series (ACS Publications)". Platinum Metals Review. 48 (4): 157–158.
doi:
10.1021/bk-2008-0985.ch007. However, its existence has been disputed.
^
abcdefghijFe(−4), Ru(−4), and Os(−4) have been observed in metal-rich compounds containing octahedral complexes [MIn6−xSnx]; Pt(−3) (as a dimeric anion [Pt–Pt]6−), Cu(−2), Zn(−2), Ag(−2), Cd(−2), Au(−2), and Hg(−2) have been observed (as dimeric and monomeric anions; dimeric ions were initially reported to be [T–T]2− for Zn, Cd, Hg, but later shown to be [T–T]4− for all these elements) in La2Pt2In, La2Cu2In, Ca5Au3, Ca5Ag3, Ca5Hg3, Sr5Cd3, Ca5Zn3(structure (AE2+)5(T–T)4−T2−⋅4e−), Yb3Ag2, Ca5Au4, and Ca3Hg2; Au(–3) has been observed in ScAuSn and in other 18-electron half-Heusler compounds. See Changhoon Lee; Myung-Hwan Whangbo (2008). "Late transition metal anions acting as p-metal elements". Solid State Sciences. 10 (4): 444–449.
Bibcode:
2008SSSci..10..444K.
doi:
10.1016/j.solidstatesciences.2007.12.001. and Changhoon Lee; Myung-Hwan Whangbo; Jürgen Köhler (2010). "Analysis of Electronic Structures and Chemical Bonding of Metal-rich Compounds. 2. Presence of Dimer (T–T)4– and Isolated T2– Anions in the Polar Intermetallic Cr5B3-Type Compounds AE5T3 (AE = Ca, Sr; T = Au, Ag, Hg, Cd, Zn)". Zeitschrift für Anorganische und Allgemeine Chemie. 636 (1): 36–40.
doi:
10.1002/zaac.200900421.
^Ni(−2) has been observed in Li2[Ni(
1,5-COD)2], see Jonas, Klaus (1975). "Dilithium-Nickel-Olefin Complexes. Novel Bimetal Complexes Containing a Transition Metal and a Main Group Metal". Angew. Chem. Int. Ed. 14 (11): 752–753.
doi:
10.1002/anie.197507521. and Ellis, John E. (2006). "Adventures with Substances Containing Metals in Negative Oxidation States". Inorganic Chemistry. 45 (8): 3167–86.
doi:
10.1021/ic052110i.
PMID16602773.
^Zn(0) has been observed; see Singh, Amit Pratap; Samuel, Prinson P.; Roesky, Herbert W.; Schwarzer, Martin C.; Frenking, Gernot; Sidhu, Navdeep S.; Dittrich, Birger (2013). "A Singlet Biradicaloid Zinc Compound and Its Nonradical Counterpart". J. Am. Chem. Soc. 135 (19): 7324–9.
doi:
10.1021/ja402351x.
PMID23600486. and Soleilhavoup, Michèle; Bertrand, Guy (2015). "Cyclic (Alkyl)(Amino)Carbenes (CAACs): Stable Carbenes on the Rise". Acc. Chem. Res. 48 (2): 256–266.
doi:
10.1021/ar5003494.
PMID25515548.
^Zn(III) has been predicted to be stable in compounds with highly stabilized borane-based trianions, but no Zn(III) candidates are known experimentally; see Hong Fang; Huta Banjade; Deepika; Puru Jena (2021). "Realization of the Zn3+ oxidation state". Nanoscale. 13 (33): 14041–14048.
doi:
10.1039/D1NR02816B.
PMID34477685.
S2CID237400349.
^Ga(0) has been observed in
Gallium monoiodide among other gallium's oxidation states
^Ge(−1), Ge(−2), and Ge(−3) have been observed in
germanides; see Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (1995). "Germanium". Lehrbuch der Anorganischen Chemie (in German) (101 ed.). Walter de Gruyter. pp. 953–959.
ISBN978-3-11-012641-9.
^As(0) has been observed; see Abraham, Mariham Y.; Wang, Yuzhong; Xie, Yaoming; Wei, Pingrong; Shaefer III, Henry F.; Schleyer, P. von R.; Robinson, Gregory H. (2010). "Carbene Stabilization of Diarsenic: From Hypervalency to Allotropy". Chemistry: A European Journal. 16 (2): 432–5.
doi:
10.1002/chem.200902840.
PMID19937872.
^As(I) has been observed in
arsenic(I) iodide (AsI); see Ellis, Bobby D.; MacDonald, Charles L. B. (2004). "Stabilized Arsenic(I) Iodide: A Ready Source of Arsenic Iodide Fragments and a Useful Reagent for the Generation of Clusters". Inorganic Chemistry. 43 (19): 5981–6.
doi:
10.1021/ic049281s.
PMID15360247.
^As(IV) has been observed in
arsenic(IV) hydroxide (As(OH)4) and HAsO−; see Kläning, Ulrik K.; Bielski, Benon H. J.; Sehested, K. (1989). "Arsenic(IV). A pulse-radiolysis study". Inorganic Chemistry. 28 (14): 2717–24.
doi:
10.1021/ic00313a007.
^Se(−1) has been observed in
diselenides(2−) (Se22−).
^A Se(0) atom has been identified using DFT in [ReOSe(2-pySe)3]; see Cargnelutti, Roberta; Lang, Ernesto S.; Piquini, Paulo; Abram, Ulrich (2014). "Synthesis and structure of [ReOSe(2-Se-py)3]: A rhenium(V) complex with selenium(0) as a ligand". Inorganic Chemistry Communications. 45: 48–50.
doi:
10.1016/j.inoche.2014.04.003.
ISSN1387-7003.
^Se(III) has been observed in Se2NBr3; see Lau, Carsten; Neumüller, Bernhard; Vyboishchikov, Sergei F.; Frenking, Gernot; Dehnicke, Kurt; Hiller, Wolfgang; Herker, Martin (1996). "Se2NBr3, Se2NCl5, Se2NCl−6: New Nitride Halides of Selenium(III) and Selenium(IV)". Chemistry: A European Journal. 2 (11): 1393–1396.
doi:
10.1002/chem.19960021108.
^Bromine(0) is present as an adduct in a copper-bromine complex, see Okrut, Alexander; Feldmann, Claus (5 March 2008). "{[P(o-tolyl)3]Br}2[Cu2Br6](Br2)—An Ionic Compound Containing Molecular Bromine". Inorganic Chemistry. 47 (8): 3084–3087.
doi:
10.1021/ic7021038.
PMID18318489.
^Br(II) is known to occur in bromine monoxide
radical; see
[1]
^
abcdefghijklmYttrium and all lanthanides except Ce and Pm have been observed in the oxidation state 0 in bis(1,3,5-tri-t-butylbenzene) complexes, see Cloke, F. Geoffrey N. (1993). "Zero Oxidation State Compounds of Scandium, Yttrium, and the Lanthanides". Chem. Soc. Rev. 22: 17–24.
doi:
10.1039/CS9932200017. and Arnold, Polly L.; Petrukhina, Marina A.; Bochenkov, Vladimir E.; Shabatina, Tatyana I.; Zagorskii, Vyacheslav V.; Cloke (2003-12-15). "Arene complexation of Sm, Eu, Tm and Yb atoms: a variable temperature spectroscopic investigation". Journal of Organometallic Chemistry. 688 (1–2): 49–55.
doi:
10.1016/j.jorganchem.2003.08.028.
^Y(I) has been observed in
yttrium(I) bromide (YBr); see Kaley A. Walker; Michael C. L. Gerry (1998). "The pure rotational spectrum of yttrium monobromide". The Journal of Chemical Physics. 109 (13): 5439–5445.
doi:
10.1063/1.477162.
^Y(II) has been observed in [(18-crown-6)K][(C5H4SiMe3)3Y]; see MacDonald, M. R.; Ziller, J. W.; Evans, W. J. (2011). "Synthesis of a Crystalline Molecular Complex of Y2+, [(18-crown-6)K][(C5H4SiMe3)3Y]". J. Am. Chem. Soc. 133 (40): 15914–17.
doi:
10.1021/ja207151y.
PMID21919538.
^Zr(−1) has been reported in [Zr(
bipy)3− (see
Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.).
Butterworth-Heinemann. p. 960.
ISBN978-0-08-037941-8. and Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (1995). "Zirconium". Lehrbuch der Anorganischen Chemie (in German) (101 ed.). Walter de Gruyter. p. 1413.
ISBN978-3-11-012641-9.), but was later shown to be Zr(+4); see Bowman, A. C.; England, J.; Sprouls, S.; Weihemüller, T.; Wieghardt, K. (2013). "Electronic structures of homoleptic [tris(2,2'-bipyridine)M]n complexes of the early transition metals (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta; n = 1+, 0, 1-, 2-, 3-): an experimental and density functional theoretical study". Inorganic Chemistry. 52 (4): 2242–56.
doi:
10.1021/ic302799s.
PMID23387926.
^
abZr(0) and Hf(0) occur in (η6-(1,3,5-tBu)3C6H3)2M (M=Zr, Hf) and [(η5-C5R5M(CO)4−, see Chirik, P. J.; Bradley, C. A. (2007). "4.06 - Complexes of Zirconium and Hafnium in Oxidation States 0 to ii". Comprehensive Organometallic Chemistry III. From Fundamentals to Applications. Vol. 4. Elsevier Ltd. pp. 697–739.
doi:
10.1016/B0-08-045047-4/00062-5.
ISBN9780080450476.
^Complexes of Nb(0) and Ta(0) have been observed, see Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (2003). "4.5.7. Niobium(0) and Tantalum(0)". In J. A. McCleverty; T.J. Meyer (eds.). Comprehensive Coordination Chemistry II: From Biology to Nanotechnology. Vol. 4 (2 ed.). Newnes. pp. 297–299.
ISBN978-0-08-091316-2.
^
abNb(I) and Ta(I) occur in
CpNb(CO)4 and
CpTa(CO)4, see Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (1995). "Tantal". Lehrbuch der Anorganischen Chemie (in German) (101 ed.). Walter de Gruyter. p. 1430.
ISBN978-3-11-012641-9. and King, R. Bruce (1969). Transition-Metal Organometallic Chemistry: An Introduction. Academic Press. p. 11.
ISBN978-0-32-315996-8.
^Tc(–1) occurs in HTc(CO)5; see John E. Ellis (2003). "Metal Carbonyl Anions: from [Fe(CO)42- to [Hf(CO)62- and Beyond†". Organometallics. 22 (17): 3322–3338.
doi:
10.1021/om030105l. Tc(–3) is erroneously reported by Greenwood.
^George, G.N.; Klein, S.I.; Nixon, J.F. (1984). "Electron paramagnetic resonance spectroscopic studies on the zero-valent rhodium complex [Rh(P(OPri)3)4] at X-and Q-band frequencies". Chemical Physics Letters. 108 (6): 627–630.
Bibcode:
1984CPL...108..627G.
doi:
10.1016/0009-2614(84)85069-1.
^Rh(VII) is known in the RhO3+ cation, see Da Silva Santos, Mayara; Stüker, Tony; Flach, Max; Ablyasova, Olesya S.; Timm, Martin; von Issendorff, Bernd; Hirsch, Konstantin; Zamudio‐Bayer, Vicente; Riedel, Sebastian; Lau, J. Tobias (2022). "The Highest Oxidation State of Rhodium: Rhodium(VII) in [RhO3]+". Angew. Chem. Int. Ed. 61 (38).
doi:
10.1002/anie.202207688.
PMID35818987.
S2CID250455408.
^Palladium(VI) has been claimed to exist in Dagani, RON (2002). "New Palladium Oxidation State?". Chem. Eng. News. 80 (2): 8.
doi:
10.1021/cen-v080n002.p008., but this has been refuted showing it is a Palladium(II).
^The Ag− ion has been observed in metal ammonia solutions: see Tran, N. E.; Lagowski, J. J. (2001). "Metal Ammonia Solutions: Solutions Containing Argentide Ions". Inorganic Chemistry. 40 (5): 1067–68.
doi:
10.1021/ic000333x.
^Ag(0) has been observed in carbonyl complexes in low-temperature matrices: see McIntosh, D.; Ozin, G. A. (1976). "Synthesis using metal vapors. Silver carbonyls. Matrix infrared, ultraviolet-visible, and electron spin resonance spectra, structures, and bonding of silver tricarbonyl, silver dicarbonyl, silver monocarbonyl, and disilver hexacarbonyl". J. Am. Chem. Soc. 98 (11): 3167–75.
doi:
10.1021/ja00427a018. Also, Ag(0) has been observed in [Ag4py2n, see Hoi Ri Moon; Cheol Ho Choi; Myunghyun Paik Suh (2008). "A Stair-Shaped Molecular Silver(0) Chain". Angewandte Chemie International Edition. 47 (44): 8390–93.
doi:
10.1002/anie.200803465.
PMID18830949.
^Cd(I) has been observed in
cadmium(I) tetrachloroaluminate (Cd2(AlCl4)2); see Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (1985). "Cadmium". Lehrbuch der Anorganischen Chemie (in German) (91–100 ed.). Walter de Gruyter. pp. 1056–1057.
ISBN978-3-11-007511-3.
^In(–5) has been observed in La3InGe, see Guloy, A. M.; Corbett, J. D. (1996). "Synthesis, Structure, and Bonding of Two Lanthanum Indium Germanides with Novel Structures and Properties". Inorganic Chemistry. 35 (9): 2616–22.
doi:
10.1021/ic951378e.
PMID11666477.
^In(−2) has been observed in Na2In, see
[2], p. 69.
^Unstable In(0) carbonyls and clusters have been detected, see
[3], p. 6.
^Sb(−2) has been observed in [Sb24−, e.g. in RbBa4[Sb2][Sb][O], see Boss, Michael; Petri, Denis; Pickhard, Frank; Zönnchen, Peter; Röhr, Caroline (2005). "Neue Barium-Antimonid-Oxide mit den Zintl-Ionen [Sb]3−, [Sb24− und 1∞[Sbnn− / New Barium Antimonide Oxides containing Zintl Ions [Sb]3−, [Sb24− and 1∞[Sbnn−". Zeitschrift für Anorganische und Allgemeine Chemie (in German). 631 (6–7): 1181–1190.
doi:
10.1002/zaac.200400546.
^Sb(I) and Sb(II) have been observed in
organoantimony compounds; for Sb(I), see Šimon, Petr; de Proft, Frank; Jambor, Roman; Růžička, Aleš; Dostál, Libor (2010). "Monomeric Organoantimony(I) and Organobismuth(I) Compounds Stabilized by an NCN Chelating Ligand: Syntheses and Structures". Angewandte Chemie International Edition. 49 (32): 5468–5471.
doi:
10.1002/anie.201002209.
PMID20602393.
^Sb(IV) has been observed in [SbCl62−, see Nobuyoshi Shinohara; Masaaki Ohsima (2000). "Production of Sb(IV) Chloro Complex by Flash Photolysis of the Corresponding Sb(III) and Sb(V) Complexes in CH3CN and CHCl3". Bulletin of the Chemical Society of Japan. 73 (7): 1599–1604.
doi:
10.1246/bcsj.73.1599.
^Te(V) is mentioned by Greenwood and Earnshaw, but they do not give any example of a Te(V) compound. What was long thought to be
ditellurium decafluoride (Te2F10) is actually bis(pentafluorotelluryl) oxide, F5TeOTeF5: see Watkins, P. M. (1974). "Ditellurium decafluoride - A Continuing Myth". Journal of Chemical Education. 51 (9): 520–521.
Bibcode:
1974JChEd..51..520W.
doi:
10.1021/ed051p520. However, Te(V) has been observed in HTeO−, TeO−, HTeO−2, and TeO−3; see Kläning, Ulrik K.; Sehested, K. (2001).
"Tellurium(V). A Pulse Radiolysis Study". The Journal of Physical Chemistry A. 105 (27): 6637–45.
Bibcode:
2001JPCA..105.6637K.
doi:
10.1021/jp010577i.
^I(IV) has been observed in
iodine dioxide (IO2); see Pauling, Linus (1988). "Oxygen Compounds of Nonmetallic Elements". General Chemistry (3rd ed.). Dover Publications, Inc. p. 259.
ISBN978-0-486-65622-9.
^I(VI) has been observed in IO3, IO42−, H5IO6−, H2IO52−, H4IO62−, and HIO53−; see Kläning, Ulrik K.; Sehested, Knud; Wolff, Thomas (1981). "Laser flash photolysis and pulse radiolysis of iodate and periodate in aqueous solution. Properties of iodine(VI)". J. Chem. Soc., Faraday Trans. 1. 77 (7): 1707–18.
doi:
10.1039/F19817701707.
^Pr(I) has been observed in [PrB4−; see Chen, Xin; Chen, Teng-Teng; Li, Wang-Lu; Lu, Jun-Bo; Zhao, Li-Juan; Jian, Tian; Hu, Han-Shi; Wang, Lai-Sheng; Li, Jun (2018-12-13). "Lanthanides with Unusually Low Oxidation States in the PrB3– and PrB4– Boride Clusters". Inorganic Chemistry. 58 (1): 411–418.
doi:
10.1021/acs.inorgchem.8b02572.
PMID30543295.
S2CID56148031.
^Pr(V) has been observed in [PrO2+; see Zhang, Qingnan; Hu, Shu-Xian; Qu, Hui; Su, Jing; Wang, Guanjun; Lu, Jun-Bo; Chen, Mohua; Zhou, Mingfei; Li, Jun (2016-06-06). "Pentavalent Lanthanide Compounds: Formation and Characterization of Praseodymium(V) Oxides". Angewandte Chemie International Edition. 55 (24): 6896–6900.
doi:
10.1002/anie.201602196.
ISSN1521-3773.
PMID27100273.
^Nd(IV) has been observed in unstable solid state compounds; see Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter,
ISBN0-12-352651-5
^
abcdeAll the
lanthanides (La–Lu) in the +2 oxidation state have been observed (except La, Gd, Lu) in dilute, solid solutions of dihalides of these elements in alkaline earth dihalides (see Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter,
ISBN0-12-352651-5) and (except Pm) in organometallic molecular complexes, see
Lanthanides Topple Assumptions and Meyer, G. (2014). "All the Lanthanides Do It and Even Uranium Does Oxidation State +2". Angewandte Chemie International Edition. 53 (14): 3550–51.
doi:
10.1002/anie.201311325.
PMID24616202.. Additionally, all the
lanthanides (La–Lu) form dihydrides (LnH2), dicarbides (LnC2), monosulfides (LnS), monoselenides (LnSe), and monotellurides (LnTe), but for most elements these compounds have Ln3+ ions with electrons delocalized into conduction bands, e. g. Ln3+(H−)2(e−).
^Dy(IV) has been observed in unstable solid state compounds; see Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter,
ISBN0-12-352651-5
^Os(−1) has been observed in Na[Os(CO)13; see Krause, J.; Siriwardane, Upali; Salupo, Terese A.; Wermer, Joseph R.; Knoeppel, David W.; Shore, Sheldon G. (1993). "Preparation of [Os3(CO)112− and its reactions with Os3(CO)12; structures of [Et4N] [HOs3(CO)11] and H2OsS4(CO)". Journal of Organometallic Chemistry. 454 (1–2): 263–271.
doi:
10.1016/0022-328X(93)83250-Y. and Carter, Willie J.; Kelland, John W.; Okrasinski, Stanley J.; Warner, Keith E.; Norton, Jack R. (1982). "Mononuclear hydrido alkyl carbonyl complexes of osmium and their polynuclear derivatives". Inorganic Chemistry. 21 (11): 3955–3960.
doi:
10.1021/ic00141a019.
^Ir(–2) has been observed in IrVO2–; see Le-Shi Chen; Yun-Zhu Liu; Jiao-Jiao Chen; Si-Dun Wang; Tong-Mei Ma; Xiao-Na Li; Sheng-Gui He (2022). "Water–Gas Shift Catalyzed by Iridium–Vanadium Oxide Clusters IrVO2– with Iridium in a Rare Oxidation State of −II". The Journal of Physical Chemistry A. 126 (32): 5294–5301.
doi:
10.1021/acs.jpca.2c03974.
^Ir(VIII) has been observed in
iridium tetroxide (IrO4); see Gong, Yu; Zhou, Mingfei; Kaupp, Martin; Riedel, Sebastian (2009). "Formation and Characterization of the Iridium Tetroxide Molecule with Iridium in the Oxidation State +VIII". Angewandte Chemie International Edition. 48 (42): 7879–7883.
doi:
10.1002/anie.200902733.
PMID19593837.
^Ir(IX) has been observed in IrO+4; see Wang, Guanjun; Zhou, Mingfei; Goettel, James T.; Schrobilgen, Gary G.; Su, Jing; Li, Jun; Schlöder, Tobias; Riedel, Sebastian (21 August 2014). "Identification of an iridium-containing compound with a formal oxidation state of IX". Nature. 514 (7523): 475–477.
Bibcode:
2014Natur.514..475W.
doi:
10.1038/nature13795.
PMID25341786.
S2CID4463905.
^Pt(−1) and Pt(−2) have been observed in the
barium platinides BaPt and Ba2Pt, respectively: see Karpov, Andrey; Konuma, Mitsuharu; Jansen, Martin (2006). "An experimental proof for negative oxidation states of platinum: ESCA-measurements on barium platinides". Chemical Communications (8): 838–840.
doi:
10.1039/b514631c.
PMID16479284.
^Pt(I) and Pt(III) have been observed in bimetallic and polymetallic species; see
Kauffman, George B.; Thurner, Joseph J.; Zatko, David A. (1967). "Ammonium Hexachloroplatinate(IV)". Inorganic Syntheses. Vol. 9. pp. 182–185.
doi:
10.1002/9780470132401.ch51.
ISBN978-0-470-13240-1.
^Au(0) has been observed, see Mézaille, Nicolas; Avarvari, Narcis; Maigrot, Nicole; Ricard, Louis; Mathey, François; Le Floch, Pascal; Cataldo, Laurent; Berclaz, Théo; Geoffroy, Michel (1999). "Gold(I) and Gold(0) Complexes of Phosphinine‐Based Macrocycles". Angewandte Chemie International Edition. 38 (21): 3194–3197.
doi:
10.1002/(SICI)1521-3773(19991102)38:21<3194::AID-ANIE3194>3.0.CO;2-O.
PMID10556900.
^Hg(IV) has been reported in
mercury(IV) fluoride (HgF4); see Xuefang Wang; Lester Andrews; Sebastian Riedel; Martin Kaupp (2007). "Mercury Is a Transition Metal: The First Experimental Evidence for HgF4". Angew. Chem. Int. Ed. 46 (44): 8371–8375.
doi:
10.1002/anie.200703710.
PMID17899620. However, it could not be confirmed by later experiments; see Young, Nigel (2016-07-12).
"Is mercury a transition metal?". University of Hull. Archived from
the original on 2016-10-12.
^Tl(−5) has been observed in Na23K9Tl15.3, see Dong, Z.-C.; Corbett, J. D. (1996). "Na23K9Tl15.3: An Unusual Zintl Compound Containing Apparent Tl57−, Tl48−, Tl37−, and Tl5− Anions". Inorganic Chemistry. 35 (11): 3107–12.
doi:
10.1021/ic960014z.
PMID11666505.
^Tl(−1) has been observed in
caesium thallide (CsTl); see King, R. B.; Schleyer, R. (2004). "Theory and concepts in main-group cluster chemistry". In Driess, M.; Nöth, H. (eds.). Molecular clusters of the main group elements. Wiley-VCH, Chichester. p. 19.
ISBN978-3-527-61437-0.
^Tl(+2) has been observed in
tetrakis(hypersilyl)dithallium ([(
Me3Si)Si]2Tl—Tl[Si(SiMe3)]2), see Sonja Henkel; Dr. Karl Wilhelm Klinkhammer; Dr. Wolfgang Schwarz (1994). "Tetrakis(hypersilyl)dithallium(Tl—Tl): A Divalent Thallium Compound". Angew. Chem. Int. Ed. 33 (6): 681–683.
doi:
10.1002/anie.199406811.
^Pb(−2) has been observed in BaPb, see Ferro, Riccardo (2008). Nicholas C. Norman (ed.). Intermetallic Chemistry. Elsevier. p. 505.
ISBN978-0-08-044099-6. and Todorov, Iliya; Sevov, Slavi C. (2004). "Heavy-Metal Aromatic Rings: Cyclopentadienyl Anion Analogues Sn56− and Pb56− in the Zintl Phases Na8BaPb6, Na8BaSn6, and Na8EuSn6". Inorganic Chemistry. 43 (20): 6490–94.
doi:
10.1021/ic000333x.
^Pb(+1) and Pb(+3) have been observed in
organolead compounds, e.g. hexamethyldiplumbane Pb2(CH3)6; for Pb(I), see Siew-Peng Chia; Hong-Wei Xi; Yongxin Li; Kok Hwa Lim; Cheuk-Wai So (2013). "A Base-Stabilized Lead(I) Dimer and an Aromatic Plumbylidenide Anion". Angew. Chem. Int. Ed. 52 (24): 6298–6301.
doi:
10.1002/anie.201301954.
PMID23629949.
^Bi(I) has been observed in
bismuth monobromide (BiBr) and
bismuth monoiodide (BiI); see Godfrey, S. M.; McAuliffe, C. A.; Mackie, A. G.; Pritchard, R. G. (1998). Nicholas C. Norman (ed.). Chemistry of arsenic, antimony, and bismuth. Springer. pp. 67–84.
ISBN978-0-7514-0389-3.
^Bi(IV) has been observed; see A. I. Aleksandrov, I. E. Makarov (1987). "Formation of Bi(II) and Bi(IV) in aqueous hydrochloric solutions of Bi(III)". Bulletin of the Academy of Sciences of the USSR, Division of Chemical Science. 36 (2): 217–220.
doi:
10.1007/BF00959349.
S2CID94865394.
^Po(V) has been observed in
dioxidopolonium(1+) (PoO+); see Thayer, John S. (2010). "Relativistic Effects and the Chemistry of the Heavier Main Group Elements". Relativistic Methods for Chemists. Challenges and Advances in Computational Chemistry and Physics. Vol. 10. p. 78.
doi:
10.1007/978-1-4020-9975-5_2.
ISBN978-1-4020-9974-8.
^Rn(IV) is reported by Greenwood and Earnshaw, but is not known to exist; see Sykes, A. G. (1998).
"Recent Advances in Noble-Gas Chemistry". Advances in Inorganic Chemistry. Vol. 46. Academic Press. pp. 91–93.
ISBN978-0-12-023646-6. Retrieved 22 November 2012.
^U(II) has been observed in [K(2.2.2-Cryptand)][(C5H4SiMe3)3U], see MacDonald, Matthew R.; Fieser, Megan E.; Bates, Jefferson E.; Ziller, Joseph W.; Furche, Filipp; Evans, William J. (2013). "Identification of the +2 Oxidation State for Uranium in a Crystalline Molecular Complex, [K(2.2.2-Cryptand)][(C5H4SiMe3)3U]". J. Am. Chem. Soc. 135 (36): 13310–13313.
doi:
10.1021/ja406791t.
PMID23984753.
^Pu(II) has been observed in {Pu[C5H3(SiMe3)23}−; see Windorff, Cory J.; Chen, Guo P; Cross, Justin N; Evans, William J.; Furche, Filipp; Gaunt, Andrew J.; Janicke, Michael T.; Kozimor, Stosh A.; Scott, Brian L. (2017). "Identification of the Formal +2 Oxidation State of Plutonium: Synthesis and Characterization ofref name="curium5" {PuII[C5H3(SiMe3)23}−". J. Am. Chem. Soc. 139 (11): 3970–3973.
doi:
10.1021/jacs.7b00706.
PMID28235179.
^Pu(VIII) has been observed in PuO4; see Nikonov, M. V.; Kiselev, Yu. M.; Tananaev, I. G.; Myasoedov, B. F. (March 2011). "Plutonium volatility in ozonization of alkaline solutions of Pu(VI) hydroxo complexes". Doklady Chemistry. 437 (1): 69–71.
doi:
10.1134/S0012500811030104.
S2CID95951175. Also see Kiselev, Yu. M.; Nikonov, M. V.; Dolzhenko, V. D.; Ermilov, A. Yu.; Tananaev, I. G.; Myasoedov, B. F. (17 January 2014). "On existence and properties of plutonium(VIII) derivatives". Radiochimica Acta. 102 (3): 227–237.
doi:
10.1515/ract-2014-2146.
S2CID100915090.
^Am(VII) has been observed in AmO−5; see
Americium, Das Periodensystem der Elemente für den Schulgebrauch (The periodic table of elements for schools) chemie-master.de (in German), Retrieved 28 November 2010 and
Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.).
Butterworth-Heinemann. p. 1265.
ISBN978-0-08-037941-8.
^
abcKovács, Attila; Dau, Phuong D.; Marçalo, Joaquim; Gibson, John K. (2018). "Pentavalent Curium, Berkelium, and Californium in Nitrate Complexes: Extending Actinide Chemistry and Oxidation States". Inorg. Chem. 57 (15). American Chemical Society: 9453–9467.
doi:
10.1021/acs.inorgchem.8b01450.
OSTI1631597.
PMID30040397.
S2CID51717837.
^Cm(VI) has been observed in
curium trioxide (CmO3) and dioxidocurium(2+) (CmO+2); see Domanov, V. P.; Lobanov, Yu. V. (October 2011). "Formation of volatile curium(VI) trioxide CmO3". Radiochemistry. 53 (5): 453–6.
doi:
10.1134/S1066362211050018.
S2CID98052484.
^Cm(VIII) has been reported to possibly occur in
curium tetroxide (CmO4); see Domanov, V. P. (January 2013). "Possibility of generation of octavalent curium in the gas phase in the form of volatile tetraoxide CmO4". Radiochemistry. 55 (1): 46–51.
doi:
10.1134/S1066362213010098.
S2CID98076989. However, new experiments seem to indicate its nonexistence: Zaitsevskii, Andréi; Schwarz, W H Eugen (April 2014). "Structures and stability of AnO4 isomers, An = Pu, Am, and Cm: a relativistic density functional study". Physical Chemistry Chemical Physics. 2014 (16): 8997–9001.
Bibcode:
2014PCCP...16.8997Z.
doi:
10.1039/c4cp00235k.
PMID24695756.
^Sullivan, Jim C.; Schmidt, K. H.; Morss, L. R.; Pippin, C. G.; Williams, C. (1988). "Pulse radiolysis studies of berkelium(III): preparation and identification of berkelium(II) in aqueous perchlorate media". Inorganic Chemistry. 27 (4): 597.
doi:
10.1021/ic00277a005.
^Hs(VIII) has been observed in hassium tetroxide (HsO4); see
"Chemistry of Hassium"(PDF). Gesellschaft für Schwerionenforschung mbH. 2002. Retrieved 2007-01-31.
^Noyes, A. A.; Pitzer, K. S.; Dunn, C. L. (1935). "Argentic salts in acid solution, I. The oxidation and reduction reactions". J. Am. Chem. Soc. 57 (7): 1221–1229.
doi:
10.1021/ja01310a018.
^Noyes, A. A.; Pitzer, K. S.; Dunn, C. L. (1935). "Argentic salts in acid solution, II. The oxidation state of argentic salts". J. Am. Chem. Soc. 57 (7): 1229–1237.
doi:
10.1021/ja01310a019.
^Fernelius, W. C. (1948). "Some problems of inorganic nomenclature". Chem. Eng. News. 26: 161–163.
doi:
10.1021/cen-v026n003.p161.
^Fernelius, W. C.; Larsen, E. M.; Marchi, L. E.; Rollinson, C. L. (1948). "Nomenclature of coördination compounds". Chem. Eng. News. 26 (8): 520–523.
doi:
10.1021/cen-v026n008.p520.