Formal oxidation states

It should be noted that the above procedure of counting valence electrons takes no account whatsoever of the concept of formal oxidation state. It is indeed rather confusing to try simultaneously to count electrons and to ascribe a formal oxidation state to the metal. While the concept of formal oxidation state is quite useful in the traditional classification of classical inorganic complexes, it is generally less helpful in discussing organo metallic and related compounds. For example in the anion [ReH9], is the rhenium in the Re +(H )9, in the Re +(H )9 or in the Re +(H-)9 state  

Since the actual charge on the metal is rarely larger than 1 (this is the Pauling Electroneutrality Principle), the unreality of assigning a formal charge far beyond these limits is clear. 

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Lead, like tin, forms only one hydride, plumbane. This hydride is very unstable, dissociating into lead and hydrogen with great rapidity. It has not been possible to analyse it rigorously or determine any of its physical properties, but it is probably PbH4. Although this hydride is unstable, some of its derivatives are stable thus, for example, tetraethyllead, Pb(C2Hj)4, is one of the most stable compounds with lead in a formal oxidation state of + 4. It is used as an antiknock in petrol. 

The principal oxides formed by Group V elements and their formal oxidation states are given below ... 

Some transition metal atoms combined with uncharged molecules as ligands (notahiv carbon monoxide. CO) have a formal oxidation state of 0. for example Ni + 4CO Ni"(CO)4. 

Chromium forms a white solid, hexacarhonyl, Cr(CO)j, with the chromium in formal oxidation state 0 the structure is octahedral, and if each CO molecule donates two electrons, the chromium attains the noble gas structure. Many complexes are known where one or more of the carbon monoxide ligands are replaced by other groups of ions, for example [CrfCOlsI] . 

Chromium is able to use all of its >d and As electrons to form chemical bonds. It can also display formal oxidation states ranging from Cr(—II) to Cr(VI). The most common and thus most important oxidation states are Cr(II), Cr(III), and Cr(VI). Although most commercial applications have centered around Cr(VI) compounds, environmental concerns and regulations ia the early 1990s suggest that Cr(III) may become increasingly important, especially where the use of Cr(VI) demands reduction and incorporation as Cr(III) ia the product. 

Ghromium(IV) and Ghromium(V) Gompounds. The formal oxidation states Cr(IV) and Cr(V) show some similarities. Both states are apparentiy intermediates in the reduction of Cr(VI) to Cr(III). Neither state exhibits a compound that has been isolated from aqueous media, and Cr(V) has only a transient existence in water (55). The majority of the stable compounds of both oxidation states contain either a halide, an oxide, or a mixture of these two. As of this writing, knowledge of the chemistry is limited. 

In addition to the above oxides M2O, M2O2, M4O6, MO2 and MO3 in which the alkali metal has the constant oxidation state 4-1, rubidium and caesium also form suboxides in which the formal oxidation state of the metal is considerably lower. Some of these intriguing compounds have been known since the turn of the century but only recently have their structures been elucidated by single crystal X-ray analysis. Partial oxidation of Rb at low temperatures gives RbeO which decomposes above —7.3°C to give copper-coloured metallic crystals of Rb902 ... 

Cation Formal oxidation state Cluster structure Point group symmetry... 

The formal oxidation state of the metal differs by 2 in these two limiting formulations (or... 

It will be convenient to describe first the binary. sulfur nitrides SjN,. and then the related cationic and anionic species, S,Nv. The sulfur imides and other cyclic S-N compounds will then be discus.sed and this will be followed by sections on S-N-halogen and S-N-O compounds. Several compounds which feature i.solated S<—N, S-N, S = N and S=N bonds have already been mentioned in the. section on SF4 e.g. F4S NC,H, F5S-NF2. F2S = NCF3, and FiS=N (p. 687). Flowever. many SN compounds do not lend themselves to simple bond diagrams, - and formal oxidation states are often unhelpful or even misleading. 

The metal is in the formal oxidation state zero. As expected, the M-C bonds are somewhat shorter than M-R bonds to alkyls, but they are noticeably longer than M-CO bonds suggesting only limited double-bond character M=C, e.g. ... 

Another example of a divalent metal of this group, but which in fact is probably entirely analogous to the dihydiides, is LaL. However, the most extensive set of examples of these metals in low formal oxidation states is provided by the binary and ternary halides produced by... 

Perhaps because of inadequate or non-existent back-bonding (p. 923), the only neutral, binary carbonyl so far reported is Ti(CO)g which has been produced by condensation of titanium metal vapour with CO in a matrix of inert gases at 10-15 K, and identified spectroscopically. By contrast, if MCI4 (M = Ti, Zr) in dimethoxy-ethane is reduced with potassium naphthalenide in the presence of a crown ether (to complex the K+) under an atmosphere of CO, [M(CO)g] salts are produced. These not only involve the metals in the exceptionally low formal oxidation state of —2 but are thermally stable up to 200 and 130°C respectively. However, the majority of their carbonyl compounds are stabilized by n-bonded ligands, usually cyclopentadienyl, as in [M(/j5-C5H5)2(CO)2] (Fig. 21.8). 

Organometallic compounds apart, oxidation states below - -2 are best represented by complexes with tris-bidentate nitrogen-donor ligands such as 2,2 -bipyridyl. Reduction by LiAlH4 in thf yields tris(bipyridyl) complexes in which the formal oxidation state of vanadium is -1-2 to —1. Magnetic moments are compatible with low-spin configurations of the metal but. 

Lower formal oxidation states are stabilized, however, by M-M bonding in ternary chalcogenides such as M MeQn, M4M6Q13 (M = alkali metal M = Re, Tc Q = S, Se) and the recently reported M gMeS. Their structures are all based on the face-capped, octahedral MeXg cluster unit found in Chevrel phases (p. 1018) and in the dihalides of Mo and W... 

Sodium amalgam reductions of M2(CO)iq give Na+[M(CO)5] and, indeed, further reduction leads to the super reduced species [M(CO)4] in which the metals exhibit their lowest known formal oxidation state of —3. On the other hand, treatment of [M(CO)5Cl] with AICI3 and CO under pressure produces [M(CO)6]" AlCl4 from which other salts of the cation can be obtained. 

This indicates a change in the formal oxidation state of the iron from -F2.25 to -F2.5, and mixed Fe /Fe species have been postulated. Flowever, it appears likely that these clusters are best regarded as electronically delocalized systems in which all the Fe atoms are equivalent. 

Hydrido complexes of all three elements, and covering a range of formal oxidation states, are important because of their roles in homogeneous catalysis either as the catalysts themselves or as intermediates in the catalytic cycles. 

The difficulty of assigning a formal oxidation state is more acutely seen in the case of 5-coordinate NO adducts of the type [Co(NO)(salen)]. These are effectively diamagnetic and so have no unpaired electrons. They may therefore be formulated either as Co -NO or Co -NO+. The infrared absorptions ascribed to the N-O stretch lie in the range 1624-1724 cm which is at the lower end of the range said to be characteristic of NO+. But, as in all such cases which are really concerned with the differing polarities of covalent bonds, such formalism should not be taken literally. 

Because they possess an odd number of valence electrons the elements of this group can only satisfy the 18-electron rule in their carbonyls if M-M bonds are present. In accord with this, mononuclear carbonyls are not formed. Instead [M2(CO)s], [M4(CO)i2] and [M6(CO)i6] are the principal binary carbonyls of these elements. But reduction of [Co2(CO)g] with, for instance, sodium amalgam in benzene yields the monomeric and tetrahedral, 18-electron ion, [Co(CO)4] , acidification of which gives the pale yellow hydride, [HCo(CO)4]. Reductions employing Na metal in liquid NH3 yield the super-reduced [M(CO)3] (M = Co, Rh, Ir) containing these elements in their lowest formal oxidation state. 

The change in formal oxidation state from osmium(VI) to osmium(II) is noteworthy [192],... 

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