Oxidation numbers of metals in coordination compounds

Example 22.1 deals with oxidation numbers of metals in coordination compounds. EXAMPLE 22.1... 

Oxidation Number of Metals in Coordination Compounds Naming Coordination Compounds... 

Whether zinc is a main-group or transition metal depends, of course, on one s definition of transition metal and main-group metal. Those who classify zinc as a main-group metal cite its (almost) exclusive oxidation number of +2 in compounds (but see Section 2.06.15.2) and the absence of a partially filled r/ shell in the metal and its compounds. Those who classify zinc as a transition metal usually note its much greater effective nuclear charge, polarizing power and its limited, but well defined, coordination chemistry. 

The electrochemistry of a number of such six-coordinate compounds [MnXL]+ and seven-coordinate compounds [MX2L] (with L = (203), R,R = Me and X = halide, water, triphenylphosphine oxide, imidazole, 1-methylimidazole or pyridine) has been investigated.551 The redox behaviour of these compounds was of interest because it was considered that the potentially -acceptor macrocycle (203 R = R = Me) may promote the formation of Mn° or Mn1 species or may yield a metal-stabilized ligand radical with the manganese remaining in its divalent state. For a number of macrocyclic ligand systems, it has been demonstrated that the redox behaviour can be quite dependent on axial ligation it was also of interest to study whether this was the case for the present systems. 

The theories of bonding in coordination compounds [3] have evolved subsequent to Werner s coordination theory (1893). Werner introduced the concept of primary and secondary valency, explaining the formation of the coordination compounds. The 18-electron rule, stating that the stable complexes with low formal oxidation states of metal ions should have 18 bonding electrons around the metal ion, became an important beginning point toward the study of the stabihty of the complexes. The 18-electron rule is significant in modem coordination chemistry as it is also supported by the molecular orbital theory. However, a smaller number of complexes with metals in low oxidation states restrict its wide applicability. An important advance in the theories of bonding in coordination compounds was the introduction of... 

X-ray absorption near edge structure (XANES) is useful in determining the coordination number and the oxidation state of metal ions (Sankar et al, 1983). In Figs. 2.16 and 2.17 we show the XANES of Co and Cu in some compounds as well as catalysts. The ls-3[Pg.99]

Only a small number of zirconium(III) and hafnium(III) complexes are known. Nearly all of these are metal trihalide adducts with simple Lewis bases, and few are well characterized. Just one zirconium(III) complex has been characterized structurally by X-ray diffraction, the chlorine-bridged dimer [ ZrCl PBu,) ]- Although a number of reduced halides and organometallic compounds are known in which zirconium or hafnium exhibits an oxidation state less than III, coordination compounds of these metals in the II, I or 0 oxidation states are unknown, except for a few rather poorly characterized Zr° and Hf° compounds, viz. [M(bipy)3], [M(phen)3] and M Zr(CN)5 (M = Zr or Hf M = K or Rb). 

The coordination numbers of metal ions range from I, as in ion pairs such as Na CI- in the vapor phase, to 12 in some mixed metal oxides. The lower limit, I. is barely within the realm of coordination chemistry, since the Na+CI km pair would not normally be considered a coordination compound, and there are few other examples. Likewise, the upper limit of 12 is not particularly important since it is rarely encountered in discrete molecules, and the treatment of solid crystal lattices such as hexagonal BaTiOj and perovskite1 as coordination compounds is not done frequently. The lowest and highest coordination numbers found in typical coordination compounds are 2 and 9 with the intermediate number 6 being the most important. 

Most of the nickel compounds in the solid state and almost all in aqueous solution contain the metal in the oxidation state +2, which, by consequence, can be considered the ordinary oxidation state for nickel in its compounds. The electronic structure and stereochemistry of nickel(II) were reviewed in 1968.6 The most stable electronic configuration of the free Ni ion is [Ar]3d8 which is also the ground state configuration in its complexes. The overwhelming majority of nickel(II) complexes have coordination numbers of four, five and six. Complexes with coordination numbers of three, seven and eight are still quite rare. 

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