Ligand properties lower oxides

No isocyanide complexes have been described for lower oxidation states, probably because the c-donating properties of this kind of ligand are too strong or the w-accepting properties too weak. 

The donor acceptor properties of isonitrile are intermediate between those of CO and tertiary phosphines, and isontrile stabilizes the complexes in their higher oxidation state more and stabilizes the complexes in their lower oxidation state less than CO does. Thus, a direct reaction of zero-valent metal carbonyls with isonitrile usually results to the formation of the partially exchanged mixed-ligand complexes e.g., Cr(CO)g reacts with RNC at 120 130°C to give only the monosubstituted complex ... 

The oxidation state of tungsten in compounds may vary between —2 and -1-6, the latter being the most common. Lower oxidation state compounds exhibit basic properties while higher ones are acidic. The maximum coordination number is 8, but it may attain 13 in coordination compounds with cyclic organic ligands. Chapter 4 treats tungsten compoimds in detail. 

A characteristic of the cytochromes c3 is a very low oxidation-reduction potential. Moreover, it is obvious from the multiheme nature of these cytochromes that the redox properties should be complex. In the simplest situation, four individual redox potentials could be expected, one for each heme. In addition to the axial ligands on the hemes, a number of factors are anticipated to influence the individual heme redox potentials. First and foremost, the environment of each heme can exert an influence on its oxidation-reduction potential. This influence will be manifested in two ways the packing of the specific amino acid side chains about each heme and the extent of solvent exposure of each heme. It is quite apparent from the structural data (Figures 1 and 2) that the four hemes, which are in nonequivalent environments, are expected to have different oxidation-reduction potentials. Moreover, at least with Miyazaki cytochrome c3> one of the hemes (heme II) is substantially more exposed to solvent, which may result in a lower oxidation-reduction potential (13). Finally, it is apparent that in a small molecule that contains four hemes within close proximity (< 18 A), heme-heme interactions, principally as a result of electrostatic interactions, are likely to influence oxidation-reduction potentials (14). Indeed, on electrostatic grounds the redox state of one heme should influence another. This influence results from the fact that addition of electrons changes the formal... 

Ru " 04], [Ru NCl4l ) conversely n acceptor ligands such as CO, PRj stabilize the lower oxidation states (e.g. [Ru""(CO)4] , [Ru (CO)5], [Ru (CO)3(PPh3)2]). Ligands which are good a donors but show no substantial 7t acceptor or donor properties (e.g. HjO, NH3) are usually associated with Ru and Ru". Table 1 summarizes the oxidation states and coordination numbers for complexes of ruthenium. 

Charge neutralization also affects the redox properties of thioether complexes. It contributes to the marked stabilization of lower oxidation states found in all cases. Thioether complexes of metal ions in high oxidation states may approach the boundaries of the electroneutrality principle. Apart from any n-acidity of the ligands, simple electrostatic considerations suggest that poor charge neutralization by the ligands disfavors higher oxidation states. 

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