Assignment of oxidation states

Chemical appHcations of Mn ssbauer spectroscopy are broad (291—293) determination of electron configurations and assignment of oxidation states in stmctural chemistry polymer properties studies of surface chemistry, corrosion, and catalysis and metal-atom bonding in biochemical systems. There are also important appHcations to materials science and metallurgy (294,295) (see Surface and interface analysis). 

The assignment of oxidation states has more a formal character in the sense of electron counting rules [145]. In this context it should, however, be justified to use at least the term low valent silicon. 

The isomer shift is considered the key parameter for the assignment of oxidation states from Mossbauer data. The early studies, following the first observation of an isomer shift for Fe203 [7], revealed a general correlation with the (formal) oxidation state of iron. However, isomer shifts have also been found to depend on the spin state of the Mossbauer atom, the number of ligands, the cr-donor and the... 

One of the earliest series of metal complexes which showed strong, redox-dependent near-IR absorptions is the well-known set of square-planar bis-dithiolene complexes of Ni, Pd, and Pt (Scheme 4). Extensive delocalization between metal and ligand orbitals in these non-innocent systems means that assignment of oxidation states is problematic, but does result in intense electronic transitions. These complexes have two reversible redox processes connecting the neutral, monoanionic, and dianionic species. 

The compounds in question are classified by oxidation state of the central metal atom. Assignment of oxidation state is normally straightforward when the formula is known, but there are cases where the choice is somewhat arbitrary. Thus bis(cyclopentadienyl)nickel, (CcH Ni, may be considered as the nickel (II) cation complexed by two cyclopentadienyl anions, or as a combination of an uncharged nickel atom with two cyclopentadienyl... 

A further class of dinitrosyl complex is that containing dithiolene and related ligands. Because of electronic delocalization, there are problems of assignment of oxidation state in... 

Use of large excesses of acyl halides results in fragmentation of 2 and 4I44). The observed inertness of the bridging thiolates is not suprising, as coordination to two metal centers would be expected to substantially reduce the nucleophilic character of the thiolate sulfur. Additionally, all reasonable assignments of oxidation states indicate that the Mo atoms in 1-5 are approximately trivalent octahedrally coordinated Mo(III) (d3) would be expected to be kinetically non-labile. 

All characterized Pd(I) compounds have Pd-Pd or Pd-M bonds. Accordingly, the paramagnetic behavior anticipated for a d configuration is not found in these multinuclear diamagnetic complexes as the expected unpaired electron is in fact involved in the formation of a metal-metal bond. There are some palladium clusters in formal oxidation states ranging from 0 to -1-1 (metal-metal bonds do not count in the assignment of oxidation states) that will be considered here. 

The rules for assigning oxidation states are given in Table 4.3. Application of these rules allows the assignment of oxidation states in most compounds. The principles are illustrated in Example 4.13. 

After having learned the lesson how to recognise typical spectroscopic patterns of the porphyrin ligand-centred electron transfer, and to discriminate them from metal-centred reaction, the assignment of oxidation states in multiredox arrays including metalloporphyrins is feasible as exemplified in Section 4.7, where covalently bound fullerene-[(TPP)Co] dyads were explored by spectroelectrochemical means. 

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