Unusual valence states

G. V. Buxton and R. M. Sellers, Compilation of Rate Constants for the Reactions of Metal Ions in Unusual Valency States, NSRDS-NBS 62, 1978 D. M. Stanbury, Adv. Inorg. Chem. 33, 69 (1989). 

Otherwise, unusual valency states are often observed in cyanide complexes. A Mn complex K5Mn(CN)6 has been reported here the stable 18-electron configuration causes the valency of manganese to take the very unusual value of one, and the compound is formed in spite of the extremely unfavourable cation anion ratio. Still more remarkable are the complex nickel cyanides. KGN and Ni(CN)2 form a complex K2Ni(CN)4, in which sixteen electrons are involved in the bond formation. The diamagnetism and the square structure of the Ni(CN)4 ion show that the bonding is due to dsp2 hybridization. 

Prior knowledge of the behaviour of a proposed intermediate under a particular set of reaction conditions is often available and facilitates experimental design. For example, species which are transient under one set of conditions (solvent, temperature) may be stable under others, and then observable by conventional methods. Similar considerations apply to structural variation, which may stabilise charge or unusual valence states. Systematic studies of the effects of variation of conditions, or of structural variation on reactivity, often permit useful extrapolation to behaviour of a proposed intermediate under the conditions in question. Importantly, if extrapolations of this kind indicate that a proposed intermediate would have a lifetime of less than 10 13 s under a particular set of reaction conditions, then that proposal must be re-evaluated. Either the mechanism involving the proposed intermediate is fundamentally flawed, or the bonding changes involved in its formation and destruction are actually concerted. 

A few works have also been published reporting unusual valence states of metal ions in lattices of such oxide semiconductors as ln203, ZnO, Sn02 [44, 55-68]. These compounds attract researchers attention because they are very perspective materials like thin films and ceramics for constructing new chemical sensors [55], as well as highly conductive thermo- and chemically stable n-type conductors (ln203). 

The bistrimethylsilylmethyl ligand has been utilized by Lappert and coworkers to stabilize a number of unusual valence states for lanthanide and transition metals (65-67). In group IV the bis(bistrimethylsilylmethyl)-germanium(II), tin(II), and lead(II) compounds have all been synthesized and the germanium and tin compound shown to be monomeric in cyclohexane solution (68, 69). 

Why use P-donor ligands instead of, for example, N-donor ligands Prominent among the reasons is their abihty to form a molecular complex that often stabilizes unusual valency states. 

While this review focused mainly on reduced metals inside zeolite cavities, the brief mention of NO , abatement by zeolite-supported Cu illustrates the potential for environmental catalysis it also opens prospectives for stabilizing elements in unusual valence states, in addition to unusual states of aggregation and complexation. 

Unusual valence states resulting from probable delocalization of charge over organic ligands appear in both transition metal and boron chemistry. 

The reactions of primary and secondary radicals with metal complexes, including bioinorganic compounds, are of the following types redox, addition and atom transfer. The products of the reactions include metal centers in unusual valency states and coordinated ligand-radicals ... 

The products of these reactions may be short lived, but they often have characteristic absorption spectra that can be detected by pulse radiolysis. Subsequent reactions, such as electron transfer and ligand labilization, can be followed kinetically with the appropriate detection technique. Reviews of the spectra, kinetics and mechanisms of complexes in unusual and unstable oxidation states are available A compilation of rate constants for the reactions of metal ions in unusual valency states is available ... 

PlatinumdD-amine complexes are oxidized and reduced to corresponding Pt(III) and Pt(I) species . Complexes of Pt with glycine and other coordinated organic molecules form in unusual valency states ". 

Radiation chemistry can he used to study reactions of free radicals and of metal ions in unusual valency states, including electron-transfer reactions. In some instances, radiation chemistry facilitates experiments that can not he studied hy photochemistry, owing to differences in the fundamental physical processes in the two methods. Procedures have heen developed to accurately determine radiolysis radical yields, and a variety of physical techniques have heen used to monitor reactions. In particular, aqueous radiation chemistry has heen extensively developed, and many free radicals can he generated in a controlled manner in aqueous solution. There are extensive literature resources for rate constants and for experimental design for a variety of radicals. 

Figure 21. Room temperature EPR spectrum of a Zr (isotopically enriched) impurity in synthetic pretulite. The observation of unusual valence states like Zr and Hr, some of which are stable in the as-grown single crystals, represents an unusual feature of the solid state chemical properties of the xenotime-stmcture orthophosphates (after Abraham et al. 1984).

The Stabilization of Unusual Valence States and Coordination Numbers by Bulky Ligands... 

It therefore seems likely that the bulky 2,5-dimethylpyrrole ligand will have considerable scope in stabilizing unusual coordination numbers and unusual valency states for the transition metals. It points the way to the development of additional ligands of this kind. 

The species H, ejq, and OH" are very reactive. Both H and e-, are strong reducing agents with redox potentials of (H+/H )=-2.3 Vnhe and E° (H20/e-,)=-2.87 V he. respectively. Therefore, both can reduce metal ions present in the solution to a state of zero valence. This process takes place through the direct reaction of the metal ion with either H" or e-, in the case of monovalent ions. In contrast, the reduction of multivalent metal ions in aqueous solutions is a multistep process where atoms in unusual valence states are initially formed. This initial reduction is followed by further reduction and agglomeration until a stable nanoparticle is obtained (Belloni et al. 1998). 

Molecules that have [ML/X Z ] classifications that are remote from the highly populated areas in an MLX plot are necessarily of considerable interest because of their uniqueness. For example, with respect to scandium chemistry, while the majority of compounds possess a ML/X3 classification (/ = 2-6), there are some very interesting examples of a zerovalent arene complexes ( j -ArH)2Sc e.g. ArH = C6H3Bu 3) which possess the MLg classification. This corresponds to a most unusual valence state for scandium, and the ability to isolate... 

Furthermore, the unusual valence state of Co" or on fully charged LiCoO or LiNiOj and Jahn-Teller-unstable Mn high-spin state on the dis-... 

The impact of the high-energy radiation (fast fission fragments and neutrons, as well as a, p and y radiation) may lead to the formation of unusual valency states, but only for a very limited volume and over a very short time. Global changes in the chemical principles are not assumed to be caused by radiation effects. 

---