Binuclear complex mixed-valence

Unusual behavior results from d7r-p7r interactions between open-shell metal ions and organic radicals as illustrated by the family of 3-D metals [Cu(R R -DCNQI)2] (DCNQI = 7V,A -dicyanoquinonediimine) (Figure 14). The halogen-bridged binuclear-metal mixed-valence complex, [Pt2(dta)4l] (dta = dithioacetate) (1-D linear chain Scheme 5a), exhibits metallic conduction above room temperature, representing the first example of a metallic halogen-bridged 1-D transition-metal complex. ... 

Rate and equilibrium constant data, including substituent and isotope effects, for the reaction of [Pt(bpy)2]2+ with hydroxide, are all consistent with, and interpreted in terms of, reversible addition of the hydroxide to the coordinated 2,2 -bipyridyl (397). Equilibrium constants for addition of hydroxide to a series of platinum(II)-diimine cations [Pt(diimine)2]2+, the diimines being 2,2 -bipyridyl, 2,2 -bipyrazine, 3,3 -bipyridazine, and 2,2 -bipyrimidine, suggest that hydroxide adds at the 6 position of the coordinated ligand (398). Support for this covalent hydration mechanism for hydroxide attack at coordinated diimines comes from crystal structure determinations of binuclear mixed valence copper(I)/copper(II) complexes of 2-hydroxylated 1,10-phenanthroline and 2,2 -bipyridyl (399). 

These results suggest that the critical factor in the substrate-mediated intermolecular interactions which occur within the close-packed DHT layer is the inherent strong reactivity of the diphenolic moiety with the Pt surface. The interaction of adsorbates with each other through the mediation of the substrate is of fundamental importance in surface science. The theoretical treatment, however, involves complicated many-body potentials which are presently not well-understood (2.). It is instructive to view the present case of Pt-substrate-mediated DHT-DHT interactions in terms of mixed-valence metal complexes (2A) For example, in the binuclear mixed-valence complex, (NH3)5RU(11)-bpy-Ru(111) (NH 3)5 (where bpy is 4,4 -bipyridine), the two metal centers are still able to interact with each other via the delocalized electrons within the bpy ligand. The interaction between the Ru(II) and Ru(III) ions in this mixed-valence complex is therefore ligand-mediated. The Ru(II)-Ru(III) coupling can be written schematically as ... 

Figure 7.48 Two cytochrome c oxidase Cua center model compounds (A) delocalized core with two i-l,3-(KN KO)-ureate bridges as reported in reference 165-and (B) a dithiolate-bridged mixed-valence binuclear copper ion complex as re in reference 166.

The other copper-only binuclear centre to be considered is the CuA or purple copper complex. It is part of the terminal oxidase in mitochondrial respiration, cytochrome c oxidase (COX). Its EPR signature, a seven-line spectrum, has since long been known to be different from the classes type 1 to 3 and arises from two copper ions in a 1.5 valence (or mixed valence) state, first proposed from EPR-analysis of a similar center in nitrous oxide (N20) reductase. There is a close correspondence between the blue and purple states of copper since each of the two copper ions in CuA can be considered as being structurally related to the mononuclear blue site coordination. 

The formation of the unsaturated intermediate Run(OEP) from the hydrido-complex RumH(OEP) is supposed to occur from a LMCT excited state. As a consequence, the dimeric [RuH(OEP)]2 with a metal-metal interaction is formed [245]. Irradiation of some systems containing Fein(Por)N3 leads to p-nitrido bridged binuclear mixed-valence complexes [(Por)Fem-N-Fe,v(Por)] [134, 162], In both cases photochemistry was used as a conventional preparative route for synthesis of the binuclear complexes. 

Although several studies indicate that HO is formed in Fenton systems according to Eq. 37 and it is responsible for the efficiency of degradative reactions, it is presently believed that other Fe(IV) or Fe(V) species like Fe03+ and ferryl complexes, are also active agents in the processes [53— 55,58, 112]. For example, Kremer [112] identified a mixed valence binuclear species, FeOFe 5+, and proposed a new mechanism for the Fenton reaction, in which Fe02+ acts as the key intermediate. 

Table 6 lists data for binuclear mixed valence pentaammine ruthenium complexes. 

Table 11 Comproportionation Constants (Xc) and Intcrvalence Charge Transfer (IVCT) Bands for Mixed Valence Binuclear Complexes [RuXL2]2L"...

Few triazenide complexes reported to date are paramagnetic. Examples studied by ESR are restricted to the low-spin osmium(III) complexes OsX2(PhNNNPh)(PPh3)2 (3), the binuclear mixed-valence rhodium salts [ Rh(/z-ArNNNAr)(CO)(PPh3) 2][PFe] (43), and the low-spin d cobalt(II) species Co(ArNNNAr)(C5H5)L (178). 

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