Osmium complexes terpyridyl

Keywords Bipyridyl Energy transfer Osmium complexes Osmium photochemistry Osmium photophysics Photosubstitution Terpyridyl... 

A number of substituted osmium(III) terpyridyl complexes are known (see Scheme 5). 

Nitrogen is, after oxygen, the most frequently encountered donor atom in the coordination chemistry of osmium. There is a large and growing body of work on the ammine, pyridine, ethylenediamine and porphyrin complexes, but the most popular and rapidly growing field of study at the present time is that of the polypyridyls , the 2,2 -bipyridyl, 1,10-phenanthroline and 2,2,2,6 - terpyridyl complexes of the metal. 

Since, like terpyridyl (p. 542), these ligands are good n acceptors by virtue of their considerable ring conjugation, it is the osmium(II) (d6) state which is the commonest and generally the most stable thus [Os(LL)3]3+ species are good one-electron oxidants. Nevertheless there are also examples of osmium(IV) and even a few of osmium(V) and osmium(VI) (the osmyl osmium(VI) complexes are considered on p. 581). 

As a tridentate conjugated ligand this would be expected, like 2,2 -bipyridyl and 1,10-phenanth-roline, to stabilize osmium(II) and this is indeed the case, with most of the reported complexes containing divalent osmium. The unsubstituted bis complex [Os(terpy)2]2+ is remarkably stable. It seems likely that terpyridyl complexes of osmium(II) are good candidates for further investigation as photosensitizers, having so far received less attention than the corresponding bipyridyl or phenanthroline species. 

Evidence has been obtained for an inner-sphere mechanism in the iso-electronic peroxodiphosphate (P208 ) ion oxidation of metal complexes. For a series of reactants of the type Fe(LL)a + and Fe(LLL)a + (where LL = bipyridyl or phenanthroline and LLL = terpyridyl), there is no parallel to the outer sphere path observed in the SaOs " oxidation of the same complexes. The mechanism is considered to involve the rate-determining partial dissociation of one of the ligands, with PaOg " (or P04 ) approaching the vacant position, followed by a subsequent fast oxidation reaction. An exception may be in the case of the complex [Ru"(phen)2pyCl]+. In the case of osmium(ii) complexes, however, no reaction takes place in the absence of catalysts since no prior dissociation occurs. 

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