Catechol complexes, osmium

As might be expected for chelating 0,0 donor ligands it is the intermediate trivalent state of osmium which is stabilized by /J-diketonates (as is the case for catechols, p. 597, and tropolonato complexes, p. 597). In the presence of rc-acceptor chelates such as bipy, phen and terpy the II state is favoured, while with halides the IV state is preferred. The apparent absence of an osmyl complex, e.g. 0s02acac2, is surprising. 

The non-innocent ligand catechol forms very stable osmium(VI) complexes, as do a number of substituted catechols. 

Reaction of 0s04 with catechol or substituted catechols Rcat in chloroform yields the deep blue diamagnetic Os(Rcat)3 species (Rcat = catechol, 4-rerf-octyl-, 4-terr-butyl-, 3,5-di-ferf-butyl-catech-ol).486 X-Ray crystal structures of Os(cat)3 and of the tris complex with 3,5-di-terf-butylcatechol show these to have D3 symmetry the Os—O distances fall within the range 1.947 to 1.985 A (mean 1.960 A) and the C—O distances are between 1.30 and 1.35 A the catecholato (02C6H4 or 02C6H2) rings are essentially planar.666 For Os(cat)3, IR, Raman, HNMR and electrochemical data were obtained the latter showed two one-electron reversible reductions, presumably to [Os(cat)3] and [Os(cat)3]2-. The diamagnetism of these formally osmium(VI) species probably arises from the distortion from octahedral to Z>3 symmetry.486... 

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