Osmium geometries

The most common oxidation states and the corresponding electronic configurations of osmium ate +2 and + (t5 ), which ate usually octahedral. Stable oxidation states that have various coordination geometries include —2 and 0 to +8 (P] The single most important appHcation is OsO oxidation of olefins to diols. Enantioselective oxidations have also been demonstrated. 

Coordination about the osmium in 47 is best regarded as distorted trigonal bipyramidal with axial triphenylphosphine ligands. The distortion is toward a square pyramidal geometry with an apical nitrosyl ligand. Coordination about Os in the six-coordinate phenylcarbene complex is octahedral, as expected. 

It is interesting to note that the decrease in metal electron density that accompanies the change from five- to six-coordinate geometry does not have a detectable effect on the metal-carbene carbon bond length in these complexes. The metal-carbyne carbon bond in several osmium carbyne... 

Returning to Fig. 10, because the majority of the alkyl groups are flexible and because the interfacial electrostatic binding geometries are unknown, the rate of falloff of kbET or Hab with semiconductor/molecule separation distance cannot be evaluated quantitatively. A curious finding that remains unexplained is that the falloff with osmium complexes is considerably weaker than with ruthenium species. Finally, although the injection reaction was not the focus of the study, spacers clearly do decrease its rate, as shown, for example, by an increase in emission quantum yield (decrease in injection efficiency) with the largest spacers. 

Figure 7. (Top) The pentagonal bipy-ramidal geometry of H4Os(PMe2Ph)3 as determined by neutron diffraction. Hydrogen atoms on the methyl and phenyl groups have been omitted for clarity. (Bottom) The central core of the molecule, viewed normal to the equatorial H4OsP plane. Note the slight bending of the phosphorus-osmium-phosphorus axis (69)...

Fig. 16.56 Structures of osmium complexes which hove seven pairs of skeletul electrons. Each copped triangular face adds twelve electrons to the total electron count, but the number of skeletal pairs remains seven. Likewise removing OstCOlj deletes twelve electrons without changing the number of skeletal pairs. The diagonal lines show alternate geometries with the same total number of electrons [From McPanlm. M Poh-kednm IWM.J. 2 9 Reproduced with permission.)...

Some of the beautiful relationships that exist between r/iuo. nitlti., ind unit lino osmium complexes tire shown in Fig. 16.56.m For Lick of space wc have not touched on many subtleties associated with geometry/bontling/electron counting procedures and the reader is encouraged to consult more advanced sources.144... 

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