Osmium complexes, ammines

Within the osmium complexes in oxidation states (II-IV) [11,12] the stability of the +4 oxidation state becomes more important. Ammine and tertiary phosphine complexes have been selected for detailed examination. 

In similar but more complicated processes, chelated a-diimines can be formed by reaction of ruthenium(III) and osmium(III) ammine complexes with biacetyl however, reduction of the metal also occurs (Scheme 49).220 A 0-diimine complex is obtained when acetylacetone undergoes reaction with hexaammineplatinum(TV) ions in basic solution (equation 39).220... 

Osmium(IV) ammines have unusually low pXa values (see Table 5) and so persist only in strongly acidic media the complexes are more acid by an order of five to six than their platinum(IV) counterparts but only slightly more acidic than the corresponding iridium(IV) complexes. This has been correlated with the decreasing t2g electron occupancy in the series PtIV-Irlv-OsIV and attributed to charge transfer from ligands to the empty n orbitals on osmium(IV). The cis isomers are more acidic than the trans, and this can be correlated with differences between the osmium(III) and osmium(IV) complexes for the isomers.55... 

Many of the ammine complexes are osmium(III) compounds the +2 state is less stable than with ruthenium, as expected, and osmium(II) compounds... 

All three elements form complex ammino-derivatives. Those of osmium have been very little investigated those of iridium are analogous to the anunino-derivatives of platinum on the one hand and to the ammincs of cobalt and chromium on the other whilst the platinum derivatives resemble those of cobalt, save that the metal in the platinic derivatives is tetravalent and not trivalent as in the cobalt-ammines. 

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