Geometrical isomerism of bis(alkyl trithiocarbonato-S,S′)bis-(triphenylphosphine) complexes of osmium-(II) and -(III)

Abstract

Complexes of types [Os(RSCS₂)₂(PPh₃)₂]^z(cis-Os^II, z= 0, 1; cis-Os^III, z=+, 1⁺; trans-Os^II, z= 0, 2; and trans-Os^III, z=+, 2⁺) have been prepared, but 1⁺ is observable only in solution. The 2⁺ salts are paramagnetic (S=½) and display rhombic EPR spectra which have been analysed; a relatively weak electronic transition observed in NIR region is assigned to excitation from one Kramers doublet to another. Electrochemical and spectral data have been used to characterise the coupled redox and isomerisation processes interlinking 1, 1⁺, 2 and 2⁺. Upon oxidation 1 affords 1⁺ which isomerises to 2⁺ relatively rapidly. On the other hand, 2⁺ can be reduced to 2 which slowly but spontaneously isomerises to 1. Rate and equilibrium data are reported. The electronic and steric factors controlling isomer preference [cis for osmium(II) and trans for osmium(III)] are noted. Compared to the present complexes, the ruthenium congeners isomerise more rapidly and more completely.