ISOMER PREFERENCE OF OXIDATION-STATES - CHEMISTRY OF THE OS(XANTHATE)2(PPH3)2Z (Z = 0, +) FAMILY

The title family consist of cis isomers of osmium(II) (z = 0; 1) and osmium(III) (z = +; 1+) and the corresponding trans isomers (2, 2+). Four xanthates, ROC(S)S- (Rx-), have been used: R = Me, Et, i-Pr, PhCH2. All complexes except 1+ have been isolated in pure state in excellent yields: 1 by the reaction of Os(PPh3)3Br2 with KRx, 2+ via oxidation of 1 with cerium(IV), and 2 by the reduction of 2+ byhydrazine hydrate. The formal potentials, E-degrees (cis) and E-degrees (trans), of the redox couples 1+ -1 and 2+ -2 are respectively approximately 0.4 and approximately 0.1 V vs SCE; the equilibrium constants K(II) and K(III) of the isomerization reactions 2 reversible 1 and 1+ reversible 2+ are approximately 10 and approximately 10(4), respectively (CH2Cl2, 303L). The metal oxidation states strongly differentiate the isomeric coordination spheres. The matched combinations are cis-Os(II) (1) and trans-Os(III) (2+). The mismatched species 1+ and 2 are unstable and isomerize spontaneously in solution. To establish isomer structures and to be the origin of the differentiation process, the X-ray structures of cis-Os(Mex)2(PPh3)2 (1a), trans-Os(Mex)2(PPh3)2 (2a), and trans-[Os(Mex)2(PPh3)2]PF6.2H2O (2a+) have been determined. 1a: space group Pbca, Z = 8, a = 10.774 (3) angstrom, b = 18.580 (7) angstrom, c = 38.043 (6) angstrom, and V = 7616 (4) angstrom3. 2a: space group P1BAR, Z = 1, a = 9.231 (4) angstrom, b = 10.466 (5) angstrom, c = 11.149 (5) angstrom, alpha = 101.33 (3)-degrees, beta = 108.66 (3)-degrees, gamma = 108.02 (3)-degrees, and V = 916.8 (7) angstrom3. 2a+: space group P1BAR, Z = 1, a = 9.766 (4) angstrom, b = 11.363 (5) angstrom, c = 11.677 (5) angstrom, alpha = 112.19 (4)-degrees, beta = 105.13 (4)-degrees, gamma = 97.17 (4)-degrees, and V = 1121.6 (9) angstrom3. The binding in the OsS4 fragment is primarily-sigma in nature. The mean Os-S distance decreases upon metal oxidation: 1a, 2.424 (5) angstrom; 2a, 2.410 (2) angstrom; 2a+, 2.378 (2) angstrom. The OsP2 fragment is subject to 5d-pi-3d-pi back-bonding, which decreases rapidly in the order 1a > 2a > 2a+, leading to a large and progressive increase in means Os-P length: 1a, 2.317 (4) angstrom; 2a, 2.365 (4) angstrom; 2a+, 2.439 (3) angstrom. The stability order 1 > 2 as well as the redox potential order E-degrees (cis) > E-degrees (trans) arises primarily from the superior Os-P back-bonding in 1 compared to 2. Due to the poor back-bonding ability of osmium (III), steric factors become controlling and this explains the stability of 2+ over 1+. Paramagnetic (S = 1/2) 2+ affords strongly rhombic EPR spectra-the axial and rhombic distortion parameters being approximately 7000 and approximately 2000 cm-1, respectively. A weak optical transition within the Kramers doublets is observable near 7600 cm-1.