OS-187 NMR-STUDY OF (ETA(6)-ARENE)OSMIUM(II) COMPLEXES - SEPARATION OF ELECTRONIC AND STERIC LIGAND EFFECTS

Osmium-187 NMR data have been collected for 37 Os(arene)X(2)L type com plexes, using inverse two-dimensional (P-31, Os-187){H-1} and (H-1, Os -187) NMR spectroscopy. In the series Os(p-cymene)X(2)PMe(3), shieldin g increases in the order Cl < Br < I < Me < H. Systematic variation of the phosphite ligand of Os(p-cymene)Cl2P(OR)(3) (R = Me, Et, (n)Bu, P h, Pr-i) reveals a linear dependence on Tolman's electronic parameter chi with metal shielding increasing as the acceptor character of the P (OR)(3) ligand increases. Conversely, the structurally related Os(p-cy mene)X(2)PR(3) types (X = Cl, I; PR(3) = PMe(3), PMe(2)Ph, P(n)Bu(3), PPh(2)Me, P(CH(2)Ph)(3), PPh(3), P(m-tolyl)(3), (PPr3)-Pr-i, PCy(3)) e xhibit a linear dependence on Tolman's cone angle theta, with shieldin g decreasing as larger phosphine ligands are introduced. Variation of the arene ligand of Os(C(6)H(5)X)Cl(2)PMe(3) (X = H, Me, Et, Pr-i, (t) Bu) shows a linear dependence of chemical shift on Taft's steric param eter E(s) for group X. In only one case, for the monohydride complexes Os(p-cymene)HClPR(3) (PR(3) = PMe(3), PMe(2)Ph, P(n)Bu(3), PPh(3), (P Pr3)-Pr-i, PCy(3)), has it been possible to separate both steric and e lectronic effects of the phosphorus ligand on the observed Os-187 chem ical shifts. There are some correlations between (1)J(Os-187, P-31) an d the acceptor character of the phosphorus ligand, and a relationship between Os-P bond lengths and coupling constants was found. Osmium rel axation rates T-1(-1) and their B-0 field dependence are reported for selected complexes. The crystal structures of four Os(p-cymene)Cl(2)L complexes (L = PMe(3) (1), P(OMe)(3) (6), PBz(3) (13), and PPh(3) (15) ) have been determined.