Structure-reactivity correlations for the dissociative uncatalyzed isomerization of monoalkylbis(phosphine)platinum(II) solvento complexes

Complexes of the type cis-[PtL2Me2] (1-14) (L = an extended series of phosphines of widely different steric and electronic properties) were synthesized, characterized, and used as precursors for the formation of cis-monoalkylplatinum(II) solvento species in methanol. The cleavage of the first platinum-alkyl bond by protonolysis is a fast process, but the sequent cis to trans isomerization of the cationic solvento species [PtL2(Me)(MeOH)](+) is relatively slow and it can be monitored using P-31 NMR or conventional spectrophotometry. A large collection of H-1 and P-31 NMR data for cis-[PtL2Me2], cis-[PtL2Me(MeOH)](+), and trans-[PtL2Me(MeOH)](+) complexes showed interesting dependencies upon the size, the sigma-donor capacity, and mutual position of the phosphines in the coordination sphere of the metal. The rate constants for isomerization of cis-[PtL2Me(MeOH)](-) were resolved quantitatively into steric and electronic contributions of the phosphine ligands, by means of correlations with parameters which reflect their sigma-donor ability (chi values) and steric requirements (Tolman's cone angles, theta). The electronic and steric profiles obtained for these reactions are discussed within the framework of a mechanism which involves dissociative loss of the solvent molecule and interconversion of two geometrically distinct 3-coordinate T-shaped 14-electron intermediates. The factors controlling the stability of these coordinatively unsaturated species are discussed. The electronic and steric influences of phosphines as ''spectator'' ligands in a dissociative process are compared with those shown by these ligands when used as nucleophiles in associative substitution processes. The activation parameters Delta H-not equal and Delta S-not equal were measured using both conventional isothermal and non-isothermal spectrophotometric kinetics.