Kinetics and thermodynamics of the reversible reaction between carbon monoxide and palladium(I) dimers containing bis(diphenylphosphino)methane

Abstract

The kinetic and thermodynamic parameters for the reversible insertion of carbon monoxide into the palladium-palladium bond of Pd/sub 2/ (dpm)/sub 2/X/sub 2/ complexes (dpm = Ph/sub 2/PCH/sub 2/PPh/sub 2/; X = Cl, Br, I, NCO) in N,N'-dimethylacetamide solution have been determined by spectrophotometric methods. The forward reaction is first order in both Pd/sub 2/ and CO and occurs on the stopped-flow time scale at around ambient temperatures, the rate constants being similar for the four systems (3.15-6.60) x 10/sup 3/ M/sup -1/ s/sup -1/ at 24/sup 0/C. The equilibrium constants, estimated by kinetic and/or static methods, decrease in the order X = NCO > Cl > Br > I (from 3.7 x 10/sup 5/ to 1.3 x 10/sup 3/ M/sup -1/ at 24/sup 0/C) and are governed largely by the first-order off-rates, which probably depend more on a weakening of the Pd-CO bonds throughout the series than changes in the metal-metal bond strength. A correlation between ln k/sub -1/ for the off-rates and the energy of a high intensity UV band of the Pd/sub 2/(dpm)/sub 2/X/sub 2/ complexes is noted. The CO addition to give Pd/sub 2/(dpm)/sub 2/(..mu..-CO)X/sub 2/ occurs because of the exothermicity of the reaction that is unfavorablemore » in terms of ..delta..S; an approximate value of 90-120 kJ mol/sup -1/ is estimated for the Pd-Pd bond strength. The systems show high selectivity for binding of CO in the presence of H/sub 2/, C/sub 2/H/sub 2/, C/sub 2/H/sub 4/, CO/sub 2/, O/sub 2/, or air. 39 references, 5 figures, 1 table.« less