The kinetics and energetics of formation of a molecular hydrogen complex involving a dinuclear ruthenium(II) centre

The reversible equilibrium conversion under H-2 of [RuCl(dppb) (mu-Cl)](2) (1) to generate (eta(2)-H-2) (dppb) (mu-Cl)(3)RUCl( dppb) in CH2Cl2 (dppb = Ph(2)P(CH2)(4)PPh(2)) has been studied at 0-25 degrees C by UV-Vis and (31)p{H-1} NMR spectroscopy, and by stopped-how kinetics; the equilibrium constant and corresponding thermodynamic parameters, and the forward and reverse rate constants at 25 degrees C have been determined. A measured Delta H degrees value of similar to 0 kj mol(-1) allows for an estimation of an exothermicity of similar to 60 kj mol(-1) for binding an eta(2)-H-2 at an Ru(II) centre; a Delta S degrees value of similar to 60 J mol(-1) K-1 indicates that in solution 1 contains coordinated CH2Cl2. The kinetic and thermodynamic data are compared to those obtained from a previously studied hydrogenation of styrene catalyzed by 1. Preliminary findings on related systems containing Ph(2)P(CH2)(3)PPh(2) and (C6H11)(2)P(CH2)(4)P(C6H11)(2) are also noted.