Journal of Molecular Catalysis, 53 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM (1989) 23 - 36 23 zyxwvuts THERMODYNAMICS OF HOMOGENEOUS HYDROGENATION PART VII. THERMODYNAMICS OF THE HOMOGENEOUS HYDROGENATION OF CYCLOHEXENE CATALYZED BY SOME WATER-SOLUBLE RUTHENIUM COMPLEXES CONTAINING n-ACIDIC LIGANDS M. M. TAQUI KHAN*, S. A. SAMAD and M. R. H. SIDDIQUI Catalysis Division, Central Salt and Marine Chemicals Research Institute, Bhavnagar 364002 (India) (Received July 6,1988;accepted October 20,1988) Summary The complexes K[ Ru(EDTA-H)Cl]ZH,O 1, [ Ru(EDTA-H)(PPh,)] 2, K[Ru(EDTA-H)(CO)] 3 and K[Ru(EDTA-H)(SnCls-)] 4 activate mole- cular hydrogen at 30 “C and 1 atm H, in 7 :3 alcohol:water mixture by heterolytic cleavage of the H-H bond to form the thermodynamically stable hydrido complexes [Ru(EDTA-H)(H)12- 5, [Ru(EDTA-H)(PPh,)(H)]2- 6, [Ru(EDTA-H)(CO)(H)]‘- 7 and [Ru(EDTA-H)(SnC1s-)(H)]3- 8 in solution characterized by their proton NMR in situ. Complexes 6 - 8 form isomeric hydrides where H is cis or zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML t r am to the n-acidic group L (L = PPhs, CO, SnCl,-), the cis species being predominant in solution. The hydrido proton peaks in complexes 5 - 8 shift downfield in the order 4 > 3 > 2 > 1, in accord with the decreasing a-acidity of the coordinated group. The homogeneous hydrogenation of cyclohexene catalyzed by com- plexes 1 - 4 was investigated in the temperature range 10 - 40 “C at 0.4 - 1 atm of H2 partial pressure. Thermodynamic parameters corresponding to the formation of the monohydrido complexes 5 - 8 and the monoolefin complexes 1 - 5 were computed. The activation parameters corresponding to the rate constants 12, and k2 for the homogeneous hydrogenation of cyclo- hexene were also calculated. The enthalpy of the formation of hydrido and olefin complexes decreases with an increase in the x-acidity of the coordinated ligand. The entropies of the hydride and olefin complex formation are large positive numbers, indicating either the dissociation of a Cl- from the coordination sphere of the metal ion in 1, or removal of a coordinated carboxylate group from the coordination positions of EDTA to accommodate H- or olefin. The catalytic activity of complexes 1 - 4 decreases in the order 4 > 3 > 2 > 1, in line with the decreasing n-acidity of the secondary group L coordi- nated to Ru(I1). The thermodynamic parameters corresponding to the steps *Author to whom correspondence should be addressed. 0304-5102/89/$3.50 0 Elseviex Sequoia/Printed in The Netherlands