THERMODYNAMIC PARAMETERS FOR THE FORMATION OF COBALT RHODIUM HEPTACARBONYL AND COBALT RHODIUM OCTACARBONYL

The thermodynamic parameters for the chemical equilibrium Co2Rh2(CO)12 + 2CO reversible 2CoRh(CO)7 in n-hexane as solvent have been experimentally determined: DELTA(r)H-degrees x = -29 +/- 3 kJ/mol (-6.9 +/- 0.6 kcal/mol) and DELTA(r)S-degrees x = -38 +/- 8 J/(mol K) (-9.1 +/- 2.0 cal/(mol K)). The Gibbs free energy of reaction is approximately DELTA(r)G-degrees x(293 K, 0.1 MPa) = -16.8 kJ/mol (-4.0 kcal/mol). These parameters were determined by regression of experimental data obtained for the temperature interval T = 268-288 K and the pressure interval P(CO) = 0.002-0.04 MPa. Under these experimental conditions, conversions of Co2Rh2(CO)12 to CoRh(CO)7 greater than 95% were observed. Further, the thermodynamic parameters for the chemical equilibrium CoRh(CO)7 + CO reversible CoRh(CO)8 in n-hexane as solvent have been experimentally determined: DELTA(r)H-degrees x = -23.5 +/- 3.4 kJ/mol (-5.7 +/- 0.8 kcal/mol), DELTA(r)S-degrees x = -71 +/- 12 J/(mol K) (-17 +/- 2.7 cal/mol K), and DELTA(r)V infinity x(288 K) = -48 +/- 21 ml/mol. The Gibbs free energy of reaction is approximately DELTA(r)G-degrees x(293 K, 0.1 MPa) = -2.0 J/mol (-0.5 kcal/mol). These parameters for the chemical equilibrium were determined by regression of experimental data obtained for the temperature interval T = 258-288 K and the pressure interval P(CO) = 1.0-10 MPa. Under the experimental conditions used, conversions of CoRh(CO)8 up to 75% were observed. Hence, the coordinatively unsaturated species CoRh(CO)7 was the predominant mixed-metal species in the present thermodynamic study. The entire system defined as dissolved Co2Rh2(CO)12, CoRh(CO)7, CoRh(CO)8, and CO in n-hexane represents only a metastable equilibrium. The homometallic metal carbonyls Co2(CO)8 and Rh4(CO)12 form upon decomposition of the above system, to reach finally an equilibrium mixture between homo- and heterometallic carbonyls; however, this rate of decomposition at these temperatures is negligibly slow. The thermodynamics of the cobalt, rhodium, and cobalt-rhodium tetranuclear metal carbonyl systems are compared. The results of the equilibrium CoRh(CO)7 + CO arrow-pointing-right-over-arrow-point-left CoRh(CO)8, together with considerations of the partial molar volume of CO, suggest an unusually large reaction volume \DELTA(r)V infinity x\ for the transformation of metal carbonyls under CO. It is shown that reaction volumes on the order of 10(2) mL/mol are realistic for the fragmentation of many metal carbonyl clusters and that there is an associated nonnegligible contribution to the molar Gibbs free energy under typically encountered reaction conditions.