Thermodynamic parameters for the formation of cobalt-rhodium heptacarbonyl and cobalt-rhodium octacarbonyl

The thermodynamic parameters for the chemical equilibrium Co₂Rh₂(CO)₁₂ + 2CO ⇄ 2CoRh(CO)₇ in n-hexane as solvent have been experimentally determined: Δ_rH°_x = -29 ± 3 kJ/mol (-6.9 ± 0.6 kcal/mol) and Δ_rS°_x = -38 ± 8 J/(mol K) (-9.1 ± 2.0 cal/(mol K)). The Gibbs free energy of reaction is approximately Δ_rG°_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 ⇄ CoRh(CO)₈ in n-hexane as solvent have been experimentally determined: Δ_rH°_x = -23.5 ± 3.4 kJ/mol (-5.7 ± 0.8 kcal/mol), Δ_rS°_x = -71 ± 12 J/(mol K) (-17 ± 2.8 cal/mol K), and Δ_rV^∞_x(288 K) = -48 ± 21 ml/mol. The Gibbs free energy of reaction is approximately Δ_rG°_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 ⇄ CoRh(CO)₈, together with considerations of the partial molar volume of CO, suggest an unusually large reaction volume |Δ_rV^∞_x| for the transformation of metal carbonyls under CO. It is shown that reaction volumes on the order of 10² 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. © 1991 American Chemical Society.