A Molecular Mechanics Model of Ligand Effects. 5. Ligand Repulsive Energy Values for Phosphines and Phosphites Bound to CpRh(CO) and the Crystal Structure of CpRh(CO)(PPh₃)

Molecular mechanics methods have been employed to compute energy-minimized structures of a series of CpRh-(CO)(PX₃) complexes, where PX₃ is CO, trialkylphosphine, trialkyl phosphite, mixed alkyl/arylphosphine, or mixed alkyl/aryl phosphite. The energy-minimized structures are applied to compute the ligand repulsive energy, E′_R. Values of E′_R are compared with relative values of E_R (Brown, T. L. Inorg. Chem. 1992, 31, 1286) in Cr(CO)₅ in which the geometric character of the metal center is substantially different from CpRh(CO). The values of E′_R in CpRh(CO) and E_R in Cr(CO)₅ correlate reasonably well (r_corr = 0.93). The slope of the linear regression of E′_R vs E_R is 1.4. The magnitude of the slope indicates that the CpRh(CO) metal center is less crowded than the Cr(CO)₅ metal center with respect to the ligands PX₃. The correlation of E′_R with cone angle is also fairly good (r_corr = 0.82). The trend in molecular mechanics energy changes of CpRh(CO) with ligand cone angle follows the same general pattern observed in Cr(CO)₅. The structure of CpRh(CO)(PPh₃) has been determined by X-ray diffraction measurement at 0 °C with E_w = 0.033. The crystals belong to the triclinic space group P1̄, with a = 10.024(2) Å, b = 10.589(2) Å, c = 11.282(2) Å, α = 72.06(1)°, β = 81.42(1)°, γ = 62.57(1)°, Z = 2, and V = 1011.2(3) ų.