Multimetallocenes. A theoretical study

Quantum chemical calculations using gradient-corrected density functional theory at the BP86 level in conjunction with TZ2P basis sets have been carried out for the multimetallocenes CpM_nCp, where M = Be, Mg, Ca, and Zn with n = 2-5. The equilibrium geometries and energetics with respect to loss of one metal atom are theoretically predicted. The nature of the metal-ligand interactions between the M_n²⁺ and (Cp^-) ₂ moieties was investigated with energy decomposition analysis (EDA). The calculations predict that the CpM_nCp species with > 2 are thermodynamically unstable with respect to loss of one metal atom except for the beryllium compounds. The beryllocenes exhibit unusual stabilities in the gas phase for the whole series CpBe_nCp up to n = 5. The calculations suggest that the energy for loss of one metal atom from CpBe₂Cp is significantly higher than from CpZn₂Cp. The energy for the metal extrusion reaction of CpBe₃Cp is much less endothermic than for CpBe₂Cp but it is still more endothermic than the reaction of CpZn₂Cp. The thermodynamic stability of the higher members CpBe ₄Cp and CpBe₅Cp toward loss of one metal atom is only slightly less than for CpBe₃Cp, while the other multimetallocenes, CpM₃Cp, CpM₄Cp, and CpM₅Cp (M = Mg, Ca, Zn), possess little extra stabilization with respect to the dimetallocenes. The calculated reaction energies which include the heats of sublimation of the metals indicate that CpBe₂Cp might become isolated in the condensed phase, while the prospect for CpCa₂Cp and CpMg₂Cp and for the higher members CpM₃Cp, CpM₄Cp, and CpM₅Cp is less likely. The analysis of the metal-ligand bonding in CpM_nCp using the EDA method suggests that the interactions between M_c ²⁺ and (Cp^-)₂ have a larger electrostatic than covalent character. The beryllocenes are more covalently bonded than the other multimetallocenes. The orbital interactions in the lower members of CpM _nCp come mainly from π orbitais, but the σ contribution continuously increases when n becomes larger and eventually may become stronger than the π contributions, which become weaker in the higher members of the series.