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 n Cp, 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 n 2+ and (Cp - ) 2 moieties was investigated with energy decomposition analysis (EDA). The calculations predict that the CpM n Cp species with n > 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 n Cp up to n = 5. The calculations suggest that the energy for loss of one metal atom from CpBe 2 Cp is significantly higher than from CpZn 2 Cp. The energy for the metal extrusion reaction of CpBe 3 Cp is much less endothermic than for CpBe 2 Cp but it is still more endothermic than the reaction of CpZn 2 Cp. The thermodynamic stability of the higher members CpBe 4 Cp and CpBe 5 Cp toward loss of one metal atom is only slightly less than for CpBe 3 Cp, while the other multimetallocenes, CpM 3 Cp, CpM 4 Cp, and CpM 5 Cp (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 CpBe 2 Cp might become isolated in the condensed phase, while the prospect for CpCa 2 Cp and CpMg 2 Cp and for the higher members CpM 3 Cp, CpM 4 Cp, and CpM 5 Cp is less likely. The analysis of the metal−ligand bonding in CpM n Cp using the EDA method suggests that the interactions between M n 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 n Cp come mainly from π orbitals, 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.