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Multimetallocenes. A Theoretical Study

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journal contribution
posted on 02.04.2020, 15:48 by Alejandro Velazquez, Israel Fernández, Gernot Frenking, Gabriel Merino
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 CpMnCp, 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 Mn2+ and (Cp-)2 moieties was investigated with energy decomposition analysis (EDA). The calculations predict that the CpMnCp 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 CpBenCp 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 CpBe4Cp 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 CpMnCp using the EDA method suggests that the interactions between Mn2+ 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 CpMnCp 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.

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