Multimetallocenes. A Theoretical Study

Abstract

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 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 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.

Isomeric Al2R4, Mg2R2 Species and Oligomerisation Products: Investigation of Al–Al and Mg–Mg σ Bonding

“…We shall then address the isoelectronic magnesium systems, which have recently been subject to several theoretical studies and discussions, especially concerning the structure and bonding of Mg 2 Cp 2 and further metal-rich Cp species. [9,10] We shall show here that even the binary species MgCl is worth to be investigated. In order to elucidate the structure and bonding of this so far nonexisting solid compound, we have investigated the structure of the MgCl oligomers, their thermodynamic behaviour with respect to disproportionation to solid magnesium metal and MgCl 2 and in relation to the formation of the hypothetical solid MgCl.…”

Mg2H2: New insight on the Mg–Mg bonding and spectroscopic study

“…Mg 2 H 2 is the simplest RMg-MgR prototype and has been partially studied several times either theoretically 2,[9][10][11][12] or experimentally. 10,11 It can be formed by Mg 2 + H 2 photolysis 13 or by the reaction of laser ablated Mg atoms with H 2 in rare gas matrices, 1,10,11,14,15 like more general MgH n and Mg 2 H 2n (n = 1,2) compounds.…”

Molecular Magnesium(I) Compounds: From Curiosity to Kudos

“…[26] However, quantum chemical considerations showed that CpBe À BeCp should be much more feasible than CpCa À CaCp. [18] These experiments initiated investigations regarding the importance of d and p orbitals at calcium for the description of CaÀX bonds in species such as CaCH 3 , CaNH 2 , CaOH, and CaF. [27] These studies support the premise that very bulky groups should be able to stabilize Ca À Ca moieties, [19] but a competition between s-and p-based interactions in calciumphenyl complexes could limit the use of phenyl groups as protective units.…”