Preparation, X-ray crystal structure and theoretical study of the bonding in [Mo 2O 4I 2L 3] (L = OPPh 3), an example of an unsymmetrical dinuclear molybdenum complex

Abstract

The reaction between the oxomolybdenum(V) complex Et 4N[MoOI 4(H 2O)] and triphenylphosphine oxide (L = OPPh 3) produces the mononuclear complex [MoOI 3L 2] and the dinuclear complex 1 [Mo 2O 4I 2L 3], the crystal structure of which has been studied by X-ray. Each metal centre is bound to a terminal and two bridging oxygen atoms; the Mo 2O 2 central fragment appears slightly disymmetric with a short MoMo distance [2.557(1) Å], in agreement with a single metalmetal bond. The whole structure is strongly asymmetric, with the three phosphorus ligands bound to a first molybdenum atom with a distorted octahedral MoO 6 coordination and the two iodo ligands bound to a second molybdenum atom which has a cis-MoOO 2I 2 square-pyramidal coordination. Extended Hückel calculations on the model [Mo 2O 4I 2L 3] (L = OPH 3), the structure of which was derived from the averaged structure of 1, have been performed to understand the structure and bonding in this unsymmetrical dimer. They show a HOMO-LUMO gap of 1.06 eV and indicate that 1 can be considered unambiguously as an Mo VMo V [and not an Mo IVMo VI] dinuclear complex. An orbital explanation is provided to rationalize why the cis conformation is generally preferred for Mo VMo V dinuclear species: such a conformation allows a stronger MoMo bond. Some predictions are made, concerning the reactivity of the MoMo bond with respect to nucleophilic and electrophilic attacks.