Structural flexibility of formally d10 [M(biphosphinine)2]q complexes

Abstract

The potential energy curves associated with the interconversion between square planar (SP) and tetrahedral (Td) conformations of the formally d¹⁰ [M(biphosphinine)₂]^q complexes (M = Ni, Pt, Co, Rh and Ru) were obtained by means of DFT calculations with the B3LYP functional. In agreement with the experimental data, the complexes with M = Ni, Pt, Co and Rh were found to adopt a distorted tetrahedral conformation, the flatness of the computed potential energy curves accounting for the experimental characterization of several independent molecules and the fluxional behaviour observed in solution for M = Co and Rh. As expected for formal d¹⁰ complexes, the tetrahedral structure was always found to be more stable than the square planar structure. However, the computed energy difference is rather small (between 3.1 kcal mol⁻¹ for M = Ru and 13.6 kcal mol⁻¹ for M = Ni) and it is shown that, for a given column of the periodic table, the lower the d orbitals on the metal centre (Ni < Pt and Co < Rh), the less favoured the square planar conformation. Finally, for M = Ru, the slight preference for the tetrahedral conformation found in the isolated complex is not preserved in the presence of two counter cations (Na⁺): in agreement with the experimental data for the uncryptated complex, the two cations were found to be located above and below the plane of a nearly square planar dianionic [Ru(biphosphinine)₂]²⁻ unit.