Electronic Structure of Bis(Dimethylmethylenephosphoranyl)dihydroborato(1–) Main-Group and Transition-Metal Complexes. A Case Study Involving Theoretical Pseudopotential ab Initio Calculations and Uv Photoelectron Spectroscopy

This contribution describes a study that uses He I/He II UV photoelectron spectroscopy and theoretical ab initio pseudopotential calculations to investigate the electronic structure of a large series of bis(di-methylmethylenephosphoranyl)dihydroborato(1–) [(L)-] complexes. The series includes the pseudotetrahedral complexes (CH3)2M(L) (M = Al, Ga) and M(L)2 (M = Be, Cd, Hg), as well as the square-planar M(L)2 (M = Ni, Pd, Pt) complexes. All complexes represent unusual examples of tetracoordinated alkyls and, in the case of homoleptic M(L)2 complexes of d8 transition-metal M(II) ions (M = Ni, Pt, Pd), are almost unique examples of square-planar alkyls. Reorganization energies in the ionic state as well as relativistic corrections in the case of the heavy atom in the Pt(L)2 complex have been taken into account to interpret the photoelectron spectra. The theoretical results provide a convincing description of the metal-ligand bonding in these complexes. In the case of the (CH3)2M(L) complexes both theoretical and experimental results indicate that their uppermost MOs can be described in terms of symmetry combinations of the ligand ylidic and methyl lone pairs perturbed by the metal subshells. This is also true in the case of the group II metal complexes. The metal-ligand bonding appears much more intriguing in the square-planar d8 metal complexes, in which the metal d subshells lead to severe perturbations not only of ligand ylidic lone pairs but also of the deep σ MOs. Detailed assignments of the photoelectron spectra are proposed on the basis of both theoretical and experimental results. A tentative comparison of bonding between the phosphorus ylide under study and the more usual hydrocarbyl complexes has been carried out.