Does back-bonding involve bonding orbitals in boryl complexes? A theoretical DFT study.

Abstract

Theoretical calculations at the DFT (B3LYP) level have been undertaken on tris- and bis(boryl) complexes. Two model d(6) complexes [Rh(PH(3))(3)(BX(2))(3) and Rh(PH(3))(4)(BX(2))(2)(+), X = OH and H] have been studied. In the model tris(boryl) complex (X = OH) we find a fac structure as a minimum, in accordance with the experimental data. The mer geometries are found to be higher in energy. Analysis of the energetic ordering in mer isomers shows that back-bonding in these complexes involves a bonding Rh-B orbital (and not a d-block orbital as usual). This surprising behavior is rationalized through a qualitative MO analysis and quantitative NBO analysis. Results on the bis(boryl) complex confirm the preceding analysis. Full optimization of unsubstituted (X = H) complexes leads to structures in which the BH(2) moieties are coupled. In the optimal geometry of the bis(boryl) complex, the B(2)H(4) ligand resembles the transition state of the C(2v)-->D(2d) interconversion of the isolated B(2)H(4) species. In the tris(boryl) complex, we find a B(3)H(6) ligand in which the B(3) atoms define an isosceles triangle with one hydrogen bridging the shorter B-B bond.