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

Theoretical calculations at the DFT (B3LYP) level have been undertaken on tris- and bis(boryl) complexes, Two model d(6) complexes [Rh(PH3)(3)(BX2)(3) and Rh(PH3)(4)(BX2)(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 cl-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 BH2 moieties are coupled. In the optimal geometry of the bis(boryl) complex, the B2H4 ligand resembles the transition state of the C-2v --> D-2d interconversion of the isolated B2H4 species, In the tris(boryl) complex, we find a B3H6 ligand in which the B-3 atoms define an isosceles triangle with one hydrogen bridging the shorter B-B bond.