Characterization of Rh–Al Bond in Rh(PAlP) (PAlP = Pincer-type Diphosphino-Aluminyl Ligand) in Comparison with Rh(L)(PMe₃)₂ (L = AlMe₂, Al(NMe₂)₂, BR₂, SiR₃, CH₃, Cl, or OCH₃): Theoretical Insight

Abstract

The unique Rh-Al bond in recently synthesized Rh(PAlP) 1 {PAlP = pincer-type diphosphino-aluminyl ligand Al[NCH₂(P ⁱPr₂)]₂(C₆H₄)₂NMe} was investigated using the DFT method. Complex 1 has four doubly occupied nonbonding d orbitals on the Rh atom and one Rh d orbital largely participating in the Rh-Al bond which exhibits considerably large bonding overlap between Rh and Al atoms like in a covalent bond. Interestingly, Rh^δ--Al^δ+ polarization is observed in the bonding MO of 1, which is reverse to Rh^δ+-E^δ- (E = coordinating atom) polarization found in a usual coordinate bond. This unusual polarization arises from the presence of the Al valence orbital at significantly higher energy than the Rh valence orbital energy. Characteristic features of 1 are further unveiled by comparing 1 with similar Rh complexes RhL(PMe₃)₂ (2 for L = AlMe₂, 3 for L = Al(NMe₂)₂, 4 for L = BMe₂, 5 for L = SiMe₃, 6 for L = SiH₃, 7 for L = CH₃, 8 for L = OMe, and 9 for L = Cl). As expected, 7, 8, and 9 exhibit usual Rh^δ+-E^δ- polarization (E = coordinating atom) in the Rh-E bonding MO. On the other hand, the reverse Rh^δ--E^δ+ polarization is observed in the Rh-E bonding MOs of 2-5 like in 1, while the Rh-Si bond is polarized little in 6. These results are clearly understood in terms of the valence orbital energy of the ligand. Because the LUMO of 1 mainly consists of the Rh 4d_σ, 5s, and 5p orbitals and the Al 3s and 3p orbitals, both Rh and Al atoms play the role of coordinating site for a substrate bearing a lone pair orbital. For instance, NH₃ and pyridine coordinate to both Al and Rh atoms with considerably large binding energy. PAlP exhibits significantly strong trans influence, which is as strong as that of SiMe₃ but moderately weaker than that of BMe₂. The trans influence of these ligands is mainly determined by the valence orbital energy of the ligand and the covalent bond radius of the coordinating E atom.