Characterization of Rh-Al Bond in Rh(PAlP) (PAlP = Pincer-type Diphosphino-Aluminyl Ligand) in Comparison with Rh(L)(PMe) (L = AlMe, Al(NMe), BR, SiR, CH, Cl, or OCH): Theoretical Insight.

The unique Rh-Al bond in recently synthesized Rh(PAlP) 1 {PAlP = pincer-type diphosphino-aluminyl ligand AlNCH(P Pr)NMe} 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-Al polarization is observed in the bonding MO of 1, which is reverse to Rh-E (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(PMe) (2 for L = AlMe, 3 for L = Al(NMe), 4 for L = BMe, 5 for L = SiMe, 6 for L = SiH, 7 for L = CH, 8 for L = OMe, and 9 for L = Cl). As expected, 7, 8, and 9 exhibit usual Rh-E polarization (E = coordinating atom) in the Rh-E bonding MO. On the other hand, the reverse Rh-E 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, 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, NH 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 SiMe but moderately weaker than that of BMe. 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.