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The recently synthesized rhodium-aluminum bimetallic complex Rh(PAlP) 1 (PAlP = pincer-type diphosphino-aluminyl ligand Al{[N(C6H4)]2NMe}[CH2P(iPr)2]2) containing a unique Rh-Al direct bond exhibits coordination flexibility because Rh and Al can play the role of coordination site for the substrate. DFT calculations of NH3, CO, and C2H4 adducts with 1 show that the Rh atom is favorable for all these substrate but the Al atom is as favorable as the Rh atom for NH3 and unfavorable for CO and C2H4. NH3 and CO prefer the coordination at the Rh-axial (Ax) site to the Rh-equatorial (Eq) site, but C2H4 prefers coordination at the Rh-Eq site to the Rh-Ax site. Consequently, two CO and C2H4 molecules coordinate with 1 at the Rh-Ax and Rh-Eq sites to afford trigonal bipyramidal complexes Rh(PAlP)(CO)2 and Rh(PAlP)(C2H4)2, which is consistent with the experimental observation of Rh(PAlP)(CO)2. Energy decomposition analysis reveals that an electrostatic term plays an important role for NH3 coordination with the Al atom of 1, because Al has a significantly large positive charge and NH3 has a much negatively charged N atom and exhibits a considerably negative electrostatic potential at the Al position. In B and Ga analogues Rh(PBP) 2 and Rh(PGaP) 3, B and Ga atoms are not good for CO and C2H4 like the Al atom in 1. NH3 adducts with 2 and 3 at the B and Ga sites are less stable than those adducts at the Rh-Ax site unlike the NH3 adduct with 1 at the Al site. This difference in the NH3 adduct between Rh(PAlP) and others (Rh(PBP) and Rh(PGaP)) arises from much less positive charges of B and Ga and a smaller atomic size of B than that of Al. These results indicate that the significantly large electropositive nature and appropriate atomic size of Al are responsible for the characteristic coordination flexibility of Rh(PAlP).

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, Rhδ--Alδ+ polarization is observed in the bonding MO of 1, which is reverse to Rhδ+-Eδ- (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 Rhδ+-Eδ- polarization (E = coordinating atom) in the Rh-E bonding MO. On the other hand, the reverse Rhδ--Eδ+ 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 4dσ, 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.