Electron Density and Molecular Orbital Analyses of the Nature of Bonding in the η-CCH Agostic Rhodium Complexes Preceding the C-C and C-H Bond Cleavages.

In our recent work, we revisited C-H and C-C bond activation in rhodium (I) complexes of pincer ligands PCP, PCN, PCO, POCOP, and SCS. Our findings indicated that an η-CCH agostic intermediate acts as a common precursor to both C-C and C-H bond activation in these systems. We explore the electronic structure and bonding nature of these precleavage complexes using electron density and molecular orbital analyses. Using NBO, IBO, and ESI-3D methods, the bonding in the η-CCH agostic moiety is depicted by two three-center agostic bonds: Rh-C-C and Rh-C-H, with all three atoms datively bound to Rh(I). IBO analysis specifically highlights the involvement of three orbitals (CC→Rh and CH→Rh σ donation, plus Rh→CCH π backdonation) in both C-C and C-H bond cleavages. NCIPLOT and QTAIM analyses highlight anagostic (Rh-H) or β-agostic (Rh-C-H) interactions and the absence of Rh-C interactions. QTAIM molecular graphs suggest bond path instability under dynamic conditions due to the nearness of line and ring critical points. Several low-frequency and low-force vibrational modes interconvert various bonding patterns, reinforcing the dynamic η-CCH agostic nature. The kinetic preference for C-H bond breaking is attributed to the smaller reduced mass of C-H vibrations compared to C-C vibrations.