NMR Shifts, Orbitals, and M···H−X Bonding in d8 Square Planar Metal Complexes

We have investigated M···H−C interactions in d 8 square planar Rh and Pt complexes involving different types of C−H bonds, as well as a M···H−N hydrogen-bonded complex, using a combination of DFT and “atoms-in-molecules” (AIM) theory. The AIM analysis shows that both M···H−C and M···H−N interactions in d 8 square planar complexes are of a closed-shell, electrostatic nature, similar to protein backbone hydrogen bonds. However, at the shortest M···H distances, both interactions have partial covalence. We find no evidence for the involvement of d z 2 orbitals in M···H−C interactions but do find evidence for the involvement of d xz / yz orbitals in M···H−C bonding. The DFT calculations reproduce well the experimental proton NMR chemical shifts, with a theory-versus-experiment correlation coefficient R 2 = 0.985. There are large downfield 1 H NMR chemical shift changes on metal bonding, accompanied by changes in shielding tensor orientations.