Chameleon-like Nature of Anagostic Interactions and Its Impact on Metalloaromaticity in Square-Planar Nickel Complexes

Anagostic C-H-M (M = a metal center) intramolecular interactions, one of the most fundamental and elusive forces in organometallic chemistry, are intuitively considered as repulsive and purely electrostatic in nature because of significant metal-hydrogen distances (similar to 2.3-3.0 angstrom). Contrary to the current state of knowledge, it is shown herein by quantum chemical computations based on the case study of new square-planar Ni-II isomers based on N-thiophosphorylated thiourea that despite significant metal hydrogen anagostic distances, the covalent-type charge delocalization contribution [Ni(d(z)(2)) -> sigma*(C-H) and sigma-(CH) -> Ni(d(z)(2))] exists and it covers, together with the London dispersion energy, up to similar to 40% of the overall anagostic stabilization. This charge delocalization component is found to amplify the metalloaromaticity phenomenon although a lack of any stabilizing charge transfer is expected at such long metal hydrogen distances (>3 angstrom). Remarkably, for the relatively short regime (<3 angstrom) of anagostic distances, the electrostatic Coulomb forces are destabilizing, which leads to the repulsive anagostic interactions, whereas, surprisingly, an increase of anagostic distance above 3 angstrom makes anagostic interactions stabilizing mostly because of attractive Coulomb forces. It shows unprecedented agostic (attractive) <-> anagostic (repulsive) transitions in ubiquitous d(8) square-planar Ni-II complexes containing elongated metal-hydrogen distances.