π‑Backbonding on Group 9 Metal Complexes Bearing an η2‑(H2O@C60) Ligand

As a measure of d-π* back-donation, IR stretching modes are, in general, used for metal carbonyl compounds. Once the ligand is replaced with olefins (metal–arene complexes), it can be analyzed crystallographically unless otherwise the C–C bond length falls into values within standard deviation. Herein, we propose a novel measure using an η2-(H2O@C60) ligand in group 9 metal–fullerene complexes, in which the stronger d-π* backbonding causes enhanced sp3-character on the coordinated olefinic bond and consequently leads to the restricted rotation for the H2O molecule as detectable by the 1H NMR relaxation time. The π-backbonding strengths were, therefore, found to follow the ascending order of atomic numbers (Co 2-H2O@C60) (Cp = cyclopentadienyl), associated with the anodic shift on oxidation potentials and hypsochromic shift on absorption edges.