Analysis of the metal–ligand bonds in [Mo(X)(NH2)3] (X = P, N, PO, and NO), [Mo(CO)5(NO)]+, and [Mo(CO)5(PO)]+

摘要

Quantum chemical calculations at the DFT level have been carried out for model complexes [Mo(P)(NH2)3] (1), [Mo(N)(NH2)3] (2), [Mo(PO)(NH2)3] (3), [Mo(NO)(NH2)3] (4), [Mo(CO)5(PO)]+ (5), and [Mo(CO)5(NO)]+ (6). The equilibrium geometries and the vibration frequencies are in good agreement with experimental and previous theoretical results. The nature of the Mo–PO, Mo–NO, Mo–PO+, Mo–NO+, Mo–P, and Mo–N bond has been investigated by means of the AIM, NBO and EDA methods. The NBO and EDA data complement each other in the interpretation of the interatomic interactions while the numerical AIM results must be interpreted with caution. The terminal Mo–P and Mo–N bonds in 1 and 2 are clearly electron-sharing triple bonds. The terminal Mo–PO and Mo–NO bonds in 3 and 4 have also three bonding contributions from a σ and a degenerate π orbital where the σ components are more polarized toward the ligand end and the π orbitals are more polarized toward the metal end than in 1 and 2. The EDA calculations show that the π bonding contributions to the Mo–PO and Mo–NO bonds in 3 and 4 are much more important than the σ contributions while σ and π bonding have nearly equal strength in the terminal Mo–P and Mo–N bonds in 1 and 2. The total (NH2)3Mo–PO binding interactions are stronger than for (NH2)3Mo–P which is in agreement with the shorter Mo–PO bond. The calculated bond orders suggest that there are only (NH2)3Mo–PO and (NH2)3Mo–NO double bonds which comes from the larger polarization of the σ and π contributions but a closer inspection of the bonding shows that these bonds should also be considered as electron-sharing triple bonds. The bonding situation in the positively charged complexes [(CO)5Mo–(PO)]+ and [(CO)5Mo–(NO)]+ is best described in terms of (CO)5Mo → XO+ donation and (CO)5Mo ← XO+ backdonation (X = P, N) using the Dewar–Chatt–Duncanson model. The latter bonds are stronger and have a larger π character than the Mo-CO bonds.