Bonding analysis of trimethylenemethane (TMM) complexes [(CO) ₃M-TMM] (M = Fe, Ru, Os, Rh⁺). Absence of expected bond paths

Quantum chemical calculations using gradient-corrected DFT methods (BP86) using effective core potentials and TZP quality basis sets have been carried out for the complexes (CO)₃M-TMM (M = Fe, Ru, Os, Rh⁺; TMM = trimethylenemethane). The calculated equilibrium geometries give trigonal pyramidal structures which are in good agreement with experimental results for the iron and ruthenium species. The TMM ligand adopts a pyramidal coordination of the central carbon atom which has a shorter distance to the metal atom than the terminal carbon atoms. The theoretically predicted bond dissociation energies of the TMM ligand are between 108.0 kcal/mol for the iron complex and 155.1 kcal/mol for the rhodium complex. The trend of the calculated TMM-M bond strength is Fe < Ru < Os < Rh⁺. The investigation of the electronic structure of the complexes (CO)₃M-TMM using MO correlation diagrams and energy partitioning methods clearly shows that the metal atom is mainly bonded to the terminal carbon atoms of the TMM ligand while the bonding to the closer central carbon atom is much weaker. This is further supported by the AIM analysis which shows that the Laplacian distribution ²ρ(r) at the terminal carbon atoms possesses local area of charge concentration which points toward the metal atom while the Laplacian distribution at the central carbon atoms is nearly undistorted. In spite of the shape of the Laplacian distribution, there are bond paths between the metal atom and the central carbon atom but not to the terminal carbon atom which comes from the different interatomic distances. This clearly shows that the absence and the existence of a bond path are not reliable criteria for chemical bonding.