Theoretical Insight into 20-Electron Transition-Metal Complexes (C 5 H 5 ) 2 TM(E 1 E 2 ) 2 (TM = Cr, Mo, W; E 1 E 2 = CO, N 2 , BF): Stabilities, Electronic Structures, and Bonding Nature

A systematic first-principles study is performed to investigate the 20-electron transition metal complexes (C 5 H 5 ) 2 TM(E 1 E 2 ) 2 (TM = Cr, Mo, W; E 1 E 2 = CO, N 2 , BF). The bond dissociation energy (De) based on (C 5 H 5 ) 2 TM(E 1 E 2 ) 2 → (C 5 H 5 ) 2 TM(E 1 E 2 ) + E 1 E 2 indicates much lower thermodynamic stability of (C 5 H 5 ) 2 TM(N 2 ) 2 because of poor binding ability of N 2 ligands. For the thermodynamic stable (C 5 H 5 ) 2 TM(E 1 E 2 ) 2 complexes (TM = Cr, Mo, W; E 1 E 2 = CO, BF), their 20-electron nature is derived from their occupied nonbonding molecular orbital mainly donated by ligands. Furthermore, charge transfer from TMs to the C 5 H 5 ligands is revealed by the atoms in molecules (AIM) theory, leading to the positive charges of the TM atoms. On the other hand, the nature of the TM-E 1 bond has been thoroughly analyzed by the energy decomposition analysis (EDA) method. The absolute value of interaction energies (|Δ E int |) between (C 5 H 5 ) 2 TM(E 1 E 2 ) and E 1 E 2 has the same trend as the corresponding bond dissociation energy and Wiberg bond orders of TM-E 1 bonds, following the order W > Mo > Cr with same ligands and BF > CO with same TM. Additionally, the largest contribution to the Δ E int values is the repulsive term Δ E Pauli . Similar contributions from covalent and electrostatic terms to the TM-E 1 bonds were found, which can be described as the classic dative bond with nearly same σ and π contributions. The stronger σ donations and π backdonations in (C 5 H 5 ) 2 TM(BF) 2 than in (C 5 H 5 ) 2 TM(CO) 2 indicate much more stability of (C 5 H 5 ) 2 TM(BF) 2 .