A theoretical study was carried out at the B3LYP level of theory to study the impact of the ligand and coordinate number on the ethylene insertion into the Rh—H bond of heterobinuclear (CO)₄Cr(m-PH₂)₂RhH(L_n) (L＝CO or PH₃, n＝1 or 2) complexes. It is shown that the bonding between ethylene and rhodium in the hexa-coordinate ethylene complexes is mainly the s-donation from ethylene to rhodium center, while the bonding between ethylene and rhodium in the penta-coordinate ethylene complexes involves the s-donation and p-back-donation. The PH₃ ligand inhibits the ethylene insertion thermodynamically. The phosphine ligand cis to the hydride ligand can enhance the ethylene insertion kinetically. The penta-coordinate pathway of the ethylene insertion is predominant. The introduction of Cr(CO)₄ moiety lowers the activation energy of the ethylene insertion.

Key words: ethylene insertion, heterobinuclear Rh-Cr complex, density functional theory, charge decomposition analysis