DFT studies of structural preference of coordinated ethylene in W(CO)₃(PX₃)2(CH₂CH₂) (X=H, CH3, F, Cl, Br, and I)

A set of phosphine complexes of the type W(CO)₃ (PX₃)₂(CH₂=CH₂) (X=H, CH₃, F, Cl, Br, and I) were investigated by density functional theory method (BP86) to examine the effect of the substituent X on the orientation of C-C vector of the ethylene ligand with respect to one of the metal-ligand bonds as well as the donation and the backdonation in the bonding ligands of phosphine and ethylene. When X=CH₃, H, F, and Cl, the ethylene C-C vector prefers to be coplanar with metal-phosphine bonds, while for the ethylene complexes containing PBr₃ and PI₃ ligands, the structural preference is coplanarity of the ethylene and the metal-carbonyl bonds. The molecular orbital calculations and natural bond orbital analysis were used to examine the structural consequences derived from these complexes. It can be concluded that the structural preferences in the complexes have a clear relation to electronic effects of phosphine ligands. Our calculations for halide phosphine complexes, particularly for PBr₃ and PI₃, allow us to conclude that in addition to electronic effects, steric factors can also affect the orientation of the ethylene ligand in complexes.