Binding and electrophilic activation of ethylene by zinc(II), cadmium(II), and mercury(II) complexes: A theoretical investigation

Abstract Density functional theory has been used to investigate binding and activation of ethylene by the group 12 metal ions Zn 2+ , Cd 2+ , and Hg 2+ in a series of [M(L)( η 2 –C 2 H 4 )] n+ complexes (where L = 2,2′ - bipyridine, N,N,N′,N′–tetramethylethylenediamine, tris(3,5-dimethyl-1-pyrazolyl)methane, 1,4,7-trimethyl-1,4,7-triazacyclononane, hyrdotris(3,5-dimethyl-1-pyrazolyl)borate, the bis(2,6-dimethylphenyl)-substituted β -diketiminate, and the iminophosphanamide ( t Bu) 2 P(NSiMe 3 ) 2 ). Structural and vibrational analyses predict an activated ethylene C=C bond in all complexes, with activation maximized by the use of neutral bidentate ligands and the Hg 2+ ion. Bond energy decomposition analysis (EDA-NOCV) shows that [M(L)] n+ –ethylene interaction energies are favorable for all complexes studied, and even more favorable than those for many of the analogous and experimentally-isolated [Cu(L)( η 2 –C 2 H 4 )] n+ complexes. Electrostatic and orbital stabilization provide roughly equal contributions to the metal–ethylene bond, while both EDA-NOCV and natural bond orbital (NBO) analysis indicate that ethylene( π )→[M(L)] n+ electron donation and orbital stabilization dominates Dewar-Chatt-Duncanson bonding in these complexes, yielding a significant positive charge on the ethylene moiety. Molecular orbital analysis confirms these findings and indicates that the electrophilic reactivity of the metal-bound ethylene moiety would be further enhanced due to substantial stabilization of the alkene π *- and π -orbitals. Finally, addition of ammonia to the bound ethylene moiety to form the metal–ammonioalkyl species is generally predicted to be thermodynamically favorable, with less bulky, neutral ligands enhancing the favorability of the reaction and yielding a lower activation barrier.