Density Functional (DFT) Study of the Anti−Syn Isomerization of the Butenyl Group in Cationic and Neutral (Butenyl)(butadiene)(monoligand)nickel(II) Complexes

The transition-metal assisted isomerization of the butenyl group, which most likely takes place via π → σ butenyl conversion followed by internal rotation around the C 2 −C 3 single bond, was studied theoretically. This was performed using the density functional theory (DFT) with cationic and neutral (butenyl)(butadiene)(monoligand)nickel(II) complexes. The important structural accommodation during the process of rotational isomerism is the pyramidalization of the carbon center associated with sp 3 hybridization. There are two conformers of the rotational transition structure for the inward and outward rotation of the electron pair which is formed during the rotation. The π → σ butenyl conversion followed by formation of the rotational transition structure is accompanied by negative charge migration, which is mainly located at the rotated sp 3 carbon atom. The isomerization barrier is strongly influenced by the ligand's donor−acceptor ability. Our calculations indicate an increased barrier due to an α-alkyl substitution of the butenyl group. The σ-butenyl rotational transition structure is stabilized by the occupation of its single vacant coordination site by an additional ligand, which therefore gives rise to lower activation energy for isomerization.