Orientational preference of two ethylene ligands bound to a nickel atom

CASSCF and CCI calculations have been performed to analyze the bonding in Ni(C₂H₄)₂. Three different relative orientations of the two olefins have been studied. It is found that a structure with D_2d symmetry, where the C-C bonds in the two olefins make a 90 degree angle to each other, gives the lowest energy. A D_2h form, with the two C-C bonds and Ni in the same plane, is 10.3 kcal/ mol higher in energy. The reason for the preference of the D_2d form is analyzed in terms of valence bond theory, and is found to be due to a d ⁸ structure with two simultaneous d bonds. A C _2v form, for which the two nickel olefin bonds make a 103 degree angle to each other and the C-C bonds are parallel to each other, is 32 kcal/mol higher in energy than the D_2d form. The low binding energy of the C _2v form is due to a poor interaction with inefficient sd hybridization.