+ is an appropriate model, and addition of an electron does little to perturb the electronic structure. In combination, the analysis suggests that 2 is best described as a 19-electron metal σ-complex derived from coordination of the two-electrons in the zinc-hydrogen bond of 1a to a 17-electron {Ni(CO)Cp} fragment. Figure . Simplified MO interaction between [NiCp(CO)] + and a Zn-H bond (isosurface values of 0.02).