WHY ARE BIS(PENTAMETHYLCYCLOPENTADIENYL) COMPLEXES, [MCP2-ASTERISK], OF CALCIUM, STRONTIUM, BARIUM, SAMARIUM, EUROPIUM, AND YTTERBIUM BENT

A molecular mechanics force field is applied to minimize the structures of [MCp*2], M = Ca, Sr, Ba, Sm, Eu, and Yb, complexes. It is found that van der Waals attractive forces between Cp* rings cause all of the molecules to be bent and the calculated and observed structures correspond to a remarkable degree. The differences in energy between the bent and linear geometries is small (<2.1 kcal mol-1), and it is predicted that the bis(pentaisopropylcyclopentadienyl) complexes of these metals should be linear.