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 structure s of [MCp2], M = Ca, Sr, Ba, Sm, Eu, and Yb, complexes. It is found t hat van der Waals attractive forces between Cp rings cause all of the molecules to be bent and the calculated and observed structures corre spond to a remarkable degree. The differences in energy between the be nt and linear geometries is small (<2.1 kcal mol-1), and it is predict ed that the bis(pentaisopropylcyclopentadienyl) complexes of these met als should be linear.