Heterobimetallic Complexes Containing Ca–Fe or Yb–Fe Bonds: Synthesis and Molecular and Electronic Structures of [M{CpFe(CO)2}2(THF)3]2 (M = Ca or Yb)

Reaction of calcium or ytterbium amalgam with [CpFe(CO)2]2 (Fp2) gave the isostructural heavy alkaline earth or divalent rare earth compounds [MFp2(THF)3]2 (M = Ca or Yb) containing two direct Ca–Fe (3.0185(6) Å) or Yb–Fe (2.9892(4) Å) bonds. Density functional theory supports experiment in finding shorter Yb–Fe than Ca–Fe distances, and Ziegler–Rauk, molecular orbital, and atoms-in-molecules analyses find the M–Fe bonding to be predominantly electrostatic in nature. The Yb–Fe interaction energy and bond critical point electron density are slightly larger than for Ca–Fe, in agreement with the shorter M–Fe bond in the former. The corresponding reaction for magnesium gave MgFp2(THF)4 with two O-bound Fp moieties and no Mg–Fe bond.