Are the Carbon Monoxide Complexes of Cp₂M (M = Ca, Eu, or Yb) Carbon or Oxygen Bonded? An Answer from DFT Calculations

DFT calculations have been performed on the CO adducts of the bivalent lanthanides, Cp(2)M(CO)(x), where M is Eu or Yb and x is 1 or 2, the alkaline earth metallocene Cp(2)Ca(CO), and the methylisocyanide adducts of Yb. The calculated nu(CO) values are in agreement with experiment for Cp(2)M(CO) when M is Ca or Eu, but in striking disagreement when the CO is bound to the metal by way of the carbon atom in CO in the case of Yb. The calculated nu(CO) values for M = Yb are brought into agreement with experiment when the CO is allowed to bond to Cp(2)Yb by way of the oxygen atom.