Solution Infrared Spectroscopic Studies on Equilibrium Reactions of CO with the Decamethylmetallocenes Cp*2MII, Where MII = Mg, Ca, Sr, Ba, Sm, Eu, Yb

The reaction of CO with the alkaline-earth decamethylmetallocenes Cp2MII, where MII is Mg, Ca, Sr, or Ba, and the bivalent lanthanide decamethylmetallocenes, where MII is Sm, Eu, or Yb, have been studied in toluene or methylcyclohexane solution in a high-pressure infrared cell. In all cases, except for Mg and Ba, the monocarbonyl complex Cp2MCO is observed to form under CO pressure. The CO stretching frequencies for Cp2CaCO (2158 cm-1), Cp2SrCO (2159 cm-1), Cp2SmCO (2153 cm-1), and Cp2EuCO (2150 cm-1) are greater than that of free CO (2134 cm-1 in toluene or methylcyclohexane). In contrast, Cp2YbCO has nuCO 2114 cm-1, below that of free CO. This 1:1 complex is formed at low CO pressure (<2 bar); at higher CO pressures the 1:2 adduct Cp2Yb(CO)2 with an even lower nuCO value of 2072 cm-1 predominates. Equilibrium constants were determined as a function of pressure and temperature for the equilibria Cp2MII(solv) + nCO(gas or solv) Cp2MII(CO)n(solv), with MII = Ca, Eu, Yb. The thermodynamic constants delta H degrees, delta S degrees, and delta G degrees for the formation of each species, extracted from the changes in the intensity of the corresponding iotaR absorption bands, indicate weak MII-CO binding. Models to account for the bonding in these metallocene CO adducts are discussed.