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 Cp*2MII, where MIIis Mg, Ca, Sr, or Ba, and the bivalent lanthanide decamethylmetallocenes, where MIIis 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 Cp*2MCO is observed to form under CO pressure. The CO stretching frequencies for Cp*2CaCO (2158 cm-1), Cp*2SrCO (2159 cm-1), Cp*2SmCO (2153 cm-1), and Cp*2EuCO (2150 cm-1) are greater than that of free CO (2134 cm-1in toluene or methylcyclohexane). In contrast, Cp*2YbCO has Vco2114 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 Cp*2Yb(CO)2with an even lower Vcovalue of 2072 cm-1predominates. Equilibrium constants were determined as a function of pressure and temperature for the equilibria Cp*2MII(solv) + nCO(gas or solv) ⇌ Cp*2MII(CO)n(solv), with MII= Ca, Eu, Yb. The thermodynamic constants ΔH°, ΔS°, and ΔG° for the formation of each species, extracted from the changes in the intensity of the corresponding IR absorption bands, indicate weak MII-CO binding. Models to account for the bonding in these metallocene CO adducts are discussed.