Solution infrared spectroscopic studies on equilibrium reactions of CO with the decamethylmetallocenes Cp*₂M^II, where M^II = Mg, Ca, Sr, Ba, Sm, Eu, Yb

Abstract

The reaction of CO with the alkaline-earth decamethylmetallocenes Cp*₂M^II, where M^II is Mg, Ca, Sr, or Ba, and the bivalent lanthanide decamethylmetallocenes, where M^II 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 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^-1 in toluene or methylcyclohexane). In contrast, Cp*₂YbCO has νCO 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 Cp*₂Yb(CO)₂ with an even lower νCO value of 2072 cm^-1 predominates. Equilibrium constants were determined as a function of pressure and temperature for the equilibria Cp*₂M^II(solv) + nCO(gas or solv) Cp*₂M^II(CO)_n(solv), with M^II = 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 ΙR absorption bands, indicate weak M^II−CO binding. Models to account for the bonding in these metallocene CO adducts are discussed.