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* 2 M 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* 2 MCO is observed to form under CO pressure. The CO stretching frequencies for Cp* 2 CaCO (2158 cm - 1 ), Cp* 2 SrCO (2159 cm - 1 ), Cp* 2 SmCO (2153 cm - 1 ), and Cp* 2 EuCO (2150 cm - 1 ) are greater than that of free CO (2134 cm - 1 in toluene or methylcyclohexane). In contrast, Cp* 2 YbCO has ν CO 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* 2 Yb(CO) 2 with an even lower ν CO value of 2072 cm - 1 predominates. Equilibrium constants were determined as a function of pressure and temperature for the equilibria Cp* 2 M II (solv) + n CO(gas or solv) ⇌ Cp* 2 M II (CO) n (solv), with M II = Ca, Eu, Yb. The thermodynamic constants Δ H °, Δ S °, and Δ G ° 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.