Coordination of Carbon Monoxide and Isocyanides to Bis(pentamethylcyclopentadienyl)ytterbium and Related Bivalent Ytterbocenes

Changes in the infrared spectra of methylcyclohexane solutions of the base-free ytterbocenes (Me5C5)2Yb, [1,3-(Me3C)2C5H3]2Yb, and [1,3-(Me3Si)2C5H3]2Yb in the presence of CO show that carbonyl adducts are formed at room temperature. The adducts are formed reversibly, and they have not been isolated. The nu-CO values are lower than that of free CO for the adducts of (Me5C5)2Yb and [1,3-(Me3C)2C5H3]2Yb, and in the case of (Me5C5)2Yb the 13C NMR resonance of the carbonyl is deshielded from that of free CO, indicating that CO is acting as a net pi-acceptor ligand in those molecules. The CO value for the adduct of [1,3-(Me3Si)2C5H3]2Yb is slightly higher than that of free CO, which demonstrates that changing the substituents on the cyclopentadienyl rings can alter the electron richness of the ytterbocene. Isocyanide complexes of these ytterbocenes and of (Me4C5H)2Yb have been isolated and fully characterized, including by X-ray crystallography for (Me5C5)2Yb(2,6-Me2C6H3NC)2, [1,3-(Me3C)2C5H3]2Yb(2,6-Me2C6H3NC)2, and [1,3-(Me3Si)2C5H3]2Yb(2,6-Me2C6H3NC)2. The solution electronic spectra (350-1000 nm) for the base-free ytterbocenes and their carbonyl and isocyanide adducts have been measured and interpreted according to a molecular orbital model. All of the data are consistent with the notion that ytterbium(II) metallocenes can act as net pi-donor fragments