6 carbon monoxide (100 psi) in pentane. The carbonyl stretching frequency (1440 cm-I) and the 13C NMR shift of the carbonyl carbon for 2 are similar to corresponding values for CpzZr (s2-COSiMe3) C1. 4d A possible explanation for the greater reactivity of 1 over CpZZr [Si (SiMeJ3] C1 toward carbon monoxide is that increased steric interactions about the metal center promote CO insertion in 1. A similar effect has been observed for Cp2Zr-[CH ( ...