is at least as bulky as a t-butyl radical; R is a structural bridge between the (C5 R4) and (R2 C5 C4 R'4) rings to impart stereorigidity and is preferably a hydrocarbyl or hydrosilyl radical having one atom of carbon or silicon to form the bridge; Q is a hydrocarbyl radical, such as an alkyl, aryl, alkenyl, alkylaryl or arylalkyl radical having 1-20 carbon atoms or is a halogen; Me is a Group II