Reversible carbon-carbon bond formation in organolanthanide systems. Preparation and properties of lanthanide acetylides [Cp*{sub 2}LnC{triple_bond}CR]{sub n} and their rearrangement products [Cp*{sub 2}Ln]{sub 2}({mu}-{eta}{sup 2}:{eta}{sup 2}-RC{sub 4}R) (Ln = La, Ce, R = Alkyl)