The catalytic activity of many organolanthanoids is controlled by the ionic radius of Ln³⁺. In general Ln(Cp*)₂X complexes that are catalysts for olefin polymerization show decreasing reactivity as La is replaced by Lu. In contrast, some reactions proceed more rapidly as the catalyst is changed from La to Lu. Discuss the principles involved in tuning the catalytic activities of organolanthanoid catalysts in terms of rates and selectivity.