A process for preparing addition polymerization catalysts having a limiting charge separated structure corresponding to the formula: LMX.sup.+ XA.sup.-, wherein: L is a derivative of a substituted delocalized .pi.-bonded group imparting a constrained geometry to the metal active site and containing up to 50 nonhydrogen atoms; M is a metal of Group 4 or the Lanthanide series of the Periodic Table of the Elements; X independently each occurrence is hydride or a hydrocarbyl, silyl or germyl group having up to 20 carbon, silicon or germanium atoms; and A.sup.- is an anion of a Lewis acid, A, having relative Lewis acidity greater than or equal to that of phenylbis(perfluorophenyl)borane, said anion being compatible with the metal cation, the steps of the process comprising contacting a derivative of a Group 4 or Lanthanide metal corresponding to the formula: i LMX.sub.2, wherein L, M, and X are as previously defined, with the Lewis acid, A, under conditions to cause abstraction of X and formation of the anion XA.sup.-.