A composition comprising a boron or aluminum containing neutral Lewis acid and a compound of formula I or Ia: wherein R1 and R2 are each independently hydrocarbyl, substituted hydrocarbyl, or silyl; Q is (i) C—R4, where R4 is hydrocarbyl, substituted hydrocarbyl, heteroatom connected hydrocarbyl, or heteroatom connected substituted hydrocarbyl, (ii) P(NH2)2, or (iii) S(NH)(NH2) or S(O)(OH); L is a monoolefin or a neutral Lewis base that can be displaced by a monoolefin; T is hydrogen, hydrocarbyl or substituted hydrocarbyl, or with L forms a &pgr;-allyl group; and M is Ni(II), Pd(II), Co(II) or Fe(II). The composition is useful as an olefin polymerization catalyst. Also described is a process for preparing a supported catalyst.