Abstract: A novel organometallic compound is disclosed which can be used, in conjunction with a trialkyl or bialkyl aluminum cocatalyst, for catalyzing the polymerization and copolymerization of ethylene without requiring the use of either methyl aluminoxane or borate as cocatalyst. The organometallic compound is represented by the formula of (C.sub.5 R.sub.n H.sub.5-n)(L)MX.sub.a Y.sub.b ; wherein: (a) C.sub.5 R.sub.n H.sub.5-n is a substituted or unsubstituted cyclopentadienyl group, in which n is an integer between 0 and 5, and R is C.sub.1 to C.sub.6 alkyl group; (b) L is tetrapyrazolyl borate, hydrotrispyrazolyl borate, dihydrobispyrazolyl borate or hydrotris(3,5-dimethylpyrazolyl) borate; (c) M is a Group IIIB, Group IVB, or Group VB transitional metal; (d) a and b are integers such that a+b=m-2, where m is the valance of M; and (e) X and Y, which can be the same of different from each other, are halogen atoms or ligands represented by the formula of ER.sup.1, where E is a Group VA or VIA element, and R.sup.