Various 3,5-substituted tris(pyrazolyl)borate nickel(II) and palladium(II) complexes have been prepared and characterised in both the solid state and solution [tris(pyrazolyl)borate = tris(3,5-dimethylpyrazolyl)borate (Tp*), tris(3-t-butyl-5-methylpyrazolyl)borate (Tp(t-Bu.Me)) or tris(3-cumyl-5-methyl pyrazolyl)borate (Tp(Cum.Me))]. The crystal structures of the tetrahedral complex [k(3) N,N',N"-Tp(t-Bu.Me)NiCl] and of the trigonal-bipyramidal derivative [k(3)N,N',N"-Tp(Cum.Me)NiCI(3-methyl-5-cumylpyrazole)] have been determined by X-ray diffraction analysis. The catalytic performance of all compounds has been tested in ethylene polymerisation. The nickel catalysts activated with methylaluminoxane (MAO) produce polyethylene with moderate to good activity, while the palladium compounds show scarce activity. (C) 2004 Elsevier B.V. All rights reserved.