The in situ Ni K-edge XAS study of the Ni(alpha-diimine)Cl-2/methylalumoxane catalyst during 1-decene polymerization indicates the formation of a complex with Ni(II) atoms bonded to C and N atoms as well as a Ni...Cl interaction at ca. 4.02 Angstrom, which suggests the formation of a pentacoordinated nickel complex as the active site. This geometry explains spectroscopic and magnetic properties of the complexes and can play an important role in the polymerization of ethylene by late transition metal catalysts. (C) 2003 Elsevier Science (USA). All rights reserved.