The catalytic activation process of the Ni-based Ziegler-Natta catalyst composed of Ni(naphthenate)2, catalytic amount of 1,3-butadiene, BF3.OEt2 and AlEt3, is spectroscopically observed by using synchrotron X-ray absorption and crystal field spectroscopies, and an optimum model of the nickel active site is proposed with the density functional theory (DFT) calculations. In the X-ray absorption near edge structure (XANES) spectrum of nickel naphthenate (NN), the position of the pre-edge at 8330.0eV and both the strong edge and the weak pre-edge peaks indicate that NN possesses high Oh symmetry and is in a divalent state. The similar XANES spectra of NNB (the mixture of NN and catalytic amount of 1,3-butadiene) and NNBF (the mixture of NNB and BF3.OEt2) indicate that 1,3-butadiene and BF3.OEt2 do not greatly disturb the coordination geometry around nickel. AlEt3 functions as the alkylating agent in the presence of BF3.OEt2, and initiates 1,3-butadiene polymerization in NNBFA (the mixture of NNBF and AlEt3) where the coordination geometry is altered from O-h - to T-d-like but nickel remains in a divalent oxidation state. In the absence of BF3.OEt2, AlEt3 works as the reducing agent in NNBA (the mixture of NNB and AlEt3). A single and well-defined peak at 1.62 Angstrom is observed for NN due to the Ni-O shell in the phase-uncorrected Fourier-transformed (Fr) EXAFS (extended X-ray absorption fine structure) spectrum of NN. A significant change in the EXAFS spectrum of NNBFA has occurred where the main peak shifts toward a higher distance with increased complexity. The complex nature of the peak at 1.75 Angstrom indicates that the coordination feature around nickel differs from the Ni-O shell and the longer bonds, Ni-C and Ni-F, are formed. The Ni-F bond of NNBFA is confirmed by the Ni-F peak, - 137 ppm, in the F-19-NMR spectrum. The three main absorption bands, around 8930, 14660 and 24940 cm(-1), are obtained with the crystal field spectra of NN, NNB and NNBF, which represent a typical octahedral structure of Ni(II) having three spin-allowed transition bands from (3)A(2g) ground state to the T-3(2g), T-3(1g)(F) and T-3(1g)(P) states, respectively. Heterogeneity of the Ziegler-Natta catalyst is evidenced by strong scattering in the UV-Vis region of NNBFA. (C) 2002 Elsevier Science B.V. All rights reserved.