This review is concerned with data on the composition and structure of inte
rmediate paramagnetic cobalt and nickel compounds in Ziegler-type metal com
plex catalysts. The problem of reactivity of the intermediate compounds is
briefly outlined. General regularities are revealed for the mechanism of fo
rmation of the metal complex catalysts. Particular emphasis is made on the
structure of intermediates whose EPR spectra explicitly exhibit the effects
of vibronic interactions. Vibronic effects are shown to play an important
role in the realization of geometric configuration for structures with the
pseudodegenerate electronic state. (C) 2000 Elsevier Science B.V. All right
s reserved.