Chelate ligands of the diazadiene type, R-N=C(R')-C(R)=N-R (dad), especially when complexed with iron, have acclaimed special interest as directive ligands for homogeneous catalytic reactions. Thus, e. g., the dimerization of butadiene or isoprene can be accomplished with (dad),Fe"', the product distribution between six-and eight-membered ring compounds depending on the size and conformation of the groups R. 2,2'-Bipyridyl and 2-pyridinealdimines coordinated to iron also give interesting catalytic systems[21. With complexes (dad)Fe(NO)2 (1) we found not only the known dimerization of isoprene to 1,4-and 2,4-dimethyl-4-vinyl-1-cyclohexene (2)13] but, depending upon R,R, also polymerization at room temperature, predominantly to 3,4-p0lyisoprene[~' ( Table I). R e(NO)z + 6 + 6 + Polymer (2) (3) ( l a ) , R = C~H S ; (Ib). R = i-C3H7To obtain information about the stereochemical conditions in the coordination sphere of the iron, we have also investigated the complexation of bulky dad-ligands in the trischelate complex ion [(dad)3Fe]2+, known previously only [' I Prof.