Abstract Tris (ethene) nickel (0) reacts with tmeda and butadiene in ether below− 40 C to yield a deep-red solution of (tmeda) Ni (η 2-C 4 H 6) 2 (3a), from which red crystals of the thermolabile dinuclear complex {(tmeda) Ni (η 2-C 4 H 6)} 2 (η 2, η 2-C 4 H 6)(3b) can be separated. Results in the formation of mononuclear 3a, which also forms upon dissociation of 3b in solution, was identified from its 1 H and 13 C NMR spectra. Above− 40 C, 3a, b ...