A novel type of resonance-stabilised dicobalt complex with a cyclopentadienylidene bridge - Synthesis, structure and electronic properties of [(C(5)R(5))Co(mu-C5H4)Co(L)(C(5)R(5))'] (L=C2H4, CO, CNtBu, PR(3), P(OMe)(3); R=H, Me)

The dinuclear mu-cyclopentadienylidene complexes [(C(5)R(5))Co(mu-C5H4)Co(C2H4)(C(5)R(5)')] (5) [5aa (R = R' = H), 5ab (R = H, R' = Me), 5ac (R = H, R(5)' = H(4)Me), 5ba (R = Me, R' = H), 5bb (R = R' = Me) and 5da (R(5) = Me(4)Et, R' = H)] were synthesised from [(C(5)R(5)')Co(eta(4)-C5H6)] (4) [4a (R = H), 4b (R = Me), 4d (R(5) = Me(4)Et)] and [(C(5)R(5)')Co(C2H4)(2)] (1) [1a (R' = H), 1b (R' = Me) and 1c (R(5)' = H(4)Me)]. In these both CH bonds of the group of coordinated cyclopentadiene are activated under mild conditions. Substitution of the ethylene Ligand in 5 by L leads to the carbonyl, isocyanide, phosphine, and phosphite derivatives [(C(5)R(5))Co(mu-C5H4)Co(L)(C(5)R(5)')] [6aa, 6ab, 6ba, 6bb (L = CO), 7aa (L = tBuNC), 8aa, 8ab (L = PMe(3)), 9aa (L = PMe(2)Ph), 10aa (L = PMePh(2)) and 11aa (L = P(OMe)(3))]. The crystal structures of 5aa, 5ab, 5ba, 6aa and 8aa have been determined. The experimental geometry is rationalised in terms of two limiting structures with mu-eta(4):eta(1) and mu-eta(5):eta(1) coordination of the bridging cyclopentadienylidene ligand. On the basis of the 18 valence electron rule, zwitterionic character is assigned to latter. This structure is preferred when L is an acceptor ligand, as in 6. In solution, 5-11 are fluxional with rapid rotation about the very short cobalt-carbene C bond. In addition the hindered rotation of the ethylene ligand in 5ab was studied by DNMR spectroscopy. In solution, a singlet-triplet equilibrium was established by variable-temperature NMR spectroscopy for 8aa. The temperature-dependent H-1 NMR line shifts were analysed by means of an isotropic shift model to give 24 less than or equal to Delta H degrees less than or equal to 32 kJ mol(-1) and 45 less than or equal to Delta S degrees less than or equal to 74 mol(-1) K-1 with the triplet state being preferred by entropy at higher temperatures.