A Novel Type of Resonance‐Stabilised Dicobalt Complex with a Cyclopentadienylidene Bridge—Synthesis, Structure and Electronic Properties of [(C 5 R 5 )Co(μ‐C 5 H 4 )Co(L)(C 5 R 5 )'] (L = C 2 H 4 , CO, CN t Bu, PR 3 , P(OMe) 3 ; R = H, Me)

The dinuclear μ‐cyclopentadienylidene complexes [(C 5 R 5 )Co(μ‐C 5 H 4 )‐Co(C 2 H 4 )(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(η 4 ‐C 5 H 6 )] ( 4 ) [ 4a (R = H), 4b (R = Me), 4d (R 5 = Me 4 Et)] and [(C 5 R' 5 )Co(C 2 H 4 ) 2 ] ( 1 ) [ 1a (R' = H), 1b (R' = Me) and 1c (R' 5 = H 4 Me)]. In these reactions, both CH bonds of the methylene 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(μ‐C 5 H 4 )Co(L)‐(C 5 R' 5 )] [ 6aa, 6ab, 6ba, 6bb (L = CO), 7aa (L = t BuNC), 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 μ‐η 4 :η 1 and μ‐η 5 :η 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 1 H NMR line shifts were analysed by means of an isotropic shift model to give 24≤ΔH° ≤ 32 kJ mol ‐1 and 45≤Δ S ° ≤74 J mol ‐1 K ‐1 with the triplet state being preferred by entropy at higher temperatures.