The first reactivity studies of mononuclear cobalt-allene complexes are reported. The isomeric cobalt-allene complexes (RS,RS,Z)-[(eta(5)-C(5)H(5))Co(1,2-eta(2)-CH(SO(2)Ph)= C=CH(CO(2)Et))] (2-Z) and (SR,RS,E)-[(eta(5)-C(5)H(5))Co(2, 3-eta(2)-CH(SO(2)Ph)=C=CH(CO(2)Et))] (3-E) undergo protonation with HBF(4) to give the cationic pi-allyl complex (eta(5)-C(5)H(5))Co(PPh(3))[exo-eta(3)-CH(CO(2)Et)CHCH(SO(2)Ph)](BF(4)) (4) and the first structurally characterized oxacobaltacyclopentadiene complex, (eta(5)-C(5)H(5))Co(PPh(3))[kappa(2)-OC(OEt)CH=C(CH(2)SO(2)Ph)](BF(4)) (5-BF(4)), respectively. Deprotonation of 5-BF(4) regenerates cobalt-allene complex 3-E, as well as 2-Z.
