Reaction of (R2PC2H4PR2)Ni(C2H4) With COT gives the mononuclear complexes (R2PC2H4PR2)Ni(eta(2)-C8H8) (R = Pr-i la, Bu-t Ib). The COT ligand in la,b is planar with alternating C-C and C=C bonds, corresponding to a formal semiaromatic [C8H8](-) ligand. Reactions of la with ((Pr2PC2H4PPr2)-Pr-i-Pr-i)Ni(eta(2),eta(2)-C6H10) and of Ib with stoichiometric amounts of {((Bu2PC2H4PBu2)-Bu-t-Bu-t)Ni}(2)(mu-C6H6) or lithium afford the dinuclear complexes {((Pr2PC2H4PPr2)-Pr-i-Pr-i)Ni}(2){mu-eta(4)(1,2,5,6):eta(4)(3,4,7,8)-C8H8} (2a) and {((Bu2PC2H4PBu2)-Bu-t-Bu-t)Ni}(2)(mu-eta(2):eta(2)-C8H8) (2b; two isomers). The COT ligand in 2a is tub-shaped and olefinic, whereas in 2b (as in la,b) it is planar and semiaromatic. The products are characterized by IR, solution and solid-state NMR spectroscopy, and by X-ray structure analysis.