Stepwise interaction of first row transition metal atoms with 1,5-cydooctadiene to give (C8H12)(2)M complexes is studied using the M06-L/DZP density functional method. The experimentally known (C8H12)(2)Ni is the thermodynamically most favorable complex, with a predicted geometry consistent with its experimental structure as determined by Xray crystallography. The other transition metal atoms from scandium to zinc also interact exothermically with 1,5-cyclooctadiene to give (C8H12)(2)M derivatives, but these exhibit lower symmetry than the S-4 symmetry exhibited by (C8H12)(2)Ni. Carbon-hydrogen activation of CH2 groups in a C8H12 ligand is predicted for most systems. Thus, conversion of (eta(2,2)-C8H12)(2)M to (eta(3,2)-C8H11)(eta(2,1)-C8H13)M, through a hydride intermediate (eta(3,2)-C8H11)(eta(2,2)-C8H12)MH, is predicted for scandium, vanadium, chromium, manganese, and cobalt. For titanium with a lowlying empty orbital, further C-H activation through a hydride intermediate (eta(6)-C8H10)(eta(2,1)-C8H13)TiH is predicted, leading ultimately to (eta(6)-C8H10)(eta(1,1)-C8H14)Ti, in which the hexahapto eta(6)-C8H10 ligand is shown by NICS to be aromatic. These two C-H activation processes on a titanium center represent the dehydrogenation of 1,5-cyclooctadiene to 1,3,5-cyclooctatriene with the second 1,5-cyclooctadiene ligand as the hydrogen acceptor. For zinc C-H activation terminates at (eta(1)-C8H11)(C8H12)ZnH, which has a C-Zn-H three-center bond. No energetically favorable C-H activation processes are predicted for the iron, nickel, and copper (eta(2,2)-C8H12)(2)M derivatives.