Irradiation of the cyclohexadienyl complexes [(eta(5)-C(6)H(6)R(1))Mn(CO)(3)] (1) (R(1) = exo-H, Me, Tol) and 2 equiv of the alkynes (RC drop CR, R = Ph, Me, Pt) with UV light in toluene or n-hexane gives the new tricyclic complexes 2. These complexes arise from sequential [5 + 2] and [3 + 2] cycloadditions of two alkyne molecules to the eta(5)-coordinated six-membered ring forming a tricycle[5.2.1.0(4,8)] deca-2,5-dien-10-yl ligand. An X-ray crystallographic study on the adduct from [(eta(5)-C6H7) Mn(CO)(3)] and two molecules of 1-phenyl-1-propyne has been performed : monoclinic, P2(1)/c, a = 8.859(2) Angstrom, b = 16.318(3) Angstrom, c = 15.358(3) Angstrom, beta = 105.79(3)degrees, R = 0.043 for 2672 independent reflections. The tetramethyl and tetraethyl complexes 2a,b react with [CPh(3)](+) to give the cations 3. The X-ray structure of the methylene species 3a is reported : monoclinic, P2(1)/c, a = 8.585(2) Angstrom, b = 13.960(3) Angstrom, c = 15.286(3) Angstrom, beta = 96.00(3)degrees, R = 0.047 for 2382 independent reflections. The tricyclo[5.2.1.0(4,8)]deca-2,5-dienes containing an exocyclic double bond at C(10) can be decomplexed from the metal in 3 by refluxing in acetonitrile.