UV photolysis of [(eta(6)-C(7)H(7)R)Cr(Co)(3)] (1, R = H, Ph, CH(CO(2)Me)(2)) and R’C = CR’ (R’ = Ph, Tol, SiMe(3), Et) in toluene or n-hexane gives the [6 + 2] cycloadducts [(eta(2:4)-C9H7(R)R’(2))Cr(CO)(3)] (2). Heating 2 in toluene releases the 7,8-disubstituted bicyclo[4.2.1]nona-2,4,7-trienes 3 and [(eta(6)-C(6)H(5)Me)Cr(CO)(3)]. Irradiation of solutions of 1 and dimethyl acetylenedicarboxylate at 0 degrees C yields the bicyclotriene as the major product but no organometallic complex. The diphenyl derivative 2a undergoes a reversible intramolecular metal migration to the eta(6)-aryl species 4. Irradiation of the azepine complex [(eta(6)-C6H6N(CO(2)Et))Cr(CO)(3)] (5) and R’C = CR’ (R’ = Ph, SiMe(3)) in toluene also forms [6 + 2] cycloadducts [(eta(2:4)-C8H6N(CO2Er)R’(2))Cr(CO)(3)] (6). The heterobicyclotrienes 7 are isolated from 6 by treatment with iodine (R’ = SiMe(3)) or by hearing in toluene (R’ = Ph). X-ray crystal structures are reported for 6b (R’ = SiMe(3)) [orthorhombic, P2(1)2(1)2(1), a = 8.871(1), b = 10.148(1), and c = 26.017(4) Angstrom, R = 0.041 for 2500 independent reflections] and a byproduct (8) that results from the endo [6 + 4] cycloaddition of N-carbethoxyazepine to 5; monoclinic, P2(1)/c, a = 10.665(9), b = 14.976(7), and c = 13.004(7) Angstrom, beta = 100.05(7)degrees, R = 0.047 for 1044 independent reflections. Heating cycloheptatriene and alkynes R’C = CR’ (R’ = SiMe(3), Me, Et) in dibutyl ether with a catalytic amount of Cr(CO)(6) or Fe-2(CO)(9) also gives good yields of the [6 + 2] cycloadducts 3, whereas the thermal reaction with dimethyl acetylenedicarboxylate in the presence of Cr(CO)(6) gives the known norcaradiene [4 + 2] adduct 9.