Photolysis of M(CO)(4)(iprop-dab) (1, M = W; 2, M = Mo; iprop-dab = 1,4-diisopropyl-1,4-diazabuta-1,3-diene) in the presence of (E)-cyclooctene (eco) afforded high yields of the unprecedented olefin-substituted derivatives fac-M(CO)(3)(iprop-dab)(eta (2)-eco) (3, M = W, 93%; 4, M = Mo, 84%), which were comprehensively characterized by IR, UV-vis, and NMR spectroscopy and by an X-ray diffraction structure analysis of 3. Quantum yield measurements at 254, 302, 365, and 405 nm revealed a gradual decrease from Phi approximate to 0.1 to 0.02 and then a sharp drop to 0.001 at 548 nm (CTML excitation). Irradiation of 2 in low-temperature matrices, as monitored by means of IR spectroscopy in the v(CO) region, was shown to yield fac-Mo(CO)(3)(iprop-dab) (5, in Ar or CO-doped Ar), fac-Mo(CO)(3)((CO)-C-13)(iprop-dab) (in (CO)-C-13-doped Ar), and fac-Mo(CO)(3)(N-2)(iprop-dab) (6, in N-2), thereby establishing that a vacant axial coordination site is created by photolytic CO dissociation, with wavelength-dependent efficiency. The involvement of the (solvated) fragment fac-M(CO)3(iprop-dab) as a key intermediate in the solution photochemistry Of M(CO)4(iprop-dab) was substantiated by laser flash photolysis of 2 in combination with time-resolved IR and UV-vis spectroscopy. (CO)-C-13-enriched samples of 1 and 2 (specifically monolabeled) as well as 3, 5, and 6 (partially labeled) were used for gathering complementary v(CO) data as a basis for energy-factored CO force field analyses. The response of the low-energy CTML electronic transitions to the loss of CO and its replacement by the olefin or N2 ligand is discussed in terms of ligand-dependent metal d(pi) level stabilization.