Density functional theory (DFT) calculations were performed on clusters [Os-3(CO)(10)(alpha-diimine)], for alpha-diimine = 2,2'-bipyridine (BPY), N-isopropyl 2-iminomethylpyridine (IMP), and N,N'-diisopropyl-1,4-diaza-1,3-butadiene (DAB), together with their spectroscopic study. This important family of clusters is known to convert upon irradiation with visible light into short-lived biradicals and long-lived zwitterions from a sigma pi* (SBLCT) excited state that has not been described accurately thus far by quantum mechanical calculations. On the basis of the combined DFT, UV-vis absorption, and resonance Raman data, the lowest-lying visible absorption band is assigned to a sigma(Os1-Os3)-to-pi*(alpha-diimine) CT transition, for alpha-diimine = bpy and IMP, and to a strongly delocalized sigma(Os1-Os3)pi*-to-sigma*(Os1-Os3)pi* transition for conjugated nonaromatic alpha-diimine = DAB. The DFT calculations rationalize the experimentally determined characteristics of this electronic transition in the studied series: (i) The corresponding absorption band is the dominant feature in the visible spectral region. (ii) The CT character of the electronic excitation declines from alpha-diimine = bpy to IMP and vanishes for DAB. (iii) The excitation energies decrease in the order alpha-diimine = DAB > BPY > IMP. (iv) The oscillator strength shrinks in the order alpha-diimine = DAB > IMP > BPY. Reference photoreaction quantum yields measured accurately for the formation of a cluster zwitterion from [Os-3(CO)(10)(IMP)] in strongly coordinating pyridine demonstrate that the optical population of the lowest-energy (1)sigma pi* and relaxed (3)sigma pi* excited states in the DFT model scheme is still capable of inducing the initial homolytic Os1-Os3 sigma-bond splitting, although less efficiently than the optical excitation into neighbor higher-lying electronic transitions due to a higher potential barrier for the reaction from a dissociative (sigma sigma*) state.