The electronic, structural and optical properties (including Spin-Orbit Coupling) of metal nitrosyl complexes [M(CN) 5 (NO)] 2-(M=Fe, Ru or Os) are investigated by means of Density Functional Theory, TD-DFT and MS-CASPT2 based on RASSCF wavefunction. The energy profiles connecting the N-bound (η 1-N), O-bound (η 1-O) and side-on (η 2-NO) conformations have been computed at DFT level for the closed shell singlet electronic state. For each structure the lowest singlet and triplet states have been optimized in order to get insight of the energy profiles describing the conformational isomerism in excited states. The energetics of the three complexes is similar with the N-bound structure being the most stable, with one exception, namely the triplet ground state of the O-bound isomer for the iron complex. The conformation isomerism is highly unfavorable in the S 0 electronic state with the occurrence of two energy barriers higher than 2 eV. The lowest bands of the spectra are assigned to MLCT NO /LLCT NO transitions with an increasing MLCT character going from iron to osmium. Two low-lying triplet states, T1 (MLCT NO /LLCT NO) and T2 (MLCT NO /IL NO), seem to control the lowest energy profile of the excited state conformational isomerism.