Various systems contg. the [ML5NO] molecule, where M = Fe, Ru, ... and L = F, Cl, ..., exhibit switching under continuous light (CW) irradn. between the ground-state nitrosyl (GS), isonitrosyl (MSI), and side-on (MSII) configurations. The metastable populations, however, are often limited to a few percent. The [Ru(py)(4)Cl(NO)](PF6)(2)center dot 1/2H(2)O system is thus a remarkable model compd. as the GS to MSI transformation is nearly complete in a single crystal. A predominant two-step photon absorption process during GS to MSI switching under blue light is revealed by visible absorption spectroscopy, although a low concn. of the transient species hinders the detn. of this process by the structural signature. During the depopulation of MSI, both two-step and direct processes are evidenced under red CW irradn. Different intermediate visible spectra revealing transient species during GS to MSI and the reverse photochem. processes are discussed in relation to MSII properties.