Necessary conditions for the photogeneration of nitrosyl linkage isomers

Abstract

We investigate the fundamental necessary conditions for optical generation of nitrosyl linkage isomers in ML₅NO compounds (M = transition metal, L arbitrary ligand) on the examples of K₃[Mn(CN)₅NO]·2H₂O and Na₂[Fe(CN)₅NO]·2H₂O. We show that the NO linkage isomers of the side-on bonded type (SII, 90° rotation of NO) and of the isonitrosyl type M–ON, where NO is O-bound to the metal M (SI, 180° rotation of NO), can be generated if two conditions are fullfilled. First the optical excitation must lead to a change in the bond between the NO group and the central metal atom M, either by a metal-to-ligand charge transfer of type d →π*(NO) or by a d→d_z² transition, which changes the σ bonding of the NO group to the metal, such that the vibrational deformation mode δ(M–N–O) can drive the system into the SII configuration. Second the excited state potential must posses a minimum close to the saddle point of the ground state surface between GS and SII, SI, or cross that surface, such that the relaxation from the excited state into the metastable minima can occur. The same is true for transfers between the two metastable states SII and SI. As a further constraint with respect to the amount of population, i.e. the number of complexes which can be transferred into SII or SI, the cross sections σ_GS,SII,SI of the states GS, SII, and SI must be considered. If σ_GS > σ_SII and σ_SI > σ_SII SII can be occupied while SI can be significantly occupied if σ_GS > σ_SI and σ_SII > σ_SI. More simply speaking the depletion rate of the metastable state should be smaller than its population rate for a given wavelength.