Local force constants and charges of the nitrosyl ligand in photoinduced NO linkage isomers in a prototypical ruthenium nitrosyl complex.

Photoinduced linkage isomers (PLI) of the NO ligand in transition-metal nitrosyl compounds can be identified by vibrational spectroscopy due to the large shifts of the (NO) stretching vibration. We present a detailed experimental and theoretical study of the prototypical compound K[RuClNO], where (NO) shifts by ≈150 cm when going from the N-bound (κN) ground state (GS) to the oxygen-bound (κO) metastable linkage isomer MS1, and by ≈360 cm when going to the side-on (κN,O) metastable linkage isomer MS2. We show that the experimentally observed N-O stretching modes of the GS, MS1, and MS2 exhibit strong coupling with the Ru-N and Ru-O stretching modes, which can be decoupled using the local mode vibrational theory formalism. From the resulting decoupled pure two-atomic harmonic oscillators the local force constants are determined, which all follow the same quadratic behavior on the wavenumber. A Bader charge analysis shows that the total charge on the NO ligand is not correlated to the observed frequency shift of (NO).