Low temperature infrared spectra of photoexcited (η-cyclopentadienyl)nitrosylnickel, [Ni(C5H5)NO]

A new metastable excited state (metastable state I, MSI) was detected when (η-cyclopentadienyl)nitrosylnickel, Ni(C5H5)NO, was irradiated at 77 K with 488.0 and 514.5 nm Ar+-laser lines. The infrared spectra of metastable states I (MSI) and metastable state II (MSII) are characterized by down-shifts of the ν(NO) bands (260 cm−1 for MSI, and 460 cm−1 for MSII). Upon heating the irradiated samples, MSII bands vanished at a lower temperature than that occuring for the MSI bands. Irradiation with the 514.5 nm line generated half of the ν(NO) bands produced after illumination with the 488.0 nm line and upon further irradiation with the 632.8 nm HeNe laser line the ν(NO)MSI band completely disappeared. Results are compared with recent X-ray diffraction and DFT studies on excited Cp*NiNO and with infrared behavior of metastable excited states of [Fe(CN)5NO]2− and [Ru(NO2)4(OH)NO]2−.