Photogeneration of Nitrosyl Linkage Isomers in Octahedrally Coordinated Platinum Complexes in the Red Spectral Range

Octahedrally coordinated platinum nitrosyl complexes [Pt(NH₃)₄(NO₃)(NO)](NO₃)₂ (1) and [Pt(NH₃)₄(SO₄)(NO)](HSO₄)(CH₃CN) (2) undergo linkage isomerization at temperatures below 130 K when excited with red light. Irradiation in the spectral range of 570−800 nm results in an inversion of the NO ligand from a Pt-NO to a Pt-ON configuration. The metastable state Pt-ON can be reverted back to the ground state (GS) Pt-NO by irradiation with blue-green or infrared light or by heating above 130 K. The characteristic shift of the ν(NO) stretching vibration from 1744 to 1815 cm⁻¹ in 1 and from 1714 to 1814 cm⁻¹ in 2 allowed the unambiguous identification of the respective nitrosyl isomers. Up to 26% of the complexes of 1 and 20% of 2 may be photochemically excited toward the metastable state (MS). Using X-ray crystallography and DFT calculations, it is shown that the Pt-NO in these {MNO}⁸ complexes exhibits a bent arrangement with a Pt−N−O angle in the range of 117−120°. As a consequence and in contrast to the known {MNO}⁶ complexes only one metastable linkage isomer Pt-ON with a correspondingly bent Pt−O−N arrangement is formed, as evidenced by spectroscopy and DFT calculations. The calculated partial density of states shows that the charge transfer transition Pt(5d) → π^★(NO) is responsible for the formation of the metastable state.