Disodium[hydroxotetranitronitrosyl]ruthenate(II), a photochromic substancewith long-lived metastable states: Structure at 110 K, spectroscopic and thermal properties

Disodiumlhydroxotetranitronitrosyl]ruthenate (II) is a photochromic compoun d excitable with blue-green light which exhibits at least one unusually lon g-lived metastable state at low temperature. At 298 K, the compound crystal lises in the space group C2/m. A reversible phase transition occurs at 240 K upon cooling, as detected by Differential Scanning Calorimetry and X-ray powder diffraction which causes a lowering of the crystal symmetry to the s pace group P2(1)/n. Synchrotron X-ray single crystal diffraction and FTIR s pectroscopy data obtained on the ground and the excited states of the title compound low temperature phase are presented.