DIFFRACTION STUDIES OF PHOTOEXCITED CRYSTALS - METASTABLE NITROSYL-LINKAGE ISOMERS OF SODIUM-NITROPRUSSIDE

The geometries of two known metastable states of sodium nitroprusside dihydrate, Na-2[Fe(CN)(5)NO]. 2H(2)O and that of the ground state have been analyzed by X-ray diffraction at 50 K, a temperature at which no decay of metastable-state concentration with time is observed. Data w ere collected on two laser-excited crystals containing populations of similar to 37% of metastable state I (MS(1)) and similar to 10% of met astable state II (MS(2)), respectively, using imaging plates and a rot ating anode source. For MS(1) the apparent geometry changes upon excit ation, determined earlier at 138 K (Pressprich, M. R.; White, M. A.; V ekhter, Y.; Coppens, P. J. Am. Chem. Sec. 1994, 116, 5233-5238), are c onfirmed. However, reinterpretation of the diffraction data points to a linear, oxygen-bound isonitrosyl geometry of this species with Fe-O and N-O distances of 1.715(5) and 1.140(7) Angstrom, respectively. The diffraction evidence points to a geometry for the MS(2) species in wh ich NO is sideways bound (eta(2)), with Fe-N, Fe-O, and N-O distances of 1.89(2), 2.07(2), and 1.14(2) Angstrom, respectively, compared with 1.6656(7) and 1.133(1) Angstrom for Fe-N and N-O in the ground state. The three geometries correspond to minima in the nondissociative end- over-end linkage isomerization pathway of a diatomic molecule proposed by Armour and Taube for N-2 (Armor, J. N.; Taube, H. J. Am. Chem. Sec . 1970, 92, 2560-2561).