Diffraction Studies of Photoexcited Crystals: Metastable Nitrosyl-Linkage Isomers of Sodium Nitroprusside

Abstract

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 were collected on two laser-excited crystals containing populations of ∼37% of metastable state I (MS 1 ) and ∼10% of metastable state II (MS 2 ), respectively, using imaging plates and a rotating anode source. For MS 1 the apparent geometry changes upon excitation, determined earlier at 138 K (Pressprich, M. R.; White, M. A.; Vekhter, Y.; Coppens, P. J . Am . Chem . Soc . 1994, 116, 5233−5238), are confirmed. 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) Å, respectively. The diffraction evidence points to a geometry for the MS 2 species in which NO is sideways bound (η 2 ), with Fe−N, Fe−O, and N−O distances of 1.89(2), 2.07(2), and 1.14(2) Å, respectively, compared with 1.6656(7) and 1.133(1) Å 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 . Soc . 1970, 92, 2560−2561).