HYDROGEN-BONDING SYSTEM IN BARIUM NITROPRUSSIDE 6.5-HYDRATE

The hydrogen-bond system in barium nitroprusside 6.5-hydrate, [Ba-2(H2 O)(10)][Fe(CN)(5)NO](2)3H(2)O], has been determined by neutron diffrac tion on monocrystals. Results show the compound to be orthorhombic, sp ace group Cmc2(1) (36), a = 16.008(43), b = 11.550(3), c = 16.648(5) A ngstrom, V = 3078(3) Angstrom(3), Z = 4. Refinement of the structure, using 973 observed structure factors, converged to the final R(W) fact or of 0.058. The 2 independent barium atoms, separated 4.60 Angstrom, share a plane of three water molecules forming dimeric tetravalent uni ts. The nitroprusside anions deviate from the C-4v ideal symmetry, but this deviation is less than that observed in other nitroprussides. Th e 10 crystallographically independent water molecules have been classi fied according to their coordination. Analysis of the H-bond strength, together with a comparison of the packing of the two known barium nit roprusside hydrates (3-hydrate and 6.5-hydrate), suggests that the wat er molecules labeled as W(1), W(7), W(8), and W(9) could be lost durin g the partial dehydration of 6.5-hydrate into 3-hydrate. (C) 1995 Acad emic Press, Inc.