Direct Evidence for O−H···NC Hydrogen Bonding in a Cyanide-Bridged Lanthanide−Transition-Metal Complex (H₂O)₆(DMF)₆Ho₂[Ni(CN)₄]₃:  An Eight-Coordinated Holmium Center with a Bicapped (Square Face) Trigonal Prismatic Geometry

Abstract

The lanthanide−transition-metal complex (H 2 O) 6 (DMF) 6 Ho 2 [Ni(CN) 4 ] 3, 1, was prepared from the reaction of Ho(NO 3 ) 3 ·6.5H 2 O with K 2 [Ni(CN) 4 ] at a 2:3 molar ratio in DMF (DMF = N, N -dimethylformamide). This compound is a model compound for studying the effects of H 2 O ligands on the coordination geometry of lanthanide centers and the tendency to form oligomers rather than extended arrays with cyanide bridges. The presence of multiple hydrate water molecules appears to inhibit the formation of an extended array complex containing cyanide bridges but favors intermolecular hydrogen bonds between oligomers. The Ho centers are trans-bridged through a [Ni(CN) 4 ] 2 - unit. Each Ho center also forms an isocyanide linkage with a terminal [Ni(CN) 4 ] 2 - unit. Holmium is eight-coordinated. It is bound to three DMF molecules and three H 2 O molecules in addition to two [Ni(CN) 4 ] 2 - units. The coordination geometry of Ho is a distorted bicapped (square face) trigonal prism. The X-ray crystal structure and IR spectra support the presence of strong-to-moderate O−H···NC hydrogen bonds. Crystal data for 1: monoclinic space group P 2 1 / n, a = 8.907(1) Å, b = 25.579(1) Å, c = 11.761(1) Å, β = 105.138(1)°, Z = 2.