Role of the H3O2 Bridging Ligand in Coordination Chemistry. 1. Structure of Hydroxoaquametal Ions

Michael Ardon, Avi Bino

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Abstract

The structures of two classical, so-called hydroxo aqua complex ions of chromium(III) and cobalt(III) were investigated by single-crystal X-ray diffraction. These structures were shown not to be mononuclear hydroxo aqua ions, with distinct OH and H2O ligands, but binuclear or polynuclear ions with symmetrical hydrogen oxide bridges (H3O2) between the metal atoms. Compound 1, cis-[Cr(bpy)2(H3O2)]2I4.2H2O, forms orthorhombic crystals, space group Pbcn, with a = 15.511 (2) Å, b = 15.115 (2) Å, c = 20.955 (3) Å, V = 4913 (1) Å3, and Z = 4. Compound 2, trans-[Co(en)2(H3O2)](ClO4)2, forms monoclinic crystals, space group P21/c, with a = 5.724 (1) Å, b = 8.146 (1) Å, c = 14.923 (2) Å, β = 99.17 (2)°, V= 687 (1) Å3, and Z = 2. The two structures were refined by full-matrix least-squares methods to residuals of R = 0.037, Rw= 0.042 and R = 0.026, RW= 0.039, respectively. In structure 1 there are two H3O2 units that are symmetry related with an O—O distance of 2.446 (5) Å. In structure 2 the cobalt atom and the central hydrogen atom of the H3O2 group reside on crystallographic inversion centers, thus forming infinite chains of — Co-H3O2-Co— with an O—O distance of 2.441 (2) Å. It is concluded that hydroxo aqua ions do not exist in the crystalline state as such: the so-called cis hydroxo aqua ions are dimers bridged by two H3O2 bridges and the so-called trans hydroxo aqua ions are polynuclear chains of metal atoms bridged by single H3O2 ligands. The mechanism of olation reactions, in the solid state and in aqueous solutions, is formulated as an elimination of water molecules from H3O2 bridges, accompanied by formation of OH bridges between the metal ions.

Original languageEnglish
Pages (from-to)1343-1347
Number of pages5
JournalInorganic Chemistry
Volume24
Issue number9
DOIs
StatePublished - 1985

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