Novel inorganic hydrogels based on the polymerization of cyanometalate transition metal complexes with [PdCl₄]^2-: A new approach to ceramic and alloy precursors

The reaction of a wide variety of cyanometalate complexes of the general form [M(CN)_x]^n- (where M = a transition metal ion) with square planar [PdCl₄]^2- in aqueous solution leads to the formation of linear polymers. Polymerization occurs via substitution of chloride ligands on the Pd(II) centers, by the nitrogen end of the cyanide ligand to generate extended bridging cyanide structures. Upon generation at room temperature polymer solutions of this type under go a sol-gel transition to generate robust hydrogels having water content in excess of 95%. in the case of the cyanocobaltate/tetrachloropalladate gel, pyrolysis at 900 °C produces ferromagnetic Pd/Co metallic alloys having novel morphological character. Materials formed with a hydrogel having a 2:1 Pd to Co stoichiometryare found to be `sponge-like'. When placed in water, the metallic matrix swells becoming pliable and holding up to seven equivalents of water per metal site. The conductivity and magnetic properties of this material are maintained in the swollen state. Sintering of the Pd/Co hydrogel in air generates the layered oxide, PbCoO₂ having a delafossite structure.