Silver halides grain precipitation

Again, the precise roles of coordination-compound chemical sensitizers, in most cases, are not understood. In fact, their effects may have little to do with their own coordination chemistry. Many simple salts of gold and other noble metals are effective sensitizers. They also may be added to solutions during silver halide precipitation to produce doped emulsions that have special properties. A variety of compounds that can act as ligands to metal ions are also effective alone as chemical sensitizers, the result of complicated oxidation-reduction, ion replacement and adsorption reactions on the silver halide grain surface. These include polyamines, phosphines and thioether- or thiol-containing compounds. The chemistry of these materials with the silver halide surface is discussed in the reference literature. 

Two grains (O.OOS mole) of (-),g9-bis(l,2-elfaanediamine)oxalatocobalt(III) iodide is suspended in 30 mL of water at 35°, 0.81 g (0.005 mole) of silver acetate is added, and the mixture is stirred for five minutes at 35°. The mixture is then filtered and the precipitated silver halide is washed with 5 mL of warm water (50°). Separately, 2.4 g (0.01 mole) of sodium cis.cis-diammine-carbonato-dicyanocobaltate(lll) dihydrate is dissolved in 10 mL of water. The solution is added to the filtrate and combined with the washings. After removal of the precipitated material, the mixed solution is kept in a refrigerator, whereupon an orange diastereoisomer of the (-I- )su form of the caifoonato complex precipitates. The product is collected on a filter and washed with cold water. Recrystallization is repeated from water until the CD peak at 24,100 cm reaches a constant value. The crystals are collected on a filter, washed with 50% cold, aqueous methanol, ethanol, and diethyl ether, and finally dried under vacuum. The yield is 0.3 g. Anal. Calcd. for [Co(ox)(en)2][Co(CN)2(C03)(NH3)2]-2H20 C, 21.44 H, 5.16 N, 22.05. Found C, 21.31 H, 5.25 N, 21.78. 

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