Citrate ligand

Coprecipitation of the metals is usually achieved from an aqueous solution of nitrates upon addition of anions such as carbonates, citrates, or oxalates (10)(24-27). First reports in this field have underlined the necessity to neutralize the pH of the solution in order to obtain complete precipitation of barium or strontium. Also, oxalate or citrate ligands may bind to two different cations. This should allow a better mixing at a microscopic level. However, care should be taken since some cations such as Y or La may precipitate as double salt complexes with alkaline ions that have been added to the solution as hydroxides in order to control the pH (24). 

A bridging citrate ligand has been suggested for a dimeric complex of Mo in which the tigand metal ratio is 1 2 and for heterodinuclear complexes of Cu"-Ni", Ni"-Zn" and Cu"-Zn". An elegant ESR study of Cu" complexation by citrate ions in aqueous solutions also shows the formation of dimeric citrate-bridged species in the pH range 7-11. 

Bidentate citrate ligand has been found in the crystal structure of )J-[triethylenetetramine-citratoCo " ] SHjO. The citrate ion forms a five-member chelate ring throu the hydroxyl group and the central carboxyl group, a coordination mode which has also been proposed for Al " solution species. 

The groups of Song and Zhang et al. have used a combination of a sol-gel method and arrested precipitation to prepare nanocrystalline Y203-based upconverters [15,16]. They first prepared a gel from yttrium and lanthanide nitrates and ammonia hydroxide that was arrested by citrate ligands. Then, they calcinated the dried gel at 800°C and obtained upconverting nanocrystalline material. It has to be stated that methods that involve annealing/calcination always result in nanocrystalline material rather than colloidally stable nanoparticles. 

Panagiotidis P, Kefalas ET, Raptopoulou CP, Terzis A, Mavromoustakos T, Salifoglou A (2008) Delving into the complex picture of Ti(IV)-citrate speciation in aqueous media synthetic, structural and electrochemical considerations in mononuclear Ti(IV) complexes containing variable deprotonated citrate ligands. Inorg Chim Acta 361 2210-2224... 

Rees TF, Daniel SR (1984) Complexation of neptunium(V) by salicylate, phthalate and citrate ligands in a pH 7.5 phosphate buffered system. Polyhedron 3 667-673... 

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