Hydrophilic and lipophilic coordinated ligands

Feigl has suggested that systematic modification of the solubilities of coordination compounds may be achieved by alteration of the hydrophilic-lipophilic balance (HLB) of the molecuks. Hydrophilic-lipophilic balance is a specific relationship between polar and nonpolar groups in a compound. By increasing the lipophilicity of a molecule, decreased solubility in water and increased solubility in organic solvents usually results. Similarly, increasing the hydrophilicity of a molecule usually results in decreased solubility in organic solvents and increased solubility in water 

Functionalization of the bipyridyl ligand with lipophilic hydrocarbon groups causes the complex to become virtually insoluble in water and highly soluble in chloroform. 

The salts, [PPN]n[M(CN)6] (M = Fe, n = 3 or 4 M = Ru, n = 4 M = Co, n = 3) and [PPN][Fe(CN)5(NO)], have been prepared by metathetical exchange reactions in aqueous solution between [PPN]C1 and the corresponding alkali metal salts of the cyanometallates7 Unlike alkali metal salts, the [PPN]+ salts are very soluble in chlorinated solvents and in most polar, aprotic solvents including acetonitrile, acetone, and dimethyl sulfoxide. 

The introduction of substituents R (R = NH2, OH, CO2H) which are capable of forming hydrogen bonds results in increased solubilities for these nitrosonaphtholato (quinonemonoximato) complexes in water or ethanol. 

Several approaches to the evaluation of the hydrophilic-lipophilic properties of chemical compounds are known from the literature. For example, in order to evaluate the HLB of surfactants, Davies developed a system based on the analysis of group numbers. The group number characterizes the contribution of each specific functional group to the energy that would be required if a solvent molecule were changed from water to an organic solvent. 

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