Formation and composition of the precipitates of various metal 8-selenoquinoline complexes

The effect of acidity on the precipitation of various bivalent metal 8-selenoquinoline and 8-mercaptoquinoline complexes has been systematically studied and compared. The metal ions were Zn(2+), Cd(2+), Pb(2+), Mn2+, Ni(2+), Cu(2+) and Co(2+). Most of the metal ions, except copper(II) and cobalt(II), precipitate as a. 1:2 complex, metal :ligand. However, in hydrochloric acid solution cadmium precipitates as CdR(2).2HCl and lead as PbR(2).2HCl or PbR.Cl. Copper(II) is reduced to copper(I) and precipitates as CuR.RH at pH above 2.0 and as CuR in strongly acidic solution. Cobalt(II) does not give a precipitate of composition but appears to precipitate as a mixture of CoR(2).RH and fixed CoR(2) or as other complexes. The reasons for the formation of the various types of precipitate are considered.