Secondary metal dithizonates

Some metals, notably copper, silver, gold, mercury, bismuth, and palladium, form a second complex (which we may term secondary dithizonates) at a higher pH range or with a deficiency of the reagent ... 

The reactions of silver(I) with 1,5-diphenyl thiocarbazone (56 dithizone = HaDz) by comparison to thiosemicarbazones have been thoroughly investigated.442,444 Dithizone has for many years been used as a colorimetric reagent for trace metal ions. Its metal complexes are of two types, the so-called primary and secondary dithizonates. Primary dithizonates are generally formed at low pHs where the ligand becomes mono depronated (pA = 4.5) but retains the NH proton. Secondary dithizonates are formed in the presence of an excess of metal and/or at higher pH values where the ligand becomes fully deprotonated. Since it has been estimated that for free dithizone pK2 > 15, the second proton obviously becomes labilized in the presence of metal ions. 

A primary or secondary dithizonate is obtained, depending on whether the dithizonate reacts as the anion of the monobasic acid (HDz ) or the dibasic acid (Dz ). Secondary dithizonates are formed by only a few metals. They are represented by such formulae as AgaDz and CuDz. Generally, acidic media and excess of dithizone favour the formation of primary dithizonates, whereas in alkaline media, and with insufficient amounts of the reagent, secondary dithizonates are formed. 

In the opinion of some authors [87,88] secondary dithizonates do not exist. In alkaline or acidic media, in the presence of excess of metal ions, mixed-ligand complexes are formed, e.g. HgCl(HDz) or Cu(OH)(HDz), where HDz is the dithizone anion.These authors negate the possibility of existence of Dz anion. 

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