Synthesis and Structure of the Zinc(II) and Cadmium(II) Complexes Produced in the Photoluminescent Enhancement of Ninhydrin Developed Fingerprints Using Group 12 Metal Salts

Abstract

Abstract The complexation reactions involving Ruhemann's purple (RP) and either zinc(II) or cadmium(II), which are made use of in the enhancement of latent fingerprints that have undergone preliminary development with ninhydrin, have been investigated under a variety of conditions. Contrary to earlier reports it is now clear that metal complexes with either a 1:1 or 1:2 metal:ligand ratio can be formed under certain conditions. Polar solvents, which cause extensive ionization of the metal salt, facilitate the formation of the 1:2 complex whereas less polar solvents allow the formation of 1:1 complexes through retention of a covalently bound anion in the primary coordination sphere. Thus, Zn(RP)2 can be prepared from seemingly any zinc(II) salt and NaRP in water or methanol. It is a highly insoluble and kinetically stable red powder with fluorescence emission bands at 590 and 640 nm. The structure of an orange 1:1 Zn:RP complex, produced as crystals from ZnCl2 and NaRP in a 5:1 chloroform:methanol mixture, has been solved by X-ray diffraction and shown to correspond to a formula of NaZn2(RP)2Cl3CH3OH. This material is very rapidly decomposed into its components by water. The previously reported 1:1 Cd:RP complexes, which are red, form under a much wider range of conditions than their zinc(II) counterparts, however, if cadmium(II) perchlorate is combined with RP in water or methanol Cd(RP)2 precipitates instead. This is a golden-brown powder showing a strong fluorescence.

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