Rhenium complexes thiocyanates

A common spectrophotometric method for the determination of rhenium involves extraction of the thiocyanate complex into isoamyl alcohol. The a-furildioxime method has similar sensitivity, the rhenium complex being measured either in the aqueous medium or in the CHCI3 extract. Methods based on ion-pairs of ReOT with basic dyes are the most sensitive. 

IR and X-ray structural analyses [64] established the N-bonding of the NCS ligand in both complexes. The rate constant Zc(Tc) for the reaction of trani-[TcO(OH2)(CN)4] with thiocyanate at 25 °C was found to be 22.2 mole s" while /t(Re), evaluated under the same conditions, was only 35 10" mole s [62,65]. The rate constant Z (Tc) turns out to be over 6000 times higher than fc(Re) of the rhenium complex. 

Re—O—Re linkage.The complexing of rhenium(iv) with thiourea (L) has been studied in HCl solutions in the presence of tin(ii) by a cation-exchange method, and equilibrium stability constants Pj—P4 for [ReOL4] -t- nCl" [ReOCl L4 -I- nh determined.The complexing of rhenium(iv) with thiocyanate ions in HCl solutions in the presence of SnClj has also been investigated and the formation of a 1 1 Re -SCN" complex confirmed. ° ... 

The irradiation of perrhenate solutions, followed by the addition of thiocyanate has been shown to produce rhenium(iv) thiocyanato-complexes.671... 

The red-orange colour formed when SnCh is added to an HCl solution containing rhenium(VlI) and thiocyanate is the basis of the spectrophotometric method for rhenium [5,22,23]. The reaction is complex, and the colour obtained depends on the concentration of the reducing agent and thiocyanate, and on the reaction time. 

The molar absorptivity of the rhenium-thiocyanate complex in isoamyl alcohol is 3.8T0" (a = 0.21) at 430 nm. The absorption spectrum is shown in Fig. 40.1. The following species interfere in the determination of rhenium by the thiocyanate method Mo, W, V, and Cr, as well as oxidizing and reducing agents. 

Fig. 40.1. Absorption spectra of the rhenium thiocyanate complex in isoamyi aicohol (1) and the rhenium a-furildioxime compiex in aqueous acetone (2)...

In addition to a-furildioxime, other oximes are used as spectrophotometric reagents for rhenium, such as dimethylglyoxime [7,48,49], a-pyridyldioxime [50], a-benzoinoxime [51], methyl-2-pyridylketoxime [52], and phenyl-2-pyridylketoxime [53]. ot-Benzoyldioxime gives with Re, in thiocyanate medium, a complex (A-max = 430 nm) extractable into isoamyl acetate [54]. In each case the colour reaction occurs in the presence of SnCh. 

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