Rhenium thiocyanates

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)...

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... 

Rhenium can be separated from many metals by precipitation with H2S in 5-6 M HCl (or 2.5-3 M H2SO4). Arsenic(in) is a suitable collector since it does not interfere in the subsequent determination of rhenium by the thiocyanate method. 

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. 

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. 

Thiocyanate, nitrate, and fluoride interfere in this method for determining rhenium. Palladium and copper in acid media sparingly soluble a-furildioximates. Normally, rhenium should be separated from Mo before the determination. 

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. 

The thiocyanate method has been used for determining rhenium in dusts and in molybdenite [51], eopper ores and eoncentrates [28,71,72], molybdenum, tungsten, and vanadium [73], and coal [16]. 

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. 

Rhenium heptaselenide, hydrogenations with — 15, 57 Rhenium heptoxide, hydrogenations with — 15, 83 Rhodanides s. Thiocyanates Rhodanines... 

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