Colorless perrhenates

Rhenium metal powder is oxidized quantitatively to perrhenic acid by dissolution in 6 M nitric acid followed by repeated evaporation with concentrated nitric acid nearly to dryness. On dilution with 25 ml. of water, neutralization with 6 M sodium hydroxide, and the addition of slightly more than the stoichiometric amount of silver nitrate dissolved in as little water as possible, silver perrhe-nate precipitates in the form of colorless crystals. These additions are made in a beaker protected from light. The solution is decanted from the product, which is dried in an oven at 110° for 8 hours. 

Trimethylsilyl perrhenate forms colorless crystals, which are very sensitive to hydrolysis and which must be handled in a dry atmosphere. Under anhydrous conditions, the compound is stable to light and oxidation. Contaminated products undergo slow decomposition at room temperature, blue reduction products of Re(VII) being formed. In an excess of water trimethylsilyl perrhenate is hydrolyzed quickly and quantitatively. The perrhenic acid formed in this reaction together with trimethylsilanol can be determined by acidimetric titration or precipitation as nitron perrhenate. Trimethylsilyl perrhenate is soluble without decomposition in all common anhydrous and aprotic solvents (benzene, cyclohexane, ethyl ether, tetrahydro-furan, chloroform, etc.). 

Rhenium. The element rhenium, atomic number 75, was fliscovered by the Gennan chemists Walter Noddack and Ida Tacke in 1925. The principal compound of rhenium is potassium perrhenate, KReO. a colorless substance. In other compounds all oxidation numbers from -f-7 to —1 are represented examples are RCgOj, ReOg, ReClg, Re02, Re20g, Re(OH)2. 

A tube 40 cm. in length and 20 mm. i.d. is fitted with a stopcock. The tube is charged with 50 g. of 50-100-mesh Dowex 50 W-Xl cation-exchange resin above a 1-cm. layer of Pyrex wool. The resin is washed with 20 ml. of 6 AT hydrochloric acid and rinsed with pure hot (100°) water until the effluent is colorless and does not give any turbidity with silver nitrate solution. (About 200 ml. of water is required.) A 10.0-g. sample (0.0356 mol) of finely ground potassium perrhenate is dissolved in 120 ml. of pure hot (100°) water. The hot solution is added to the resin in four 30-ml. portions and the column is then rinsed with pure water (boiling, if undissolved potassium perrhenate is observed). The effluent is collected at the rate of 15 to 20 ml. per minute. The initial neutral portion of the effluent (ca. 50 ml.) is discarded and the fraction which is acid to litmus (ca. 130 ml.) is collected in a 250-ml. beaker. 

The product can be recrystallized from hot hexamethyldisiloxane under nitrogen. Colorless crystals, Mp 79.5—80.5°C. The compound sublimes in a vacuum at 65—80°C/1.0 Torr. The transparent crystals are monoclinic, space group C2/c. The trimethyl-silyl group is attached to a single oxygen atom of the perrhenate group, with distances Si-0 and Re—O at 1.69(8) and 1.67(8) A, respectively, and an angle Si-0—Re at 164°. 

Trimethylgermanyl perrhenate is also a colorless crystalline solid, mp 108°C, sublimes at 115—125°C/1.5 Torr (25). Trimethyl-stannyl perrhenate is a colorless crystalline powder, mp 213°C (decomposition) (28). Their preparation follows similar protocols. 

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