Rhenium perrhenic acid

Salts of perrhenic acid may be obtained in acid—base reactions, and may include the tetrahedral ReO anion or the octahedral anion ReO , eg, in Ba (ReOg)2 [13598-09-9], Ammonium perrhenate and perrhenic acid, as well as rhenium metal, are sold by the primary suppHers of this element. 

World production of rhenium in 1993 and 1994 was estimated to be 34 and 35 metric tons, respectively. The price of rhenium metal in 1995 varied in the range 825— 1600/kg. Price is based on form, purity, and quantity. The price for the most commonly used salt, NH ReO, was 550— 770/kg. Dirhenium decacarbonyl and perrhenic acid are also commercially available. 

Knapp, F. F., Jr. Lisic, E. C. Mirzadeh, S. Callahan, A. P. Tungsten-188/carrier-free rhenium-188 perrhenic acid generator system. World Pat. Appl. 219541, 1992. 

Rhenium is attacked by neither hydrochloric acid nor by cold sulfuric or hydrofluoric acid. However, oxidizing acids, such as nitric acid or hot sulfuric acid, vigorously react with the metal forming perrhenic acid, HRe04. The metal is oxidized by hydrogen peroxide in ammoniacal solution forming ammonium perrhenate, NH4Re04. 

Salts of the perrhenate anion (including perrhenic acid) are not only the single most important group of starting materials for the synthesis of other rhenium compounds but are also of importance in catalysis and other commercial processes. Although they are formally coordination complexes of Revn and they appear quite extensively in the chemical literature (e.g. see Chemical Abstracts), much of the interest in them is outside the confines of the topic of coordination chemistry as it is commonly accepted. Accordingly, an effort has been made here to keep the coverage of their chemistry to a minimum. 

Complexes of rhenium (V) have generally been prepared by careful reduction of perrhenic acid or perrhenates or by oxidation of rhenium(IV) compounds. - These procedures involve some difficulties, as in the former case compounds of rhenium (IV) may be produced, and in the latter, complete oxidation to perrhenate may occur. 

The complexes described here, which are very easy to prepare in a pure state, are useful starting materials for the preparation of most compounds of rhenium (V) and should be considered with perrhenic acid, potassium hexahalorhe-nate(IV) salts, rhenium(III) chloride, rhenium(V) chloride, and dirhenium decacarbonyl as basic source materials in rhenium chemistry. The procedure is a modification of that originally used by Freni and Valenti, although the nature of the compounds obtained was clarified only by subsequent studies. - ... 

A solution of perrhenic acid may be prepared by any of the following methods. (1) Three grams of rhenium metal... 

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. 

Rhenium is commonly separated as the heptasulfide and isolated as potassiiun perrhenate, a substance which is sparingly soluble in water. The ammonium salt, which is desirable for the preparation of pure rhenium " and rhenium compounds, is less readily obtained than is the potassium salt because of the pronounced solubility of ammonium perrhenate in water. Ammonium perrhenate can be obtained readily by neutralizing a solution of perrhenic acid with ammonia. 

Two methods for the preparation of rhenium (II I) iodide are described below. In one procedure (A), perrhenic acid, obtained by reaction of 30% hydrogen peroxide on powdered rhenium metal, is reduced directly to the triiodide by the action of concentrated hydriodic acid and ethanol at elevated temperatures. The second procedure (B) utilizes the controlled thermal decomposition of rhenium(IV) iodide in an atmosphere of iodine. The tetraiodide is prepared by reduction of perrhenic acid with concentrated aqueous hydriodic acid, followed by dehydration of the initial product. 

Rhenium(VII) oxide is a canary-yellow solid, forming as hexagonal crystals. It begins to sublime at 250° under 1 atmosphere of pressure and melts at 297° in a closed tube. It is very soluble in water and in such basic solvents as ethers, esters, alcohols, dioxane, and pyridine. It is extremely deliquescent, absorbing water to form perrhenic acid (HRe04). The product may be analyzed using the tetraphenylarsonium chloride procedure. ... 

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