The perrhenate ion is the anion with the formula ReO−
4, or a compound containing this ion. The perrhenate anion is tetrahedral, being similar in size and shape to perchlorate and the valence isoelectronic permanganate. The perrhenate anion is stable over a broad pH range and can be precipitated from solutions with the use of organic cations. At normal pH, perrhenate exists as metaperrhenate (ReO−
4), but at high pH mesoperrhenate (ReO3−
5) forms. Perrhenate, like its conjugate acid perrhenic acid, features rhenium in the oxidation state of +7 with a d0 configuration. Solid perrhenate salts takes on the color of the cation.
Contents
Typical perrhenate salts are the alkali metal derivatives and ammonium perrhenate. These salts are prepared by oxidation of rhenium compounds with nitric acid followed by neutralization of the resulting perrhenic acid.
Basicity of the anion
In inorganic chemistry, perrhenate anion is also used as a weakly coordinating anion. It is a weaker base than Cl−
or Br−
but stronger than ClO−
4 or BF−
4. Unlike the related permanganate, perrhenate is nonoxidising. Silver perrhenate reacts with trimethylsilyl chloride to give the silyl "ester" (CH3)3SiOReO3.
Reactions of perrhenates
The perrhenate ion reacts with the cyanide to give trans-[ReO2(CN)4]3−. With sulfide sources such as hydrogen sulfide, one obtains black ReS2 and Re2S7. These species form via the intermediacy of ReO3S−. Heating ammonium perrhenate gives Re2O7 and then ReO2.
The chemistry of the perrhenate ion is similar to that of the pertechnetate ion TcO−
4. For this reason, perrhenate is sometimes used as a carrier for trace levels of pertechnetate, for instance in nuclear medicine scanning procedures.