The perrhenate ion is ReO4− and a perrhenate is a compound containing this ion. Salts of perrhenic acid are known as perrhenates. The perrhenate anion is tetrahedral, as shown by IR and Raman spectroscopy. As in the parent acid rhenium has an oxidation state of +7 with a d0 configuration.[4]
The perrhenate anion is stable over a broad pH range and can be precipitated from solutions with the use of organic cations. At high pH meso ReO53− forms.[1]
The perrhenate anion is also used as a weakly coordinating anion. It is a weaker base than Cl− or Br− but stronger than ClO4− or BF4−.[4] Solid perrhenate salts takes on the color of the cation.[2] Of the alkali metals salts, KReO4 is common and is the basis of the recovery of rhenium compounds from natural ores and laboratory residues.[6] Structurally the behavior of ReO4− resembles that of perchlorate, ClO4−: perrhenate and perchlorate salts are often isomorphous.
Typical perrhenate salt formation using alkali earth metals. HReO4 + KCl (solid) → KReO4 + HCl [4]
Silver and other metals can also be used:[5] AgReO4 + (CH3)3SiCl → (CH3)3SiOReO3 + AgCl
The perrhenate ion will also react with the cyanide ion. ReO4− + KCN + N2H5OH → K3[ReO2(CN)4]
Different sulfur containing reactions include the formation of thio-perrhenates with the perrhenate ion ReO4− + H2S → ReO3S− + H2O. [7]
Using one of the most common salts, potassium perrhenate to produce a very useful rhenium compounds that is very stable and allows for numerous further reactions.[6]
Perrhenate is analogous to permanganate but it has little oxidizing power.