Rhenium is a dense, silvery-gray transition metal with one of the highest melting points and excellent high-temperature strength; it is used in superalloys and catalysts.
The electron configuration of rhenium is [Xe] 4f14 5d5 6s2. This half-filled 5d subshell contributes to its stability and unique chemical behavior among the transition metals.
Rhenium exhibits a wide range of oxidation states from -1 to +7, the widest among all transition metals. The most common oxidation states are +4, +6, and +7. In rhenium(VII) oxide (Re2O7) and perrhenic acid (HReO4), the element is in its highest oxidation state, +7.
Rhenium has an extremely high melting point (~3186 °C) and excellent resistance to creep and oxidation at elevated temperatures. Adding 3–6% rhenium to nickel-based superalloys significantly improves their high-temperature strength, making them ideal for jet engines and gas turbines.
Rhenium is relatively inert at room temperature but forms various oxides and halides at higher temperatures. For example, in air it oxidizes slowly to form rhenium(VII) oxide:
\(\mathrm{4\,Re(s) + 7\,O_2(g) \rightarrow 2\,Re_2O_7(s)}\)
This oxide can dissolve in water to yield perrhenic acid (HReO4).
Key applications include:
Rhenium forms several oxides such as ReO2, ReO3, and Re2O7. The heptoxide (Re2O7) is particularly important because it forms perrhenic acid (HReO4) when dissolved in water and serves as a precursor to many rhenium compounds used in catalysis.
The high melting point of rhenium (~3186 °C) arises from its strong metallic bonding, which involves overlap of 5d orbitals and high cohesive energy. Only tungsten and carbon (in graphite form) have comparable or higher melting points.
Rhenium is a rare element and is not found freely in nature. It occurs as a trace impurity in molybdenite (MoS2) and other copper sulfide ores. It is mainly obtained as a by-product from molybdenum and copper refining.
Rhenium and its compounds are considered to have low toxicity. However, fine powders and volatile compounds like Re2O7 should be handled carefully as they may irritate the skin, eyes, or respiratory tract. Standard safety measures are recommended during handling.
When rhenium reacts with chlorine gas at elevated temperatures, it forms rhenium(V) chloride:
\(\mathrm{2\,Re(s) + 5\,Cl_2(g) \rightarrow 2\,ReCl_5(s)}\)
ReCl5 is a dark-colored compound used as a precursor in rhenium chemistry and vapor deposition processes.