Rhodium

Rhodium sits in the periodic table with the symbol Rh and the atomic number 45. This transition metal is very rare, hard, and resistant to corrosion, with a dark silvery-white appearance. It qualifies as a noble metal and belongs to the platinum group. Only one naturally occurring isotope exists for rhodium: 103Rh. In nature, it typically appears as a free metal or alloyed with similar metals; it is seldom found as a chemical compound, though it does occur in minerals like bowieite and rhodplumsite. Among precious metals, it ranks as one of the rarest and most valuable. It falls into group 9 (the cobalt group) and is used to enhance jewelry.

Rhodium is obtained from platinum or nickel ores alongside other platinum group metals. William Hyde Wollaston discovered it in 1803 while working with one such ore, naming it after the rose color of one of its chlorine compounds.

The biggest use of rhodium—consuming roughly 80% of global production—is as a catalyst in three-way catalytic converters for automobiles. Because it resists corrosion and most aggressive chemicals, and because it is so scarce, rhodium is usually alloyed with platinum or palladium for coatings that withstand high temperatures and corrosion. White gold often gets a thin rhodium plating to improve its look, and sterling silver is frequently rhodium-plated to prevent tarnishing.

In nuclear reactors, rhodium detectors measure neutron flux levels. Other applications include asymmetric hydrogenation to create drug precursors and processes for making acetic acid.

**History**

The name rhodium comes from the Greek word *rhodon*, meaning 'rose.' Wollaston discovered it in 1803, shortly after he found palladium. He started with crude platinum ore, likely from South America. His method involved dissolving the ore in aqua regia, then adding ammonium chloride to precipitate platinum as ammonium chloroplatinate. Adding zinc caused a mixture of copper, lead, palladium, and rhodium to precipitate. Dilute nitric acid dissolved the copper and lead. After dissolving the remaining material in aqua regia again and adding sodium chloride, the solution was evaporated to yield rose-red Na₃[RhCl₆]·nH₂O. Extracting with hot ethanol and then adding zinc displaced the rhodium from the ionic compound, leaving free rhodium metal.

For decades, this rare element saw only minor uses. By the turn of the century, thermocouples containing rhodium measured temperatures up to 1800 °C, showing exceptional stability between 1300 and 1800 °C.

The first major application was electroplating for decoration and corrosion-resistant coatings. Demand for rhodium surged in 1976 when Volvo introduced the three-way catalytic converter. Earlier catalytic converters used platinum or palladium, but the three-way design used rhodium to cut down NOx emissions.

**Characteristics**

Rhodium is hard, silvery, durable, and highly reflective. It does not normally form an oxide, even when heated. Oxygen is absorbed from the air only at rhodium's melting point and is released when it solidifies. Compared to platinum, rhodium has a higher melting point and a lower density. Most acids do not attack it: it is completely insoluble in nitric acid and dissolves only slightly in aqua regia—a property used to separate it from platinum ore.

Rhodium sits in group 9 of the periodic table, but its ground state valence electron configuration is unusual for that group, with just one electron in its outermost s orbital. The same pattern appears in niobium (41) and ruthenium (44).

**Chemical properties**

Common oxidation states for rhodium are +3 and +1. Complexes with oxidation states 0, +2, and +4 are also well known. A few compounds reach higher oxidation states, such as rhodium pentafluoride (a tetramer with the formula Rh₄F₂₀) and rhodium hexafluoride.

Three rhodium oxides exist: Rh₂O₃ (a paramagnetic black powder), RhO₂ (black when anhydrous, green as a hydrate), and RhO₃ (stable only as a gas). A rhodium sulfide, Rh₁₇S₁₅, occurs naturally as the rare mineral miassite. Synthetic RhₓSᵧ compounds serve as catalysts in applications like H₂-Br₂ fuel cells.

All Rh(III) halides are known. Many rhodium complexes start from RhCl₃·3H₂O, the hydrated trichloride, though anhydrous RhCl₃ is largely inert. Other rhodium(III) chlorides include sodium hexachlororhodate (Na₃RhCl₆) and pentaamminechlororhodium dichloride ([Rh(NH₃)₅Cl]Cl₂). These are used in recycling and purifying this very expensive metal. Heating a methanolic solution of hydrated rhodium trichloride with sodium acetate yields blue-green rhodium(II) acetate, Rh₂(O₂CCH₃)₄, which contains a Rh–Rh bond. This complex and related rhodium(II) trifluoroacetate have drawn attention as catalysts for cyclopropanation reactions.

When heated with triphenylphosphine in ethanol, hydrated rhodium trichloride converts to RhCl(P(C₆H₅)₃)₃. This square planar complex, known as Wilkinson's catalyst, is a common early example of a well-defined homogeneous catalyst for hydrogenating alkenes.

**Organorhodium compounds**

Rhodium is known for many organometallic derivatives. Rhodium(I) complexes are widely used as catalysts, including BINAP-Rh(I), DIPAMP-Rh(I), and BDPP-Rh(I).

Cyclopentadienyl complexes of rhodium have been studied as analogues of ferrocene. The parent compound is rhodocene, which participates in an unusual monomer-dimer equilibrium. Related cyclopentadienyl compounds include the Rh(I) and Rh(III) half-sandwich complexes (C₅H₅)Rh(CO)₂ and pentamethylcyclopentadienyl rhodium dichloride dimer (C₅(CH₃)₅RhCl₂)₂. The latter is prepared by reacting rhodium trichloride trihydrate with pentamethylcyclopentadiene in hot methanol.