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Group 9, by modern IUPAC numbering, is a group (column) of chemical elements in the d-block of the periodic table. Members of Group 9 include cobalt (Co), rhodium (Rh), iridium (Ir) and meitnerium (Mt). These elements are among the rarest of the transition metals, and as of 2025 rhodium and iridium are the only non-radioactive metals with higher prices per weight than gold.
Like other groups, the members of this family show patterns in electron configuration, especially in the outermost shells, resulting in trends in chemical behavior; however, rhodium deviates from the pattern.
History
"Group 9" is the modern standard designation for this group, adopted by the IUPAC in 1990.
Swedish chemist Georg Brandt (1694–1768) is credited with discovering cobalt <abbr>c.</abbr> 1735, showing it to be a previously unknown element, distinct from bismuth and other traditional metals. Brandt called it a new "semi-metal". He showed that compounds of cobalt metal were the source of the blue color in glass, which previously had been attributed to the bismuth found with cobalt. Cobalt became the first metal to be discovered since the pre-historical period. All other known metals (iron, copper, silver, gold, zinc, mercury, tin, lead and bismuth) had no recorded discoverers.
Rhodium
thumb|upright|left|[[William Hyde Wollaston]]
Rhodium was discovered in 1803 by William Hyde Wollaston, soon after he discovered palladium. He used crude platinum ore presumably obtained from South America. His procedure dissolved the ore in aqua regia and neutralized the acid with sodium hydroxide (NaOH). He then precipitated the platinum as ammonium chloroplatinate by adding ammonium chloride (). Most other metals like copper, lead, palladium, and rhodium were precipitated with zinc. Diluted nitric acid dissolved all but palladium and rhodium. Of these, palladium dissolved in aqua regia but rhodium did not, and the rhodium was precipitated by the addition of sodium chloride as . After being washed with ethanol, the rose-red precipitate was reacted with zinc, which displaced the rhodium in the ionic compound and thereby released the rhodium as free metal.
Iridium
Chemists who studied platinum dissolved it in aqua regia (a mixture of hydrochloric and nitric acids) to create soluble salts. They always observed a small amount of a dark, insoluble residue. In 1803, British scientist Smithson Tennant (1761–1815) analyzed the insoluble residue and concluded that it must contain a new metal. Vauquelin treated the powder alternately with alkali and acids and obtained a volatile new oxide, which he believed to be of this new metal—which he named ptene, from the Greek word ptēnós, "winged". Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.
Meitnerium
Meitnerium was first synthesized on August 29, 1982, by a German research team led by Peter Armbruster and Gottfried Münzenberg at the Institute for Heavy Ion Research (Gesellschaft für Schwerionenforschung) in Darmstadt. The team bombarded a target of bismuth-209 with accelerated nuclei of iron-58 and detected a single atom of the isotope meitnerium-266:
: + → +
This work was confirmed three years later at the Joint Institute for Nuclear Research at Dubna (then in the Soviet Union). Bacteria in the stomachs of ruminant animals convert cobalt salts into vitamin B, a compound which can only be produced by bacteria or archaea. A minimal presence of cobalt in soils therefore markedly improves the health of grazing animals, and an uptake of 0.20 mg/kg a day is recommended, because they have no other source of vitamin B.
Proteins based on cobalamin use corrin to hold the cobalt. Coenzyme B<sub>12</sub> features a reactive C-Co bond that participates in the reactions. In humans, B<sub>12</sub> has two types of alkyl ligand: methyl and adenosyl. MeB<sub>12</sub> promotes methyl (−CH<sub>3</sub>) group transfers. The adenosyl version of B<sub>12</sub> catalyzes rearrangements in which a hydrogen atom is directly transferred between two adjacent atoms with concomitant exchange of the second substituent, X, which may be a carbon atom with substituents, an oxygen atom of an alcohol, or an amine. Methylmalonyl coenzyme A mutase (MUT) converts MMl-CoA to Su-CoA, an important step in the extraction of energy from proteins and fats.
See also
- Iron group
- Platinum group