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Chemical properties

Rhodium belongs to group 9 of the periodic table, but exhibits an atypical ground state valence electron configuration for that group. Like neighboring elements niobium (41), ruthenium (44), and palladium (46), it only has one electron in its outermost s orbital.

The common oxidation state of rhodium is +3, but oxidation states from 0 to +7 are also observed.

Unlike ruthenium and osmium, rhodium forms no volatile oxygen compounds. The known stable oxides include Rh O, RhO, RhO · x H O, Na RhO, Sr LiRhO and Sr NaRhO. Halogen compounds are known in nearly the full range of possible oxidation states. Rhodium(III) chloride, rhodium trifluoride, rhodium pentafluoride and rhodium hexafluoride are examples. The lower oxidation states are stable only in the presence of ligands.

The best-known rhodium-halogen compound is the Wilkinson's catalyst chlorotris(triphenylphosphine)rhodium(I). This catalyst is used in the hydroformylation or hydrogenation of alkenes.