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Encyclopedia of Physical Science and Technology EN010b-481 July 14, 2001 18:45

476 Noble Metals (Chemistry)

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Rhodium(II), d , is present in dinuclear metal–metal Rhodium(V), d , derivatives are known as the fluoride
bonded complexes of the type [Rh 2 (CH 3 COO) 8 ] · H 2 O, and chloride salts and the complex Cs[RhF 6 ].
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with bridging acetate groups. Rhodium(VI), d , appears in RhF 6 and K 2 [RhO 4 ].
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Rhodium(III), d , is the most common oxidation state.
The water soluble hydrate of RhCl 3 and the hexaquo com- 7. Osmium
plex [Rh(H 2 O) 6 ] 2 (SO 4 ) 3 are valuable starting materials
for other rhodium compounds. The most common lig- The chemistry of osmium and ruthenium is similar but,
ands are the halides (except iodide), cyanide, thiocyanide, except for a few areas, their chemistry does not parallel
amines and other nitrogen donors, nitrate, sulfate, car- that of iron. Figure 6 illustrates some of the chemistry of
boxylate, and polydentate donors such as ethylenedi- ruthenium, which in general also applies to osmium. The
aminetetraacetic acid (EDTA). Few compounds contain- osmium derivatives are usually more stable than the ruthe-
ing phosphine and arsine ligands are known. Rhodium(III) nium analogs, and osmium has more stable higher oxida-
has an extensive organometallic chemistry involving car- tion states. Osmium is found in the (−II)–(VIII) states
bonyl, thiocarbonyl, alkyl, aryl, π-allyl, cyclopentadienyl, with (III), (IV), and (VI) as the most common, showing a
olefin, and diene ligands. The alkyls and aryls are often preference for six-coordination.
prepared by oxidative addition to rhodium(I) compounds. The metal will react readily with oxygen, chlorine,
The rhodium(III) olefin complexes are unstable and very or fluorine to yield the respective products. The metal
few are known. will also react slowly with nitric acid to give OsO 4 .
Rhodium(−I) is found in complexes with π-acceptor Osmium(0) compounds involve several carbonyls, as well
ligands to stabilize the low oxidation state. Examples of asphosphine,amine,arene,dienecomplexes,andthewell-
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this state are [Rh(CO) 2 (PPh 3 ) 2 ] , and [Rh(PF 3 ) 4 ] . studied osmium clusters. The compounds are usually pre-
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Rhodium(II), d , is present in dinuclear carboxylato pared by reducing an osmium salt in the presence of the
complexeswithfourbridgingligandsandRh–Rhbonding. appropriate ligand and, if necessary, a halide acceptor such
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Rhodium(IV), d , is known in several fluoride, chlo- as copper, silver, or zinc salts.
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ride, and oxide compounds. Rhodium(IV) can be obtained Osmium(III), d , compounds consist of the halide salts,
either by oxidation of rhodium(III) with ozone, sodium except fluoride, and complexes involving halides, amines,
bimuthate, ceric sulfate, or by anodic oxidation, or by re- and acetylacetonate, as well as some mixed ligand species
duction of rhodium(VI). involving phosphines and arsines. These complexes are

FIGURE 6 Representative ruthenium chemistry.