Hexadecacarbonylhexarhodium is a metal carbonyl cluster with the formula Rh<sub>6</sub>(CO)<sub>16</sub>. It exists as purple-brown crystals that are slightly soluble in dichloromethane and chloroform. It is the principal binary carbonyl of rhodium.
Rh<sub>6</sub>(CO)<sub>16</sub> was first prepared by Walter Hieber in 1943 by carbonylation of RhCl<sub>3</sub>÷3H<sub>2</sub>O at 80–230 ðC and 200 atm carbon monoxide with silver or copper as a halide acceptor. Hieber correctly formulated the compound as a binary carbonyl, but suggested the formula Rh<sub>4</sub>(CO)<sub>11</sub>, i.e., CO/Rh ratio of 2.75. The correct formula and structure was subsequently established by Dahl et al. using X-ray crystallography. The correct CO/Rh ratio is 2.66.
Relative to the original preparation, the carbonylation of a mixture of anhydrous rhodium trichloride and iron pentacarbonyl was shown to give good yields of Rh<sub>6</sub>(CO)<sub>16</sub>. Other compounds of rhodium are also effective precursors such as [(CO)<sub>2</sub>Rh(ü-Cl)]<sub>2</sub> and rhodium(II) acetate:
It also arises quantitatively by thermal decomposition of tetrarhodium dodecacarbonyl in boiling hexane:
At least some of the CO ligands can be displaced by donor ligands.
Rh<sub>6</sub>(CO)<sub>16</sub> catalyzes a number of organic reactions including hydrogenation and hydroformylation.