Both rhodium trichloride
and tribromide are easily carbonylated
and subsequently reduced by refluxing formic acid-hydrohalic acid mixture to
give almost quantitative yields of the rhodium(1) halocarbonyl anions [Rh(CO)2X2]- (X = Cl,Br).
It has been shown that the
reaction proceeds by the intermediate formation of the rhodium(111)
halocarbonyl anions [Rh(CO)X5]2-
followed by slow reduction to the rhodium(1) complexes. Evaporation of the
respective solutions leads to almost quantitative recovery of the new rhodium(111)
halocarbonyls Rh(CO)X3 and the well known
rhodium(1) compounds [Rh(C0)2X]2.
Iododicarbonylrhodium(1) could not be isolated by this method and in fact
the only products which could be isolated were triiodocarbonylrhodium(111),
Rh(CO)I3, and its corresponding anion;
this shows that although the carbonylation reaction
had occurred the subsequent reduction did not proceed. In solution the rhodium(1)
complexes [Rh(C0)2X2]- oxidize
to give [Rh(CO)X5]2-.
Caesium salts of these rhodium(111) anions are readily isolated, but addition
of caesium salts to the rhodium(1) solutions did not give the expected Cs[Rh(CO)2X2] but instead the octahedral
complexes Cs2[Rh(C0)2(H2O)X3].
All of these changes in
composition and oxidation state have been followed in solution, as well as in
the isolated solid products, by infrared spectroscopy.