Carbonyl halides of the Group VII transition metals. I. Halocarbonyls and halocarbonyl anions of rhenium(I)

The rhenium(1) halocarbonyls Re(CO)5X (X = Cl, Br, I) may be readily prepared by refluxing a solution of the free hexahalorhenate(1V) acid in hydrohalic acid with formic acid. Visible absorption spectral studies indicate that formation of the halocarbonyls takes place following an initial reduction to rhenium(111), but no halocarbonyls of this intermediate oxidation state were isolated. The concentration of rhenium, as well as the relative amounts of hydrohalic and formic acids in the reaction mixture, are important factors for efficient production of the halocarbonyls. If the halocarbonyls are refluxed in formic acid containing a decreased proportion of hydrohalic acid, decarbonylation occurs giving the halocarbonyl anions [Re(CO)4X]2- which can be isolated as their caesium salts or as the parent halocarbonyls [Re(CO)4X]2. Continued refluxing of the formic acid solution causes still further decarbonylation giving the halocarbonyl anions [Re(CO)3X3]2- (X = Cl, Br) which may also be isolated as their caesium salts. Alternatively, evaporation to dryness of these solutions without addition of cations gives the new rhenium(1) halocarbonyl aquo complexes Re(CO)3(H2O)2X (X = Cl, Br). In the case of the iodo system, decarbonylation did not proceed to the tricarbonyl stage.