Photochemistry of acetylacetonato-, trifluoroacetylacetonato-, and hexafluoroacetylacetonato-dicarbonyl-rhodium and -iridium complexes in frozen gas matrices at 12 K. Infrared spectroscopic evidence for carbon monoxide dissociation and chelate ring-opening processes

Abstract

Infrared spectroscopic evidence, including ¹³CO labelling and energy-factored CO force-field fitting, is presented to show that on photolysis of [M(CO)₂L](L = RCOCHCOR′; R,R′= CH₃ or CF₃; M = Rh or Ir) complexes at high dilution in CH₄, Ar, CO, and N₂ matrices at 12 K new species [M(CO)L], [M(CO)₂(L*)](L*= unidentate form of L), and [M(CO)(N₂)L] are produced. The results are discussed in relation to the mechanisms of thermal reactions in solution. In particular, the reversibility of the reaction [M(CO)₂L]⇌[M(CO) L]+ CO and the dominance of this reaction indicates that the hitherto discounted dissociative pathway should be reconsidered. Some evidence is also found for the chelate ring opening to give [ M(CO)₂(L*)] species, but no evidence was found for expanded-co-ordination-number species corresponding to the previously proposed associative mechanism for ligand-exchange and substitution reactions.