Mechanisms of dirhenium decacarbonyl substitution reactions: crossover experiments with dirhenium-185 decacarbonyl and dirhenium-187 decacarbonyl

Abstract

/sup 185/Re/sub 2/(CO)/sub 10/ and /sup 187/Re/sub 2/(CO)/sub 10/ were prepared separately and then utilized in combination in crossover experiments to probe for fragmentation to mononuclear rhenium species in thermal and photochemically initiated substitution reactions. For the CO-Re/sub 2/(CO)/sub 10/ exchange reaction, a reaction separately analyzed for /sup 13/CO-/sup 12/CO interchange, no crossover was observed at 150/sup 0/C after 14 half-lives of reaction (14 h). Similarly, the thermal reaction sequences of Re/sub 2/(CO)/sub 10/ + P(C/sub 6/H/sub 5/)/sub 3/ reversible Re/sub 2/(CO)/sub 9/P(C/sub 6/H/sub 5/)/sub 3/ + CO and Re/sub 2/(CO)/sub 9/P(C/sub 6/H/sub 5/)/sub 3/ + P(C/sub 6/H/sub 5/)/sub 3/ reversible Re/sub 2/(CO)/sub 8/(P(C/sub 6/H/sub 5/)/sub 3/)/sub 2/ + CO were examined at 150/sup 0/C (maintaining a CO pressure of approx. 560 to 640 mm). No crossover was detectable in either Re/sub 2/(CO)/sub 10/ or Re/sub 2/(CO)/sub 9/P(C/sub 6/H/sub 5/)/sub 3/ (relative to blank experiments). Hence, phosphine substitution reactions proceed without a detectable formation of mononuclear rhenium species. These observations support a CO dissociative mechanism. A model based upon this mechanism can accurately reproduce the mass spectra observed during /sup 13/CO-/sup 12/CO interchange. In the absence of a CO atmosphere, /sup 185/Re/sub 2/(CO)/sub 10/ and /sup 187/Re/sub 2/(CO)/sub 10/ formedmore » /sup 185/Re/sup 187/Re(CO)/sub 10/, and this interchange was nearly complete at 150/sup 0/C within 14 to 16 half-lives. All photochemically initiated reactions with the two labeled decacarbonyls led to complete crossover within short reaction times. It appears then that the primary mode of reaction for Re/sub 2/(CO)/sub 10/ under photolysis conditions involves Re-Re bond scission as an early elementary step. Also, the reversible steps leading to the precursor(s) to Re/sub 2/(CO)/sub 10/ decomposition include scission of the rhenium-rhenium bond.« less