Electron transfer catalyzed substitution in carbonyl complexes. VI: Highly variable rates of substitution by tetracyanoethylene

Abstract Rates of substitution of tetrahydrofuran (THF) or trimethyl phosphite by tetracyanoethylene (TCNE) in organometallic complexes (THF)W(CO) 5 , (THF)Cr(CO) 5 , [P(OMe) 3 ]Cr(CO) 2 (C 6 Me 6 ), (THF)Mn(CO) 2 (C 5 Me 5 ) and (THF)Mn(CO) 2 (C 5 H 4 Me) in THF solution, have been determined. As indicated by the calculated second-order rate constants k , the manganese complexes ( k > 1900 M −1 s −1 ) react by a factor of at least 10 4 more rapidly than the W(CO) 5 complex ( k = 0.043 M −1 s −1 ). Measurements of recombination kinetics of photodissociated (acpy)Mn(CO) 2 (C 5 Me 5 ) (acpy = 4-acetylpyridine) in THF show that the substitution by TCNE of THF in the corresponding solvent complex proceeds faster by a factor of 2 × 10 4 . Of the two chromium complexes, the pentacarbonyl/THF system has a value of k of 0.39 M −1 s −1 whereas the phosphite ligand in [P(OMe) 3 ]Cr(CO) 2 (C 6 Me 6 ) is substituted by TCNE with k = 1.59 M −1 s −1 . The results and their correlation with electrochemical data support a self-induced homogeneous electron transfer mechanism: it is proposed that electron transfer between the reaction partners TCNE and the organometallic precursor leads to substitutionally-labile 17 valence electron complexes as essential intermediates in the catalytic chain. Efficient oxidation of the precursors by the TCNE-substituted 17 VE species is possible because of intramolecular metal-to-TCNE electron transfer, especially in the σ-coordinated products with mixed carbonyl/carbocycle ligands. For Part V, see ref. 1.