Theoretical studies of inorganic and organometallic reaction mechanisms. 19. Substitution reaction in cyclopentadienyl metal dicarbonyls Academic Article uri icon

abstract

  • Density functional calculations (B3LYP) of the associative reaction of (5-C5H4X)M(CO)2 with PR3 (X = H, Me, Cl; M = Co, Rh, Ir) show that transition-state (TS) effects dominate the reaction rate. Steric factors are less important than electronic ones, unless both the substituents on the cyclopentadienyl (Cp) and the entering ligand are sizable enough to hinder the reaction. Both electron-withdrawing substituents on the Cp ring and electron-donating substituents on the entering ligand facilitate CO substitution. In the associative mechanism, the Cl substituent on Cp lowers the barrier by 3-5 kcal/mol, while the Me substituents on PR3 lower the barrier by 7-10 kcal/mol. Generally, the ring slipping/folding occurs by lifting two (or three) Cp carbons away from metal by an average value of 0.54 , and the two most distant carbons take on more double-bond (C=C) character. For the first time this study identifies the origin of ring slippage and quantifies the degree of ring distortion at the TS by both the M-C bond length changes (ring slip) and C-C-C ring plane angle (ring fold). The calculations show that the Cp ring in the TS has an average folding angle of 12 and slips toward a coordination mode between 3 and 2. A single ring substituent takes one particular position with respect to the entering and leaving ligand. Although these group 9 systems clearly react by an associative mechanism, group 7 systems, such as CpM(CO)3 (M = Mn, Tc, Re), undergo substitution reactions by a dissociative mechanism, with significantly higher barriers. The difference in substitution reaction mechanisms for these two 18 e- systems (CpM(CO)3 group 7 and CpM(CO)2 group 9) arises from several factors: (1) the group 7 systems are already six-coordinate and an associative transition state will be more crowded; (2) the group 9 systems are five-coordinate d8 structures, which can access a low-energy four-coordinate structure with an orbital available for the attacking ligand.

published proceedings

  • ORGANOMETALLICS

author list (cited authors)

  • Fan, H. J., & Hall, M. B.

citation count

  • 21

complete list of authors

  • Fan, HJ||Hall, MB

publication date

  • December 2001