Dissociative substitution mechanisms

Ideally, the measurement of obs as a function of Xe concentration would have provided strong evidence for the type of mechanism, since for a dissociative mechanism, obs l/[Xe], whereas an associative mechanism would have no dependence on [Xe], However, it is not possible to vary the concentration of Xe in liquid Xe, and so pre-exponential factors A obtained from Arrhenius plots were used to differentiate the two mechanisms. The value of logA was found to lie within the expected range for a unimolecular dissociation reaction, and it was concluded that this reaction occurs by a dissociative substitution mechanism in liquid Xe. The Arrhenius plot therefore gave an estimate of the W—Xe BDE, AHw—Xe = 35.1 0.8 kJ mol . ... 

The situation is considerably different for the iron(III) analog (TPPS)Fe "(H20)2 which is hexacoordinate in aqueous solution. In this case the reaction with NO is considerably slower (kon = 4.5 x 10 s ) and kojr (500 s ) is sufficiently large to be measurable by the flash photolysis method as well [92]. Temperature and hydrostatic pressure effects were probed and AH, AS and AV values of 69 + 3 kj mol , 95 10 J mol K and +9+1 cm mol were determined for the "orf reaction and 76 + 6 kJ mol , 60 + 11 ) mol and 18 + 2 cm mol respectively, for the off reaction [93j. The activation parameters for the on reaction compare very favorably with those measured for exchange between coordinated and solvent water for aqueous solutions of (TPPS)Fe "(H20)2 [94] and indicate that kinetics for the reaction of NO with this complex are dominated by the lability of the coordinated water. Furthermore, the large and positive values of AVj and ASj point to a dissociative substitution mechanism as described in Eq. (6.43). 

Ligand substitutions at 18-electron, coordinatively saturated complexes typically occur by dissociative substitution mechanisms. Because this reaction mechanism begins with bond cleavage, these reactions are often slower than the associative substitutions of 16-electron complexes. For example, the 16-electron complex Rh(acac)(ethylene)2 reacts with ethylene (1 atm) by an associative mechanism with a rate constant of 10 s" at 25 but the related 18-electron complex CpRh(ethylene)2 reacts by dissociative substitution with a rate constant that is about 10 times slower (about 4 X 10" ° s at 25... 

Dissociative substitution mechanism (Section 4.3) cr-Acceptor and rr-donor metal orbitals (see Section 1.4)... 

Dissociative substitution mechanism (Section 4.4) Dewar-Chatt model of M(C=C) bonding involving weak back donation (Section 5.1) Electron configuration (Section 1.4) a-Acceptor and ir-donor metal orbitals (see Section 1.4)... 

More recently, Grubbs et al. obtained a refined mechanistic picture of the initiating step by conducting a 31P NMR spectroscopic study of the phosphine exchange in precatalysts 12-A. These investigations revealed that substitution of the phosphine proceeds via a dissociative-associative mechanism, i.e., a 14-electron species 12-B is involved that coordinates the alkene to give a 16-electron species 12-C (Scheme 12) [26a]. Increased initiation rates are observed if the substituents R and the phosphine ligands PR3 in precatalysts... 

Chloride substitution kinetics of [NiniL(H20)2]3+, and its protonated form [NiniL(H20)(H30)]4+, where L = 14 -oxa-1,4,8,11 -tetraazabicy-clo[9.5.3]nonadecane, yield fyn20)2 = 1400 M 1s 1 and (h2o)(H3o+) = 142M 1s V The reverse, chloride dissociation, reactions have (h2o)ci = 2.7 s 1 (h3o+)ci = 0.22 s All four reactions occur through dissociative interchange mechanisms, like earlier-studied substitutions at nickel(III) (359). 

The large and positive Avalues and, particularly the large and positive AV values obtained (21) for kon and represent signatures for a substitution mechanism dominated by ligand dissociation, for the ferri-heme complexes, i.e.,... 

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