Induced ligand substitution

In addition to the type of electron transfer reaction, shown in Equations 6.142-6.145, there are examples where pure MLCT excited states induce ligand substitutions by associative or dissociative mechanisms. A well-established example of a MLCT excited state-mediated ligand labilization reaction is shown in Equation 6.149.136... 

In solution, at room temperature Ru(bpy) does not undergo a photo-induced ligand substitution over a wide pH range, whereas at 95°C complete lab Llization occurs in a few hours. In acidic solution Cr(bpy)3 is photo-inert. At room temperature at neutral or alkaline pH, a bidentate complex is formed rapidly... 

Whereas thermally induced ligand substitution reactions with unreactive substrates such as Fe(CO)5 require forcing conditions and tend to give low yields and mixtures of products, the addition of catalysts such as CoCl2 gives single substitution products, Fe(CO)4(PR3), in high yield.8... 

Molecular motion based on the electrochemical-induced ligand substitution reaction has been reported in a copper(I) rotaxane (Figure 37). This rotaxane is composed of a 2,9-diphenyl-l,10-phenanthroline-containing cyclic poly ether ring, a tetraphenylmethane-stoppered molecular string that contains another 2,9-disubstitnted phenanthroline chelate... 

I propose to develop and apply such methods, based on ultrafast X-ray absorption spectroscopy, to study the ultrafast molecular motions of organometallics in solutions. In particular, initial studies will focus on photo-induced ligand dissociation and substitution reactions of transition metal carbonyls and related compounds in various solvent systems. 

Electron Transfer-induced and Electron Transfer-catalyzed Ligand Substitution... 

Figaro 59 Ligand substitution pathways for [Cu(f-butylsalim)2] (a) water induced and (b) ligand induced987... 

The most important observation in the pre-steady-state kinetics of the CN system is that after a short lag (100 msec) there is a phase (lasting about 3 sec) where the evolution of H2 is linear and only after these 3 sec does CN reduction occur. This long lag prior to CN reduction would correspond to 18 to 20 electron transfer steps from the Fe protein. More realistically this delay probably involves a CN -induced modification of the enzyme, such as a ligand substitution reaction (this modified state of the enzyme is designated as. E in Figure 21). However, this modification step is too slow to be part of the steady-state cycle for CN reduction. Also, it cannot be a slow activation of the enzyme prior to turnover, since the onset of H2 evolution is the same in both the presence and the absence of CN . Steady-state observations indicate that cyanide binds to a more oxidized form of the MoFe protein than binds N2, but that state cannot be defined because of the long lag phase. 

Co-C bond weakening and homolysis is induced by interaction of the coenzyme with the enzyme (e.g., through frans-axial ligand substitution or conformational distortion of the corrin ring). 

N-donor induced disproportionation of [Fe(CO)3(PR3)2]+ (R = Me, Bu, Cy, Ph) as well as halide induced disproportionation of [M(CO)3(PCy3)2]+ (M = Fe, Ru, Os) has been interpreted in terms of nucleophilic attack being rate determining.103 104 The rate data led to the conclusion that the reactivity of these 17-electron complexes is only weakly dependent on the metal, and the suggestion was made that periodic trends in 17-electron systems are generally attenuated in comparison to those for 18-electron analogues. However, it was noted previously that W > Cr by ca. 106 1 for substitution in [CpM(CO)3]. A direct comparison of the rate of associative ligand substitution at a 17-electron center as a function of the metal for a complete triad (Cr, Mo, W) was reported for the reaction in Eq. (20).14... 

Excess phosphine does not induce further substitution of CO in IV, except when the allyl moiety consists of a cyclohexenyl group. However, bidentate ligands, e.g., diphosphines - , diarsines and mixed phosphine-arsines displace two CO molecules ... 

---