Photosubstitution associative ligand

An associative ligand photosubstitution reaction is also a chemical quenching process if the excited reactant complex is... 

The second photochemical reaction which was studied was the reaction of CotCO NO with Lewis base ligands L (J 6 ). The observed solution phase photochemical reaction is carbonyl photosubstitution. This result initially did not appear to be related to the proposed excited state bending. Further reflection led to the idea that the bent molecule in the excited state is formally a 16 electron coordinatively unsaturated species which could readily undergo Lewis base ligand association. Thus, an associative mechanism would support the hypothesis. Detailed mechanistic studies were carried out. The quantum yield of the reaction is dependent on both the concentration of L and the type of L which was used, supporting an associative mechanism. Quantitative studies showed that plots of 1/ vs. 1/[L] Were linear supporting the mechanism where associative attack of L is followed by loss of either L or CO to produce the product. These studies support the hypothesis that the MNO bending causes a formal increase in the metal oxidation state. 

A Study of the photophysical and photochemical properties of alkylidyne complexes such as W(CPh)Cl(CO)2(tmeda) was reported by Bocarsly et al. (see also Section II,B) 24). Quenching experiments with aromatic hydrocarbons indicate that a significant amount of triplet character is associated with the emissive excited state. Bimolecular oxidative and reductive quenching is observed as well. Methylviologen dichloride is reduced and iV.A, A, A, -tetramethyl-p-phenylenediamine is oxidized by W(CPh)Q-(CO)2(tmeda) on irradiation with 488 nm light. Photosubstitution of two CO ligands in W(CPh)Cl(CO)(dppe) by dppe to give W(CPh)Cl(dppe)2 was observed as well. 

These reactions may be identified by the dependence of the quantum yield for the reaction on the concentration of entering ligand. An exemplary case, in which the photosubstitution of CO occurs partially by the associative mechanism outlined above, was studied by Keeton and Basolo (59) ... 

It is worthwhile to consider which primary photoprocesses might be involved in these mechanisms. The intermolecular photoinduced isomerizations, like photosubstitutions, may involve either dissociation or association of a ligand as a primary process (7) ... 

Croup VI Carbonyl complexes In the case of substitution of neutral ligands by neutral ligands, pressure effects can be better correlated with the intrinsic volume changes associated with the mechanism. One such study dealt with the photosubstitution reactions of the hexacarbonyls M(CO)6 (M = Cr, Mo, W) to give M(CO)sL (Eq. 6.22) and M(CO)4L2, where L is a ligand such as pyridine [61]. For each M, Oco decreased with increasing pressure. Under the risky assumption that kn is independent of P, the pressure dependence of laser flash photolysis techniques have shown that CO loss to form the 5-coordinate intermediate M(CO)s occurs in less than 1 ps. For this reason, one cannot treat the ligand substitution pathway from the reactive ES in terms of mechanisms elucidated for bound excited states. Instead the positive... 

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