Metal-ligand bond breaking

By analogy with thermal processes (92), photoinduced isomerization s may occur either by intramolecular or intermolecular mechanisms. Intramolecular isomerizations are further classified either as "twisting" mechanisms, which involve no metal-ligand bond breaking, or as bond-rupture mechanisms. The latter term applies to reactions of chelate compounds in which one of the ligands is for a short time partially dissociated. Intermolecular mechanisms involve species other than those that make up the reactant complex. 

The kinetics of racemization of the [Co(ox)3] anion in the presence of perchloric, hydrochloric, or sulphuric acid are consistent with an Al mechanism. Some features of this acid catalysis do not correspond with metal-ion-catalysed racemization of this complex, which may mean that different mechanisms are involved. An obvious difference would be between an intramolecular twist involving no metal-ligand bond breaking on the one hand, and a mechanism with a unidentate oxalato-complex as intermediate on the other. ... 

The removal of coordinated water molecules — deaquation — is more complicated than simple dehydration and involves metal-ligand bond breaking and generation of a vacant coordination site. Coordination of an anion, for example, Eqs. 12.7-12.8 8... 

Borabenzene metal complexes, like their cyclopentadienyl counterparts, are not readily amenable to breaking of the metal-ligand bond. Such bond breaking will occur more easily in complexes with antibonding electrons. By fortuitous chance the cobaltocene route to borabenzene chemistry (Section V,A) provided 19-e complexes with the useful property of inherently weakened metal-ligand bonds. Thus most of this section centers on the reactivity of Co(C5H5BMe)2 (7) and Co(CsH5BPh)2 (13). 

Two important exceptions to the generalization that LMCT transitions occur at high energy in organometallic complexes occur in complexes with non-dative (covalent) metal-ligand bonds, and in complexes with high oxidation-state metal centers. As an example of the former, complexes with M-alkyl bonds oftentimes have photochemi-cally accessible cr (M-alkyl) — metal CT states. Depopulation of the cr (M-alkyl) orbital will weaken and break the metal-alkyl bond. (This type of transition corresponds to transition 3 in Figure 1.) An example of this type of photoreactivity is provided by coenzyme B12 and its numerous model complexes (Equation (8)). 

The equilibrium lies to the right, and the rate constants determined, in C6D6, by H NMR spectroscopy are those for the regeneration of the catalytically active species. Exchange between the methyl groups of free Al(CH3)3 and those in the heterobimetallic species was slow on the NMR timescale. The rate constants (presumably obtained at an unstated room temperature) were independent of the value of [Al(CH3)3] and decreased in the order Ti > Zr > Hf, which was reasonable if the dissociation involves breaking a metal-ligand bond. Kinetics experiments conducted with substrates in which Cp had been replaced by ethenebis(indenyl) (EBI) or ethenebis(tetrahydroindenyl) indicated that the zirconocene methyl cation will be a superior carboalumination catalyst. 

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