Transition metal-ligand bond dissociation

The determination of the heat capacity of a substance as a function of the temperature is by itself a very important application of DSC, because it may lead to values of the thermodynamic functions S%, //-" — //q, and Gy, mentioned in chapter 2. An example is the study of C6o carried out by Wunderlich and co-workers [271], The application of DSC in the area of molecular thermochemistry has been particularly important to investigate trends in transition metal-ligand bond dissociation enthalpies. The typical approach used in these studies, and its limitations, can be illustrated through the analysis of the reaction 12.27, carried out by Mortimer and co-workers [272] ... 

The development of comprehensive models for transition metal carbonyl photochemistry requires that three types of data be obtained. First, information on the dynamics of the photochemical event is needed. Which reactant electronic states are involved What is the role of radiationless transitions Second, what are the primary photoproducts Are they stable with respect to unimolecular decay Can the unsaturated species produced by photolysis be spectroscopically characterized in the absence of solvent Finally, we require thermochemical and kinetic data i.e. metal-ligand bond dissociation energies and association rate constants. We describe below how such data is being obtained in our laboratory. 

This monograph contains enthalpies of formation, heat capacities, entropies, and metal-ligand bond dissociation enthalpies of organometallic compounds of transition and main group elements. 

Ng, F. T. T., Rempel, G. L., and Halpem, J., 1982, Ligand effects on transition-metal alkyl bond-dissociation energies, J. Am. Chem. Soc. 104 621n623. 

Squires, R. R. Gas-Phase Transition Metal Negative Ion Chemistry, Chem. Rev. 1987, 87, 623-646. Tables with electron affinities, proton affinities, and metal-ligand bond dissociation enthalpies. 

Energy-minimized conformations from molecular mechanics are useful to estimate the energetics of metal-ligand bond dissociation. A plausible thermal dissociation reaction for generic zero-valent metal carbonyl complexes that corresponds to the onset of the glass transition is... 

Transition metal centered bond activation reactions for obvious reasons require metal complexes ML, with an electron count below 18 ("electronic unsaturation") and with at least one open coordination site. Reactive 16-electron intermediates are often formed in situ by some form of (thermal, photochemical, electrochemical, etc.) ligand dissociation process, allowing a potential substrate to enter the coordination sphere and to become subject to a metal mediated transformation. The term "bond activation" as often here simply refers to an oxidative addition of a C-X bond to the metal atom as displayed for I and 2 in Scheme 1. 

The Met+—L bond dissociation energies were measured for some Zn-containing gas phase cations using different methods ". Armentrout reviewed guided ion beam studies of transition metal-ligand thermochemistry and reported the following... 

Cdn(Por) and HgII(Por), obtained by metallation of porphyrins with metal acetate in pyridine,17" resemble the Zn analogues in coordination behaviour. They are easily dissociated from the macro-cyclic ligand, and form five-coordinate complexes with N-donor ligands.18 In this group, the metal—ligand bonds, both cis and axial, become weaker as the metal becomes heavier (Zn > Cd > Hg) in contrast to other transition metal series but similar to other main group series (equation 23). 

Whereas homolytic ligand dissociation is not commonly observed for inorganic complexes, it has been identified as an important process in organometallic chemistry where it is favored by the characteristic weakness of transition metal-alkyl tr-bonds. Recent determinations yield metal alkyl band dissociation energies for CH3— Mn(CO)s (ca. 120 kJ/mol) and for several alkylcobalt complexes (ca. 80-100 kJ/mol) . Homolytic dissociation of such complexes results in the formation of free radicals and in the opening up of free radical catalytic pathways, e.g., for hydrogenation". Important biochemical examples of free radical catalytic mechanisms, initiated by the homolytic dissociation of a transition metal-carbon bond (i.e., the 5 -deoxyadenosy 1-cobalt bond of coenzyme 8,2) are provided by the coenzyme B,2-promoted rearrangements (see Section... 

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