Transition metal complexes computational studies

The question as to whether a transition metal complex of type 4 is best described as an alkene 7T-complex 4A or as a metallacyclopropane 4B, which is of practical importance, has been addressed in several computational studies on the relationship between alkene 7T-complexes and three-membered rings [48—52]. It has been concluded that the titanium complexes of type 4 are best represented as titanacydopropanes, i.e. by resonance structure 4B, if one is willing to accept the notion that 4A and 4B are limiting resonance forms [52],... 

Reaction rates are macroscopic averages of the number of microscopical molecules that pass from the reactant to the product valley in the potential hypersurface. An estimation of this rate can be obtained from the energy of the highest point in the reaction path, the transition state. This approach will however fail when the reaction proceeds without an enthalpic barrier or when there are many low frequency modes. The study of these cases will require the analysis of the trajectory of the molecule on the potential hypersurface. This idea constitutes the basis of molecular dynamics (MD) [96]. Molecular dynamics were traditionally too computationally demanding for transition metal complexes, but things seem now to be changing with the use of the Car-Parrinello (CP) method [97]. This approach has in fact been already succesfully applied to the study of the catalyzed polymerization of olefins [98]. 

With the advent of appropriate DFT-based methods, NMR properties of transition-metal compounds have now become amenable to theoretical computations (8). Suitable density functionals have been identified (9) which permit calculations of transition-metal chemical shifts with reasonable accuracy, typically within a few percent of the respective shift ranges. Thus, it is now possible to investigate possible NMR/reactivity correlations for transition-metal complexes from first principles several such studies have already been undertaken (10,11,12). 

Rene Fournier is studying atomic clusters238 and transition metal complexes.239 He is using a combination of density functional methods, tight-binding models, and molecular simulations with empirical interaction potentials, as part of a research program designed to study materials by computations on simple model systems. 

To end this section, it is worthwhile mentioning the recent TDDFT study by Autschbach et al [107] on the electronic and circular dichroism (CD) spectra of several chiral Werner complexes, since it represents the first application of TDDFT to the computation of the circular dichroism (CD) spectra of transition metal complexes. The absorption and CD spectra of the... 

Abstract The applications of hybrid DFT/molecular mechanics (DFT/MM) methods to the study of reactions catalyzed by transition metal complexes are reviewed. Special attention is given to the processes that have been studied in more detail, such as olefin polymerization, rhodium hydrogenation of alkenes, osmium dihydroxylation of alkenes and hydroformylation by rhodium catalysts. DFT/MM methods are shown, by comparison with experiment and with full quantum mechanics calculations, to allow a reasonably accurate computational study of experimentally relevant problems which otherwise would be out of reach for theoretical chemistry. 

Recently, the borylene complex [p-B(NMe2) (r 5-C5H5)Mn(CO)2 2] (1) was subject to detailed computational studies and the theoretically predicted and the experimentally derived structural parameters were found to be in very good agreement (Table I).117 Density functional theoretical studies have concluded that borylenes BX can be viable ligands in the design of transition metal complexes, which are thermodynamically stable with... 

Several systematic experimental and computational studies have compared the sigma-donating abilities of NHCs and tertiary phosphines for a variety of transition-metal complexes [8-17]. As illustrative examples, analyses of the nickel-carbonyl complex 1 and iridium carbonyl complex 2 (Fig. 1) re-... 

The determination of the charge distribution in a molecule, needed here for the latter term, (Ges), has been a considerable problem in force field calculations, especially for transition metal compounds (see Sections 3.2.6 and 3.3.6). Most promising but not yet fully tested for transition metal complexes are semi-empiri-cal quantum-mechanical methods[ 103,1041. Future studies might show whether a combination of approximate methods for the computation of charge distributions and solvation will lead to a reliable approximation of solvation parameters of coordination compounds. 

---