Organometallic theoretical modeling

It should now be possible to study interactions between many other interesting surfaces and adsorbates. In particular, it will be important to try to model the surface interactions of organometallic dyes like N3. There are, however, many factors which are yet to be accounted for, including solvent and thermal effects, as well as defects and surface states. With increasing computer power, improved theoretical models and better computational schemes, quantum chemistry has good prospects of becoming a valuable tool in the field of photoelectrochemistry. 

Computations in the gas-phase Theoretical modeling is typically performed by organometallic chemists using the framework of pervasive density functional theory (DFT). The vast majority of these quantum chemical calculations are traditionally performed in the gas-phase (vacuum) and quite frequently by using simplified molecular models. The most typical motivation for this is "to reduce computational time."... 

In this section, some representative case studies are discussed with the aim of illustrating the capabilities of the theoretical models sketched in the preceding sections for a reliable description of the structure, spectroscopy, and thermodynamics of small-to-larger organometalic systems, isolated in the gas phase and embedded in more complex environments. 

When we transfer the theoretical aspects of these models to or nometallic chemistry, we are aware of the fact that, e.g. when altering the number of rr-electrons by two (67T-, 4ir- or 2Tr-electrons) in the elementary steps of organometallic reactions, we have, step-by-step to await a change in the type of process (S or A) (in analogy to Fig. 1 in Scheme 2.1-3). On the other hand, we also can learn that the energies of the FMO s change systematically, going from 6n- to 4ir- to 27r-electron stems (from e.g. bis-ir-allyl- to rr-allyl-a-allyl to bis-o-allyl-ntetal-complexes). 

The first section summarizes simply the essential features of the different types of molecular orbital calculations currently used to solve theoretical problems in organometallic chemistry. A critical comparison of these calculations is also given. The second section discusses the bonding in organometallic complexes and draws on recent computational results and develops the chemical and structural implications of bonding models based on perturbation theory arguments. 

The course of modern organometallic chemistry has been greatly influenced by three simple generalizations the Dewar-Chatt-Duncanson synergic bonding model for metal-olefin complexes (40, 72) Pauling s electroneutrality principle (174), and the 18-electron or inert gas rule (202). In this section the impact of recent theoretical calculations on these important generalizations will be evaluated. 

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Thirdly, it will be important to gain more direct information on the stability of outer-sphere precursor states, especially with regard to the limitations of simple electrostatic models (Sect. 4.2). One possible approach is to evaluate Kp for stable reactants by means of differential capacitance and/or surface tension measurements. Little double-layer compositional data have been obtained so far for species, such as multicharged transition-metal complexes, organometallics, and simple aromatic molecules that act as outer-sphere reactants. The development of theoretical double-layer models that account for solvation differences in the bulk and interfacial environments would also be of importance in this regard. 

A later paper by workers associated with the Takasago process reported on ab initio MO calculations for the catalytic cycle. Their work suggested that steps b and c (Scheme 9.22) involved an intramolecular OA of C-H, followed by a RE in which the hydride ligand attached itself to the terminal carbon of the allyl group. See M. Yamakawa and R. Noyori, Organometallics, 1992, 11, 3167. It is not clear which pathway is correct because the theoretical study used only PH3 and not BINAP to model the Rh-catalyzed isomerization. See also C. Chapuis, M. Barthe, and J.-Y. de Saint Laumer, Helv. Chim. Acta, 2001, 84, 230. 

Some early proposals for the modes of adsorption of thiophenes on metal sulfides have been probed by comparisons with the structures of well-characterized metal complexes this has allowed the identification of the most reasonable alternatives and of new possibilities not previously considered. Tlieoretical studies on such complexes at increasing levels of sophistication have also contributed in an Important manner to provide a clear and consistent picture of the different possible bonding modes of thiophenes to metal centers. When these theoretical and experimental results from molecular chemistry are combined with the information available from surface techniques and heterogeneous catalysis, the chemisorption of this type of organosulfur compounds on metal sulfides arises as a very well understood phenomenon. This is no doubt one of the most important achievements of the organometallic modeling approach to HDS chemistry. 

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