Open-shell transition-metal complexe molecules

It can thus be anticipated that theoretical research into the description of open-shell molecules, and especially into open-shell Transition-metal complexes and clusters, will continue to be important and successful in the future. 

Semiempirical methods have played and still play the major role in the elucidation of chemical bonding, the calculation of molecular geometries, and for the determination of other properties of larger molecules and extended systems. While ab-initio calculations have supplemented them for small and, increasingly, for medium-sized molecules, semiempirical methods still remain the better alternative even for relatively small molecules, when information on excited electronic states is demanded. This holds for open shell systems such as transition metal complexes, and, in particular, when heavier atoms are involved and/or spin-orbit coupling has to be considered. 

To cope with the problems of a mechanistic description of CC, we will first analyze three basic questions the nature of the differences in behavior between central atoms on the one hand and organogenic atoms on the other hand, which results in limitations for the MM techniques when applied to molecules of CC. Getting an idea of the source of these differences tentatively allows us to address further questions developing an adequate MM-like scheme for CCs of nontransition metals and non-metals which will be able to reproduce fine structural features of the mutual ligand influence characteristic for this class of molecules. Next we turn to the most complex problem - developing a hybrid modeling technique which would allow us to cover complexes of transition metals with open 7-shells. 

Recent advances in the techniques of photoelectron spectroscopy (7) are making it possible to observe ionization from incompletely filled shells of valence elctrons, such as the 3d shell in compounds of first-transition-series elements (2—4) and the 4/ shell in lanthanides (5, 6). It is certain that the study of such ionisations will give much information of interest to chemists. Unfortunately, however, the interpretation of spectra from open-shell molecules is more difficult than for closed-shell species, since, even in the simple one-electron approach to photoelectron spectra, each orbital shell may give rise to several states on ionisation (7). This phenomenon has been particularly studied in the ionisation of core electrons, where for example a molecule (or complex ion in the solid state) with initial spin Si can generate two distinct states, with spin S2=Si — or Si + on ionisation from a non-degenerate core level (8). The analogous effect in valence-shell ionisation was seen by Wertheim et al. in the 4/ band of lanthanide tri-fluorides, LnF3 (9). More recent spectra of lanthanide elements and compounds (6, 9), show a partial resolution of different orbital states, in addition to spin-multiplicity effects. Different orbital states have also been resolved in gas-phase photoelectron spectra of transition-metal sandwich compounds, such as bis-(rr-cyclo-pentadienyl) complexes (3, 4). 

Because the convenience of the one-electron formalism is retained, DFT methods can easily take into account the scalar relativistic effects and spin-orbit effects, via either perturbation or variational methods. The retention of the one-electron picture provides a convenient means of analyzing the effects of relativity on specific orbitals of a molecule. Spin-unrestricted Hartree-Fock (UHF) calculations usually suffer from spin contamination, particularly in systems that have low-lying excited states (such as metal-containing systems). By contrast, in spin-unrestricted Kohn-Sham (UKS) DFT calculations the spin-contamination problem is generally less significant for many open-shell systems (39). For example, for transition metal methyl complexes, the deviation of the calculated UKS expectation values S (S = spin angular momentum operator) from the contamination-free theoretical values are all less than 5% (32). 

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