Open-shell transition metal ions

Open-shell transition metal ions and complexes... 

Open-shell transition metal ions play central roles in many modem research fields such as catalysis, molecular magnetism, or bioinorganic chemistry, to name only a few. Central to this... 

The multifaceted behavior of transition metal complexes calls for not only theoretical explanations within a common conceptual framework but also theoretical tools that are powerful enough to predict the chemical and magnetic behavior of open-shell transition metal ions. Specifically, one looks for theoretical methods to calculate geometries and relative energies for stable species and transition states as well as for methods that allow one to determine spectroscopic parameters with sufficiently predictive accuracy. 

Acknowledgements Close collaboration with Richard Catlow, Julian Gale and Peter Battle over the past 6 years was vital for our developments in the field of GAs and the new interatomic potentials for modelling open-shell transition-metal ions. This and financial support from the E.P.S.R.C. are gratefully acknowledged. I am also grateful for useful discussions with Alexey Sokol and provision of visualisation software by Accelrys Inc. 

Thus the ammonia acts to effectively sequester the open shelled transition metal ion, creating a relatively uninteresting classical metal complex that acts as a counterion for a number of complicated sulfosalt frameworks [127,128]. 

Several papers have appeared in press in the past three years using the B3LYP functional under PBC, as embodied in CRYSTAL98 [30]. The systems studied include several compounds of main group elements [40-57], compounds containing transition metal ions in formal electronic configuration d ° [58, 59], and compounds with one or more open shell transition metal ions in electronic configuration d " (n O) [60-64]. 

Abstract The concept of stable superatoms, molecular species which mimic the shell closures emphasised by Lewis and Kossel, has become an important paradigm of stability in cluster chemistry. In this review we discuss recent work, both experimental and theoretical, on the family of endohedral clusters M Ex, where M is a transition metal ion and E is a member of group 14 (Si, Ge, Sn, Pb). The structural chemistry within this family is very varied, ranging from deltahedral motifs for the heavier tetrels to open 3-connected stmctures such as the hexagonal prism in Cr Sii2- We explore the arguments that have been presented to rationalise these structural trends and their implications for chemical bmiding. 

The important feature of magnetic insulators is that, being nonmetallic, they have a band gap and possess unpaired electrons. They show crystal-field transitions due to the presence of open-shell (d") ions. Mott proposed that electron repulsion can be responsible for the breakdown of the normal band properties of transition-metal... 

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). 

For Cu+ (d °) and open-shell metal ions, the absorption spectra in the visible spectral range are totally different, and weak bands (e of the order of 10 M cm ) are found up to 600-700 nm. Cu.5+ exhibits the expected wide MLCT absorption (see above), whereas the bands observed for Ni-+ (d ) and Co + (d ) are assigned to metal-centered (MC) or ligand-to-metal-charge-transfer (LMCT) transitions. Finally, the Pd + (d ) catenate has to be considered a special case since it is actually a... 

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