Crystal field descriptions

On the other hand, this MO description matches a crystal field description of a Cu(II) ion insofar as the five highest occupied MO s are concerned four of these are doubly occupied and have a predominant copper 3 d character (65 % at least), whereas the fifth orbital contains the unpaired electron and still has 50% copper 3 d character. The very small MuUiken charge on the copper atom, which does not fit in the description of a Cu(II) ion, is caused by the partial occupation of the 4 p orbitals, which effect is neglected in the crystal field theory. 

In a simple crystal-field description, a spin-quartet ground state (S = 3/2) of iron(III) is energetically stabilized over the spin-sextet (S = 5/2) when the energy separation... 

However, one also has to take into consideration that in a molecular orbital description the number of Ti(3dz )-C(2p) a bonds and Ti(3r/ , 3dyz)-C(2p) it bonds between the respective Ti -3(5 and C-2p states is reduced due to the missing carbon atoms. The Tii -3d orbitals not participating in bonds are destabilized and shifted from the region of the p bands to a position just below or at the bottom of the d bands. On the other hand, the loss of carbon partners will shift T -3d states involved in antibonding Ti(3rf)-C(2p) interactions in the stoichiometric compound downward, leading qualitatively to the same result as the crystal field description. 

For transition metal complexes, techniques derived from a crystal-field theory or ligand-field theory description of the molecules have been created. These tend to be more often qualitative than quantitative. 

We will discuss the crystal field model here. It assumes that the bonding between metal "Crystal field" isn t a very descriptive term,... 

A complete description of the effects of a crystal field upon a d" ion would include similar analysis of the behavours of all terms arising for that d" configuration. Box 3-7 summarizes the case for d, and in Box 3-8, we illustrate a method of using Fig. 3-19 to determine Dq and B values from real spectra. 

The single-electron crystal-field picture may be a crude over-simplification because Coulomb repulsion and SOC can lead to a situation where the ground term is a composite mixture of = 1/2, 3/2, and 5/2 states that does not derive from a single configuration [71, 72] the corresponding physical description is obtained from proper quantum chemical calculations [73-75]. 

The electron spin resonance (ESR) technique has been extensively used to study paramagnetic species that exist on various solid surfaces. These species may be supported metal ions, surface defects, or adsorbed molecules, ions, etc. Of course, each surface entity must have one or more unpaired electrons. In addition, other factors such as spin-spin interactions, the crystal field interaction, and the relaxation time will have a significant effect upon the spectrum. The extent of information obtainable from ESR data varies from a simple confirmation that an unknown paramagnetic species is present to a detailed description of the bonding and orientation of the surface complex. Of particular importance to the catalytic chemist... 

Electronic transitions like insulator-metal transitions, magnetic order-disorder transitions, spin transitions and Schottky-type transitions (due to crystal field splitting in the ground state in/element-containing compounds) profoundly influence the phase stability of compounds. A short description of the main characteristics of these transitions will be given below, together with references to more thorough treatments. 

These properties of the d-shell chromophore (group) prove the necessity of the localized description of d-electrons of transition metal atom in TMCs with explicit account for effects of electron correlations in it. Incidentally, during the time of QC development (more than three quarters of century) there was a period when two directions based on two different approximate descriptions of electronic structure of molecular systems coexisted. This reproduced division of chemistry itself to organic and inorganic and took into account specificity of the molecules related to these classical fields. The organic QC was then limited by the Hiickel method, the elementary version of the HFR MO LCAO method. The description of inorganic compounds — mainly TMCs,— within the QC of that time was based on the crystal field... 

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