Crystal field theory compounds

Color from Transition-Metal Compounds and Impurities. The energy levels of the excited states of the unpaked electrons of transition-metal ions in crystals are controlled by the field of the surrounding cations or cationic groups. Erom a purely ionic point of view, this is explained by the electrostatic interactions of crystal field theory ligand field theory is a more advanced approach also incorporating molecular orbital concepts. 

The method mentioned has been appUed to ClFe(Et2dfc)2 first, because for this compound, and some related ones, an abnormally large QS has been observed, while the crystal field theory predicts QS = 0 for this intermediate spin (S = 3/2) compound. 

For ionic compounds, crystal field theory is generally regarded a sufficiently good model for qualitative estimates. Covalency is neglected in this approach, only metal d-orbitals are considered which can be populated with zero, one or two electrons. To evaluate (Vzz)vai 4t the Mdssbauer nucleus, one may simply take the expectation value of the expression — e(3cos 0 — for every electron in a valence orbital i/, of the Mdssbauer atom and sum up,... 

In the case of covalent compounds, crystal-field theory is a poor model for estimating electric field gradients because of the extensive participation of ligand atomic orbitals in the chemical bonds. MO calculations are a much better choice, since the corresponding interactions are considered, and realistic (noninteger) population numbers are obtained for the central metal as well as the ligand atomic orbitals. 

Another typical example for anisotropic covalency is found in five-coordinate ferric compounds with intermediate spin S = 3/2 (also discussed in Sect. 8.2). Crystal field theory predicts a vanishing valence contribution to the EFG, whereas large quadrupole splittings up to more than 4 mm s are experimentally found. 

Although Chapter 25 does not address directly why some compounds with coordination 4 are tetrahedral and some are square planar, it is possible to surmise that the answer lies with (1) Crystal Field Theory and the energies of the d orbitals involved bonding and (2) how many unpaired electrons the metal complex has. 

Note to the student The AP chemistry exam does not emphasize complex ions or coordination compounds. There is nothing on the AP exam that involves the concepts of crystal-field theory, low versus high spin, valence bond theory, or other related areas. If you understand the questions presented here, then you are basically "safe" in this area of the exam. Most high school AP chemistry programs do not focus much on this area of chemistry because of time constraints. 

Adsorption of Ag on the surface of PdO is also an interesting option offered by colloidal oxide synthesis. Silver is a well-known promoter for the improvement of catalytic properties, primarily selectivity, in various reactions such as hydrogenation of polyunsaturated compounds." The more stable oxidation state of silver is -F1 Aquo soluble precursors are silver nitrate (halide precursors are aU insoluble), and some organics such as acetate or oxalate with limited solubility may also be used." Ag" " is a d ° ion and can easily form linear AgL2 type complexes according to crystal field theory. Nevertheless, even for a concentrated solution of AgNOs, Ag+ does not form aquo complexes." Although a solvation sphere surrounds the cation, no metal-water chemical bonds have been observed. 

Nevertheless, the application of crystal field theory to natural compounds... 

Analysis of the valence-band spectrum of NiO helped to understand the electronic structure of transition-metal compounds. It is to be noted that th.e crystal-field theory cannot explain the features over the entire valence-band region of NiO. It therefore becomes necessary to explicitly take into account the ligand(02p)-metal (Ni3d) hybridization and the intra-atomic Coulomb interaction, 11, in order to satisfactorily explain the spectral features. This has been done by approximating bulk NiO by a cluster (NiOg) ". The ground-state wave function Tg of this cluster is given by,... 

Why do titanium(III) salts hydrolyze to a smaller extent than titanium(IV) salts Using the crystal field theory, explain why titanium(III) compounds are coloured, while titanium(IV) ones are oolourless. 

In broad terms, there are two very different theories which have been used to interpret the properties of transition-metal compounds MO theory and crystal field theory. ... 

Crystal field theory has its origins in Hans Bethe s famous 1929 paper Splitting of terms in crystals. In that paper Bethe demonstrated what happens to the various states of an ion when it is placed in a crystalline environment of definite symmetry. Later, John Van Vleck showed that the results of that investigation would apply equally well to a transition-metal compound if it could be approximated as a metal ion surrounded by ligands which only interact electrostatically with the... 

The central concern of crystal field theory is what happens to the electronic states of an ion when it is placed in some perturbing symmetric environment. If we assume that a set of ligands placed symmetrically about a transition-metal ion interact only electrostatically with the electrons of that ion, then the answer to this question is relevant to our study of transition-metal compounds. 

Ion-dipole forces are important in solulions of ionic compounds in polar solvents where solvated species such as NatOH,) and F(H 20) (for solutions of NaF in H.O) exist. In the case of some metal ions these solvated species can be sufficiently stable to be considered as discrete species, such as [Co(NHj)6]j+. Complex ions such as the latter may thus be considered as electrostatic ion—dipole interactions, but this oversimplification (Crystal Field Theory sec Chapter 11) is less accurate than are alternative viewpoints. 

The model that largely replaced valence bond theory for interpreting the chemistry of coordination compounds was Ihe crystal field theory, first proposed in 1929 by Hans Bethe.11 As originally conceived, it was a model based on a purely electrostatic... 

Chapter 11 Coordination Chemistry Bonding, Spectra, and Magnetism 387 Bonding in Coordination Compounds 391 Valence Bond Theory 391 Crystal Field Theory 394 Molecular Orbital Theory 413 Electronic Spectra of Complexes 433 Magnetic Properties of Complexes 459... 

The VSEPR approach is largely restricted to Main Group species (as is Lewis theory). It can be applied to compounds of the transition elements where the nd subshell is either empty or filled, but a partly-filled nd subshell exerts an influence on stereochemistry which can often be interpreted satisfactorily by means of crystal field theory. Even in Main Group chemistry, VSEPR is by no means infallible. It remains, however, the simplest means of rationalising molecular shapes. In the absence of experimental data, it makes a reasonably reliable prediction of molecular geometry, an essential preliminary to a detailed description of bonding within a more elaborate, quantum-mechanical model such as valence bond or molecular orbital theory. 

The preceding consideration of crystal field theory leads one to ask what are the factors which control the magnitude of A . If we can understand these, we should be able to predict successfully some of the properties of co-ordination compounds. The magnitude of the crystal field experienced will depend both upon the nature of the metal ion and of the co-ordinated ligands. If we concentrate upon the latter factor, what are the properties of... 

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