Transition metal complexes coordination crystal field theory

Crystal field theory is a simple electrostatic model that treats the transition metal ion and the ligands as point charges. It is very successful in predicting in a very simple way the role of the 3d electrons in determining the properties of complex ions. Crystal field theory ignores the fact that the ligands form coordinate bonds with the transition metal ion via their lone pairs. 

Color is a spectacular property of coordination complexes. For example, the hexaaqua cations of 3 transition metals display colors ranging from orange through violet (see photo at right). The origin of these colors lies in the d orbital energy differences and can be understood using crystal field theory. 

Copenhagen interpretation, quantum mechanics, 266 Coulomb s law, 94 Crystal field theory, transition metal complexes, 149-152 Curvilinear coordinates, overview, 80, 86-88... 

Crystal field theory was developed, in part, to explain the colors of transition-metal complexes. It was not completely successful, however. Its failure to predict trends in the optical absorption of a series of related compounds stimulated the development of ligand field and molecular orbital theories and their application in coordination chemistry. The colors of coordination complexes are due to the excitation of the d electrons from filled to empty d orbitals d-d transitions). In octahedral complexes, the electrons are excited from occupied t2g levels to empty Cg levels. The crystal field splitting Ao is measured directly from the optical absorption spectrum of the complex. The wavelength of the strongest absorption is called Amax and it is related to Ao as follows. E = hv, so Ao = hv = Because en-... 

Crystal field theory and ligand field theory are indispensible tools for interpreting the spectra of transition metal complexes in terms of coordination interactions and geometry. MCD is useful for identifying electronic... 

Scientists have long recognized that many of the magnetic properties and colors of transition-metal complexes are related to the presence of d electrons in the metal cation. In this section we consider a model for bonding in transition-metal complexes, crystal-field theory, that accounts for many of the observed properties of these substances. Because the predictions of crystal-field theory are essentially the same as those obtained with more advanced molecular-orbital theories, crystal-field theory is an excellent place to start in considering the electronic structure of coordination compounds. 

A satisfactory theory of bonding in coordination compounds must account for properties such as color and magnetism, as well as stereochemistry and bond strength. No single theory as yet does aU this for us. Rather, several different approaches have been applied to transition metal complexes. We will consider only one of them here—crystal field theory— because it accounts for both the color and magnetic properties of many coordination compounds. 

We have seen that the crystal-field model provides a basis for explaining many features of transition-metal complexes. In fact, it can be used to explain many observations in addition to those we have discussed. Many lines of evidence show, however, that the bonding between transition-metal ions and ligands must have some covalent character. Molecular-orbital theory (Sections 9.7 and 9.8) can also be used to describe the bonding in complexes, although the application of molecular-orbital theory to coordination compounds is beyond the scope of our discussion. The crystal-field model, although not entirely accurate in all details, provides an adequate and useful first description of the electronic structure of complexes. 

Transition-metal complexes may also be interpreted from the viewpoint of molecular orbital theory, which assumes that the cation and ligand coordinate by overlap of atomic orbitals to form molecular orbital and crystal-field theories predict an arrangement of the same number of electrons in orbitals with energies in the same order, even though they consider the nature of the bonding in the complex from different points of view. 

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