Bonds and crystal field theory

Color of Complexes Valence Bond and Crystal Field Theories... 

XBLATIONSHIP OF THE GENERAL MOLECULAR-ORBITAL TREATMENT TO THE VALENCE-BOND AND CRYSTAL-FIELD THEORIES... 

The AH for your transition will involve bond breaking and making as well as geometry change. Table 3.5 and crystal field theory can be used to estimate enthalpy changes associated with geometry changes. 

Various theories of chemical bonding such as Hiickel aromatic theory in organic chemistry and crystal field theory in inorganic chemistry are successful primarily because of the full use that they make of s)unmetry properties of molecules and complex ions. Arguments based on symmetry are very powerful since they usuaUy supply answers of a yes or no variety, compared to the maybe answers of most methods used in discussing chemical bonds. 

Partially filled external, or frontier, orbitals are available for bonding with the ligands. The presence of the free orbitals allows for the formation of a covalent complex. This is because the electrons that are donated by the ligands are placed in the vacant orbitals, and this is why the d-block atoms form covalent bonds with ligands. And this is why the transition metals form the majority of organometallic complexes that are known. This is important later when talking about d-orbital splitting and crystal field theory. 

Crystal field theory has since been surpassed by ligand field theory because it takes into greater account the presence of covalent bonding. Nonetheless, crystal field theory is still a powerful theory, although simple, as it explains magnetism, electronic spectra and binding strengths of transition metal complexes. 

A theory for translocation, length of the H bond and crystal field effects... 

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. 

Restricting ourselves to a six-coordinated system and to the valence bond theory and crystal field theory, it is possible to illustrate the bonding in complexes and to designate the nomenclature using [CoFe] and [Co(NH3)e] + as examples. It is first necessary to know that [CoFs] is paramagnetic with four unpaired electrons, whereas [Co(NH3) ] + is diamagnetic. On the basis of the valence bond theory the electronic structures are designated as ( 5) or cPsp (6) hybridizations. 

There are two major theories of bonding in d-metal complexes. Crystal field theory was first devised to explain the colors of solids, particularly ruby, which owes its color to Cr3+ ions, and then adapted to individual complexes. Crystal field theory is simple to apply and enables us to make useful predictions with very little labor. However, it does not account for all the properties of complexes. A more sophisticated approach, ligand field theory (Section 16.12), is based on molecular orbital theory. 

The effects of the bonding electrons upon the d electrons is addressed within the subjects we call crystal-field theory (CFT) or ligand-field theory (LFT). They are concerned with the J-electron properties that we observe in spectral and magnetic measurements. This subject will keep us busy for some while. We shall return to the effects of the d electrons on bonding much later, in Chapter 7. 

In all these discussions, we separate, as best we might, the effects of the d electrons upon the bonding electrons from the effects of the bonding electrons upon the d electrons. The latter takes us into crystal- and ligand-field theories, the former into the steric roles of d electrons and the geometries of transition-metal complexes. Both sides of the coin are relevant in the energetics of transition-metal chemistry, as is described in later chapters. 

The spectrochemical series was established from experimental measurements. The ranking of ligands cannot be fully rationalized using crystal field theory, and more advanced bonding theories are beyond the scope of general chemistry. 

In the final section of this chapter, we shall attempt to give a brief rationalization of the regularities and peculiarities of the reactions of non-labile complexes which have been discussed in the previous sections. The theoretical framework in which the discussion will be conducted is that of molecular orbital theory (mot). The MOT is to be preferred to alternative approaches for it allows consideration of all of the semi-quantitative results of crystal field theory without sacrifice of interest in the bonding system in the complex. In this enterprise we note the apt remark d Kinetics is like medicine or linguistics, it is interesting, it js useful, but it is too early to expect to understand much of it . The electronic theory of reactivity remains in a fairly primitive state. However, theoretical considerations may not safely be ignored. They have proved a valuable stimulus to incisive experiment. 

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