Spectroscopic Study of Cu(Ii) Complexes: Crystal Field Theory
Experiment 4
Spectroscopic study of Cu(II) Complexes: Crystal Field Theory
Objective:
To study the effect of ligands on crystal field splitting energy ( E)
Theory
The color of coordination compounds of the transition elements is one of their characteristic properties. These colors are due to the absorption and subsequent emission of light in the visible part of the spectrum. Light in this region of the spectrum caused promotion of d–electrons from a lower to a higher energy level. The spectra which result are gen erally referred to as Electronic Spectra. Note that electronic transitions may also be effected by ultraviolet light. Because of the size of the quanta involved, electronic transitions in molecules are always accompanied by vibrational and rotational chang es, and hence a band spectrum is observed. In general, the bands which arise are much broader than bands in an infrared spectrum and are little used for identification purposes.
The crystal field theory of bonding in transition metal complexes has help appreciably to rationalize many of the physical properties of such complexes. Much of the data required for crystal field theory calculation is obtained from a study of the absorption spectra of transition metal complexes.
Regularly six–coordination is most readily pictured by placing the ligands at the plus and minus ends of the three coordinate axes. An electron in the d X2 - y2 and dz2 orbitals is therefore most effected by the field of the ligands and is raised in energy relative to an electron in the dxy, dyz and dzx orbitals. The combined energy level diagram is therefore composed of two upper orbitals, of equal energy, and three lower orbitals, which are also degenerate (Figure 4.1).
The energy zero is conveniently taken as the weighted mean of the en ergies of these two sets of orbitals; the lower trio are thus stabilized by -2/5 E while the upper pair are destabilized by
3/5E, where E is the total energy separation.
Spectroscopic study of Cu(II) Complexes: Crystal Field Theory
Objective:
To study the effect of ligands on crystal field splitting energy ( E)
Theory
The color of coordination compounds of the transition elements is one of their characteristic properties. These colors are due to the absorption and subsequent emission of light in the visible part of the spectrum. Light in this region of the spectrum caused promotion of d–electrons from a lower to a higher energy level. The spectra which result are gen erally referred to as Electronic Spectra. Note that electronic transitions may also be effected by ultraviolet light. Because of the size of the quanta involved, electronic transitions in molecules are always accompanied by vibrational and rotational chang es, and hence a band spectrum is observed. In general, the bands which arise are much broader than bands in an infrared spectrum and are little used for identification purposes.
The crystal field theory of bonding in transition metal complexes has help appreciably to rationalize many of the physical properties of such complexes. Much of the data required for crystal field theory calculation is obtained from a study of the absorption spectra of transition metal complexes.
Regularly six–coordination is most readily pictured by placing the ligands at the plus and minus ends of the three coordinate axes. An electron in the d X2 - y2 and dz2 orbitals is therefore most effected by the field of the ligands and is raised in energy relative to an electron in the dxy, dyz and dzx orbitals. The combined energy level diagram is therefore composed of two upper orbitals, of equal energy, and three lower orbitals, which are also degenerate (Figure 4.1).
The energy zero is conveniently taken as the weighted mean of the en ergies of these two sets of orbitals; the lower trio are thus stabilized by -2/5 E while the upper pair are destabilized by
3/5E, where E is the total energy separation.
References: 1. Mackay, K.M., Mackay, R.A. and Henderson, W., Modern Inorganic Chemistry, 5th ed., Blackie Academic & Professional, UK, 1996. 2. Pass, G. and Sutcliffe, H. Practical Inorganic Chemistry : Preparations , Reaction and Instrumental Methods, Chapman and Hall, London, 1974. 3. Potts, R.A. (1974) Journal of Chemical Education, 51, 539 .