Spectrochemical series In accordance with the amount of

Spectrochemical series In accordance with the amount of splitting in octahedral complexes, ligands form the following so-called spectrochemical series: I Br Cl F C 2 O 42 H 2 O NCS py NH 3 en dipy NO 2 CN CO. Two groups are distinguished according to the amount of splitting in octahedral complexes: weak- and strong-field ligands.

In the spectrochemical series, the ligands to the left of NH 3 usually form weak field (high-spin, or spin free, complexes), and those to the right form strong field (low-spin, or spin paired, complexes). Depending on the electron structure of the central ion and the position of the ligand in the spectrochemical sreies, complexing results in a certain gain in energy, which is known as the Energy of Stabilization by the Crystal Field (ESCF).

Energy of Stabilization by the Crystal Field (ESCF) The gain in energy (ESCF) is maximum if the t 2 g sublevel is fully occupied by electrons (strong field, configuration 6 t 2 g), while the eg sublevel remains vacant. (splitting energy = 6 x 2/5 o = 12/5 o) On the other hand, the ESCF is zero if both t 2 g and eg sublevels are filled completely, d 10, (for both strong and weak fields). When electrons occupy both t 2 g and eg levels, electron configuration 6 t 2 g 4 eg is achieved. So that a compensation of the energy between the two d-sublevels (t 2 g and eg) and the net result will be zero:

Energy of 6 t 2 g = 6 x 2/5 o = 12/5 o Energy of 4 eg = 4 x 3/5 o = 12/5 o The total energy (ESCF) = 12/5 o + (– 12/5 o) = 0 Also, ESCF equals zero for the high spin octahedral complexes (weak field) whose central atom has the electron configuration d 5 (or 3 t 2 g 2 eg): ESCF = 3 x 2/5 o + (– 2 x 3/5 o) = 0

Examples: For high-spin (spin free) complexes The highest value of the ESCF is attained for Cr 3+, that is of d 3 electron configuration. Electrons will occupy the t 2 g sublevel only, i. e. 3 t 2 g. So that, ESCF being equals to 3 x 2/5 o = 6/5 o , which is a very high value. If the number of electrons at the d level of the central ion is increased or decreased, for example in going to high-spin complex with configuration d 4 = 3 t 2 g 1 eg, the ESCF will be lowered: ESCF of 3 t 2 g 1 eg = 3 x 2/5 o – 3/5 o = 3/5 o

Examples: For low-spin (spin paired) octahedral complexes: The ESCF is maximum for the Co 3+, that is d 6 electron configuration: 6 t 2 g 0 eg. So, ESCF of 6 t 2 g 0 eg = 6 x 2/5 o = 12/5 o. Any increase or decrease in the number of electrons at the d sublevel leads to lower the ESCF; ESCF of 6 t 2 g 1 eg (Ni 3+ and Co 2+) = 6 x 2/5 o – 3/5 o = 9/5 o ESCF of 5 t 2 g 0 eg (Mn 2+ and Fe 3+) = 5 x 2/5 o = 10/5 o

List of References for this Course: 1 - A New Concise Inorganic Chemistry J. D. LEE 2 - Methodological Aspects of the Course in Inorganic Chemistry L. I. Martynenko and V. I. Spitsun 3 - Fundamental Concepts of Inorganic Chemistry E. S. Gilreath 4 - Coordination Compounds S. F. A. Kettle 5 - Advanced Inorganic Chemistry: A Comprehensive Text Cotton and Wilkinson 6 - Principles of Chemistry Davis, Gailey and Whitten