Page 7 - Microsoft Word - Lund BioinorgChem 08.doc

7

Tutorial: Coordination compounds (2): Ligand-field splitting and spin state

II
6
Example: Fe (d ): In an octahedral (O h) field, the degeneracy of the five d-orbitals is lifted.
Depending on the strength of the ligand field, the ligand field stabilisation energy (i.e. the
energy set free as all of the electrons are accommodated in the orbitals of lower energy) can be
(i) less and (ii) more than the energy needed for electron pairing. In the case of (i), i.e. aqua
ligands, a high-spin complex is formed; in the case of (ii), i.e. cyanido ligands, a low-spin
2+
complex is formed. Asymmetrically occupied orbital sets, as in the case of [Fe(H 2O) 6] , result
in further stabilisation through symmetry lowering: Jahn-Teller distortion.

[Fe(H O) ] 2+
2
6
Jahn-Teller distortion
Energy [Fe(CN) ] 4-
6

weak
perturbation perturbation
under O h
under D 4h strong
spheric perturbation
disturbance
under O h

undisturbed OH 2
CN
H 2 O OH 2 NC CN
H 2 O OH 2 NC CN
CN

OH 2

Tutorial: Coordination compounds (3): Classification of ligands;and the chelate effect

+
-
Series of ligand strengths: Halides ≈ {S} < {O} < {N} < CN < NO ≈ CO

Pearson classification (soft and hard): hard metal centres (usually early and transient transition
3+
6+
metals in high oxidation states, e.g. Mo and Fe ) prefer hard ligand (i.e. more electronegative
+
ones, such as oxygen-based donors), soft metal centres (late transition metals, e.g. Cu ) prefer
soft ligands (such as cysteinate). There are many exceptions from this “rule“.

Chelate effect: Stabilisation of a complex by multidentate ligands. The chelate effect is an
entropic effect (high entropy = high disorder [increase of particle number]). Example: The
3+
6-
complex formed between the siderophore enterobactin (ent , a hexadentate ligand) and Fe is
3-
6-
3+
particularly stable: [Fe(H 2O) 6] + ent → [Fe(ent)] + 6H 2O.