Non-innocent Ligand

Non -innocent ligands in chemistry a certain type of ligands in complex compounds - lead to the oxidation state of the central atom can not be determined - especially with transition metals.

  • 6.1 Notes and references

History

Complexes with non -innocent ligands have been known since the 1960s. Then square- dithiol planar bonds to nickel, palladium and platinum were synthesized. The term is, however, more widespread, since these complexes since only attracted greater attention, which is mainly due to their redox properties only since the 1990s.

Definition

The term was introduced in 1966 by CK Jorgenson. He defined it as " ligands are innocent whenthey allow oxidation states of the central atom to be defined. ", Resulting in German means something like: " A ligand is innocent, if he allows that the oxidation state of the central atom unambiguously determined is. "

That is, is a metal with a non -innocent ligands have a connection, so the physical oxidation state of the central atom is not determined ( not to be confused with the formal oxidation state ).

Physical oxidation state

Explanation

The physical oxidation state describes the actual electronic state spectroscopy and is examinable, so it is often referred to as spectroscopic oxidation state. Although it is true often in agreement with the formal oxidation state, but this need not necessarily be the case. Especially in free-radical organic ligand is different from the formal oxidation state of the physical.

As an example of the phenoxy radical ligand is complexed with iron in d5 - configuration. If now the formal oxidation state is determined, the ligand has to be removed with closed-shell configuration. It is not enough to leave only the unshared electron pair of the ligand, but the iron atom must also release an electron from its d-shell, so that the phenoxy ligand is no longer present as a radical. This gives the iron d4 - configuration, and is therefore loaded 4-way after removal of the ligand, and thus receives the formal oxidation state IV However, spectroscopic investigations reveal a physical oxidation state of III.

Methods of investigation

For direct determination of the physical oxidation state are:

  • Atomic Absorption Spectroscopy
  • Photoelectron spectroscopy ( PE) and X-ray photoelectron spectroscopy (XPS),
  • Mössbauer spectroscopy
  • NMR

Problem in determining

For complexes with non -innocent ligands, there are only small energy differences between the HOMO (d ( π ) orbital ) of the metal and the LUMO ( π * orbital ) of the ligand. This results in a continuous electron transfer between the central atom and a ligand, which in turn leads to a constant change in the electron state of the metal, which is related to the oxidation stage.

Not any charge-transfer complex is non -innocent. However, this is a necessary condition in order to observe a non -innocent behavior.

Examples

The simplest example of a non -innocent ligand is NO. In a compound with iron, it can not determine whether the ligand is present just positive or just negative charge.

Other well-known non -innocent ligands are the quinone derivatives. Thus, ligand having the formula [Y -CR = CR -Y ] 2 -, or Y = CR -CR = Y often non -innocent, in which case Y = O, S, NR ', NH, and R, R' = alkyl or aryl.

Properties and Applications

Of particular interest is the fact that in many non -innocent complexes in an oxidation or reduction of the central atom remains the same and instead changes the structure of the ligand.

For example, the oxidative addition in catalytic reactions is important as a bond -activating step. In oxidative addition reactions, a molecule with bond breakage to a metal outsourced to:

Could be introduced such a reaction by transition metal complexes. For this to work, however, a complex meet the following three conditions: It must be two free coordination sites and electron-deficient metal centers ( <16 e-) possess. In addition, the central atom must have potential achievable oxidation states which differ by a difference of 2.

The oxidative addition of chlorine to zirconium has already been carried out using a non -innocent complex. However, the oxidation state of the central atom remained. Instead, the ligands were oxidized.

Furthermore, these complexes are of interest for materials research, as might solids with unique electronic properties could be synthesized. Comparable properties with molecular structure for these metals, superconducting materials or magnetic crystals.

Swell

  • P. Chaudhuri, CN Verani, E. Bill, E. Bothe, T. Weyhermüller, K. Wieghardt - J. Am. Chem Soc. (2001) 123, 2213th
  • C. G. Pierpont - Coord. Chem Rev. (2001), 216-217, 99
  • C. G. Pierpont - Coord. Chem Rev. (2001), 219-221, 415
  • A. Mederos, S. Domini Dominguez, R. Hernandez -Molina, J. Sanchiz, F. Brito - Coord. Chem Rev. (1999), 193-195, 913
HOMO/LUMO
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