Tie back

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Sigma binding (left) compared to pi back binding (right).

Backbinding or return binding is a term from molecular orbital theory that refers to the bonding relationships in complexes of transition metals . In principle, depending on the type of central atom and the ligands, σ, π and δ back bonds are possible, which can occur together with σ and π bonds. The terms σ-, π- and δ-acceptor bonds and σ- and π-donor bonds are synonymous with this . Above all, π backbondings play an important role in complex chemistry .

A π back bond occurs when the central metal atom has an excess charge due to σ bonds in the ligands and this is compensated for by the π acceptor ligands (also π acid ligands ) with restructurable multiple bonds having electrons from the d orbitals of the Record central atom. Formally, this charge equalization , which stabilizes the complex, can be described with mesomeric limit formulas , whereby the metal atom strives for a number of outer electrons according to the 18-electron rule . Such π back bonding normally occurs in transition metals with low oxidation states (0 or negative).

So z. B. in the case of nickel tetracarbonyl Ni (CO) 4, a charge transfer from the ligand to the central atom occurs through the overlap of an occupied σ orbital of the CO molecule with an empty d orbital of the metal atom. In addition, the π * orbital on the CO molecule provides an unoccupied π molecular orbital with a low energy level , which can form an electron acceptor π bond with an occupied π-type d orbital of the central atom. At the same time, electron transfer from the central atom to the ligand is connected, which can also be seen in the change in the formal charge for Ni from 4− (without π backbonding) to 0 (with π backbonding).

The occupation of antibonding ligand orbitals of the π acceptor, for example, lowers the bond order of CO somewhat, which in many carbonyl complexes leads to an increase in the atomic distance of CO from 112.8 pm to 115 pm. Typical π acceptor ligands are cyanide C≡N - , carbon monoxide C≡O, fulminate C≡N − O - , isonitriles C≡N − R, acetylenide C≡C 2− and the nitrosyl cation N≡O + .


  1. AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 , pp. 1245-1246.
  2. NN Greenwood and A. Earnshaw. Chemistry of the elements . 1st corrected edition, Weinheim, Basel, Cambridge, New York: VCH, 1990, page 390, ISBN 3-527-26169-9 .