Pi backbonding

Back-bonding or bond return is a term from the molecular orbital theory, which refers to the bonding in complexes of transition metals. In principle, σ -, π - and δ - return bonds are possible depending on the type of the central atom and the ligand, which may occur together with σ - and π - Hinbindungen. So synonymous, the terms σ -, π - and δ - Akzeptorbindungen, or σ - and π -Donat Orbi Dungen. Especially π - back bonding plays an important role in coordination chemistry.

A π - back-bonding occurs when the central metal atom has an excess charge by σ - Hinbindungen of the ligands and this is compensated by receiving the π -acceptor ligands with umstrukturierbaren multiple bonds electrons from the d orbitals of the central atom. Formally, this complex stabilizing the charge compensation are described with mesomeric formulas, wherein the metal atom aims at a number of outer electrons according to the 18- electron rule. Such π - back donation usually occur when transition metals with low oxidation states (0 or negative).

So, for example, occurs when nickel carbonyl Ni ( CO) 4 by overlap of an occupied σ - orbital of the CO molecule with an empty d- orbital of the metal atom charge transfer from the ligand to the central atom. In addition to the π * orbital on the CO molecule is an energetically low- lying unoccupied π molecular orbital available, which can form an electron - π - bonding with a filled d orbital of the π - type of the central atom. Simultaneously, an electron transfer is connected from the central atom to the ligand, which is also reflected in the change in the formal charge for Ni of 4 - can (without π - back-bonding ) to 0 seen ( with π back-donation ).

Due to the antibonding ligand orbitals of the π - acceptor, for example, the bond order of CO is somewhat reduced, resulting in many carbonyl to an extension of the atomic distance of 112.8 pm CO at 115 pm. Typical π -acceptor ligands cyanide C ≡ N, C ≡ O carbon monoxide, fulminate C ≡ N -O- C ≡ N -R isonitriles, Acetylenid C ≡ C2 and the Nitrosylkation N ≡ O .

Swell

• Chemical Bonding