Dewar–Chatt–Duncanson model

[2][3] The alkene donates electron density into a π-acid metal d-orbital from a σ-symmetry bonding orbital between the carbon atoms.

The metal donates electrons back from a (different) filled d-orbital into the empty π* antibonding orbital.

Both of these effects tend to reduce the carbon-carbon bond order, leading to an elongated C−C distance and a lowering of its vibrational frequency.

The β-diketiminato aluminum(I) complex Al{HC(CMeNAr)2} (Ar = 2,6-diisopropylphenyl), which bears an Al-based spx lone pair, reacts with alkenes and alkynes to give alumina(III)cyclopropanes and alumina(III)cyclopropenes in a process analogous to the formation of π-complexes by transition metals.

[6][7] However, in most cases, the backbonding interaction is absent in these complexes due to the lack of energetically accessible filled orbitals for backdonation, resulting in π-complexes that dissociate readily and are therefore more challenging to observe or isolate.

Orbital interactions in a metal-ethylene complex. On the left, a filled pi-orbital on C 2 H 4 overlaps with an empty d-orbital on the metal. On the right, an empty pi-antibonding orbital on C 2 H 4 overlaps with a filled d-orbital on the metal