User:Macbaband/sandbox

Pi Ligands

Pi-ligands are those that bond to a metal via donating electron density to the metal from their σ-orbitals and their π-orbitals and by the metal center’s donation of electron density into the π* orbitals of the ligands. The degree of π* back-donation from the metal depends upon the energy level of the ligand’s π* orbitals, the lower the energy, the easier it is for the metal to donate electron density. According to molecular orbital theory, it is the metal’s d-electrons that share common symmetry with ligands’ p-orbitals that lead to direct bonding with the metal while those that lie in between the axes of the ligands, including dxz and dyz can donate electron density into the empty π* orbitals. The metal orbitals that the ligand participates in backhanding with will help determine the orientation of the ligands in space around the molecule. Pi bonding is not limited to a metal's d-orbitals, but can also occur between f-orbitals and pi-electron density as in more exotic organometallic compounds including uranocene, where δ-bonds play an important role.

Bonding and Pi-Backbonding in a generic metal-ethylene system. Image adapted from Pfenning, Brian W., Principles of Inorganic Chemistry (2015) p. 642

These type of ligand are anionic and thus act to increase the oxidation state of the metal center. When determining the likely stability of a metal complex using an ionic method of electron counting, each “double-bond” that participates in bonding donates two electrons to the metal center. In an olefin ligand, especially conjugated arenes, not all of the carbons participate directly in bonding. The number of carbon atoms that participate in bonding in an arene ring is referred to as the hapticity symbolized ηⁿ where n is the number of carbon atoms that are directly bonded to the metal center. This allows ligands of this type to play unique roles in organic mechanisms such as what's referred to as "ring slippage" where, in a ligand substitution, the hapticity of a ligand is reduced followed by the addition of a new ligand to the metal center which is then followed by an increase in the hapticity and the rejection of a different ligand.