text: Owing to the vacant orbital present in Z-ligands, many have incomplete octets which allow them to readily accept a pair of electrons from other atoms. A Z‑function ligand interacts with a metal center via a dative covalent bond, differing from the L‑function in that both electrons are donated by the metal rather than the ligand.  As such, Z-ligands donate zero electrons to a metal center because they tend to be strong electron acceptors. 
  Although many Z-ligands are Lewis acids, they behave as neutral ligands in the complex without contributing to the overall charge present on the complex. But since the metal uses two of its electrons in forming the metal-ligand bond, the Z-ligand raises the valence of the metal center by two units. This means that presence of the Z-ligands change the d configuration of the complex without changing the total e count.
  A Z-ligand is usually accompanied by an L-ligand, as the presence of the L-ligand adds stability to the complex. As the electrons are being donated from the central metal atom to the Z-ligand, the L-ligand donates its pair of electrons to the metal atom. This unique type of bonding existing between two different ligands and the metal atom renders the complexes stable when present with a strong sigma donor ligand. In such complexes, the L and Z ligands can be written in terms of X. For example, if one Z-ligand is accompanied by one L type ligand, it can be written as a complex containing two X type ligands; i.e. MLZ type complex becomes an MX type.