What type of complex forms when a polyentate ligand binds to a metal with more than one donor atom?

A chelate complex forms when a polyentate ligand coordinates to a metal ion through multiple donor atoms. These ligands have multiple functional groups capable of donating electron pairs to the metal. Because they can form several bonds with the same metal center, they create a stable ring structure in the complex, which is characteristic of chelation.

The stability of chelate complexes arises from the "chelate effect," where the formation of a ring structure lowers the system’s free energy compared to a situation where the same ligand exists in a monodentate form. This increased stability is crucial in many biological systems and industrial applications.

In contrast, monodentate complexes involve ligands with only one donor atom, meanings they can only bond at a single site. Mixed-ligand complexes, meanwhile, contain different types of ligands, not specifically a combination of donor atoms from a single polyentate ligand. An ambidentate ligand refers to a ligand that has more than one potential donor atom, but it binds through only one of those sites at any time.

Therefore, recognizing that a chelate complex specifically involves a polyentate ligand coordinating through multiple donor atoms to form stable structures provides clarity on why this answer is the most appropriate.