What do ligands have that allows them to bind to metal ions?

Ligands possess lone pairs of electrons that enable them to bind effectively to metal ions. This is due to the electron-deficient nature of metal ions, which typically have open coordination sites where they can accept electrons. When a ligand donates one of its lone pairs to the metal ion, a coordinate (or dative) bond is formed. This type of bond is characterized by the metal receiving both electrons from the ligand, creating a stable metal-ligand complex.

Lone pairs are particularly efficient at forming coordinate bonds because they consist of two electrons that are not involved in any other bonding interactions, allowing them to be readily available for donation. This ability to form such bonds is critical in various applications, including catalysis, drug design, and the formation of complex ions in solution.

Other options such as single bond electrons or double bond electrons do not accurately describe the mechanism through which ligands bind to metal ions in the context of coordinate bonding. While free electrons suggest an availability, they lack the specificity associated with lone pairs, which have a defined structure and inherent capacity for bonding in coordination chemistry.