What is a molecule that binds to a central metal atom to form a coordination complex called?

A molecule that binds to a central metal atom to form a coordination complex is referred to as a ligand. Ligands are typically ions or neutral molecules that have one or more pairs of electrons that they can donate to the metal atom, thereby stabilizing the complex through coordinate covalent bonds.

In coordination chemistry, ligands can vary in size, charge, and the number of binding sites they possess. This ability to coordinate with a metal atom is crucial for the formation of complex structures and is central to many biochemical processes, industrial applications, and medicinal chemistry.

Other terms provided in the options do not define a molecule that binds to a metal atom. Isotopes refer to variants of elements that have the same number of protons but different numbers of neutrons, ions are charged particles that result from the loss or gain of electrons, and neutral atoms are electrically neutral entities consisting of protons, neutrons, and electrons. These concepts are foundational in chemistry, but they do not pertain specifically to the definition of ligands in the context of coordination complexes.