Transition metals are distinguished by their ability to form complex ions through interactions with ligands—molecules or ions that donate electron pairs.

This unique characteristic of transition metals stems from their electronic configuration, which includes partially filled d orbitals. These d orbitals have the capacity to accept pairs of electrons from ligands. Ligands can take many forms, such as water molecules, ammonia, or chloride ions, and must possess at least one pair of electrons available for donation to the transition metal ion. This interaction leads to the formation of a coordinate bond, also referred to as a dative covalent bond, between the ligand and the transition metal ion.

The structure formed as a result of this bonding is known as a complex ion. At the center of this complex ion is the metal ion, which is surrounded by the ligands. The number of ligands that can bond with a metal ion is influenced by both the size and charge of the metal ion and the ligands involved. This variability can result in diverse geometrical arrangements for complex ions, including common shapes such as square planar and octahedral.

The formation of complex ions is a fundamental property of transition metals and plays a crucial role in many of their distinctive characteristics. For example, the vibrant colors seen in transition metal compounds are often attributed to the presence of complex ions. Furthermore, the ability to form complex ions is vital in numerous biological systems and industrial applications. A notable example is found in the human body, where the iron in hemoglobin forms a complex ion with oxygen molecules, facilitating the transportation of oxygen throughout the bloodstream.

In summary, transition metals form complex ions by accepting electron pairs from ligands into their d orbitals, creating coordinate bonds that lead to a variety of structures and properties.