Question

Why are substances that contain complex ions often called coordination compounds?

Short answer

Coordination compounds are called so because they contain complex ions, which consist of a central metal atom or ion that is coordinated by ligands via coordinate covalent bonds.

Step-by-step solution

Understanding Coordination Compounds

Coordination compounds are substances that consist of a central metal atom or ion (typically a transition metal) surrounded by molecules or anions, often referred to as ligands. These ligands 'coordinate' to the metal through coordinate covalent bonds, where both electrons in the bond originate from the same atom (the ligand).

Identifying Complex Ions

Within a coordination compound, the entire cluster of the central metal and its surrounding ligands is called a complex ion. This entity can often form by accepting electron pairs from the ligands to the metal, which creates the metal-ligand bonds.

Explaining the Terminology

The term 'coordination' refers to the coordinate covalent bonds between the metal and the ligands within these compounds. Therefore, because these substances contain complex ions that result from this coordination, they are commonly referred to as coordination compounds.

Key concepts

Complex Ions

Complex ions are fascinating entities in the realm of chemistry. They consist of a central metal ion surrounded by molecules or ions called ligands. These ligands can be a variety of species like water, ammonia, chloride ions, and many others. Imagine a metal ion at the center of a sphere and ligands attaching to it from different directions, like satellites orbiting a planet. These ligands donate electron pairs to the metal ion and create a more stable structure called a coordinate complex. These formations are crucial in various fields, from industrial catalysis to biological systems, such as the hemoglobin in our blood that contains a complex ion with iron at its heart.

Coordinate Covalent Bonds

Coordinate covalent bonds are unique in their formation. Unlike a typical covalent bond where two atoms share a pair of electrons, in a coordinate covalent bond, both shared electrons are donated by the same atom. In the context of complex ions, the ligands are the electron donors and the metal ion is the electron acceptor. This generous donation results in a stable bond that helps maintain the integrity of the complex ion. One of the classic examples is the ammonia molecule bonding to a metal ion where nitrogen, with its lone pair of electrons, forms a coordinate bond with the metal.

Transition Metals

Transition metals are like the celebrities of the periodic table when it comes to forming coordination compounds. They are the chemical elements found in the d-block of the periodic table and include metals like iron, copper, and nickel. Transition metals have this special ability due to their electronic configuration that allows them to accept electrons into their d orbitals. They can exhibit various oxidation states and form colored compounds, which is why you often see coordination compounds appearing as vividly colored substances. These properties make them ideal for use as catalysts or even pigments.

Ligands

Ligands are the supporting cast to the star metal ion in a coordination compound. They are ions or molecules with at least one unshared pair of electrons and they can be as simple as a water molecule or as complex as a large organic molecule. When ligands attach to the central metal ion through coordinate covalent bonds, they can dramatically change the chemical and physical properties of the metal ion. Essentially, ligands are the reason coordination compounds are so versatile. They govern the reactivity, stability, and overall architecture of these compounds. Without ligands, the fascinating world of coordination chemistry wouldn't exist.