8: Coordination Chemistry- Structure and Isomers

Introduction

Coordination compounds are important to all areas of chemistry, engineering, the life and environmental sciences, and beyond. In the synthetic laboratory catalytic amounts of coordination compounds enable organic chemists to synthesize new compounds selectively and in high yield under mild condition. Applied industrially, coordination compound catalysts serve as vital catalysts that facilitate the conversion of raw petrochemical or bio-derived feedstocks into useful industrial and consumer products. Without them life as we know it would be impossible as many biochemical systems are coordination complexes. Examples include the hemoglobin which transports oxygen around our bodies and the myoglobin which stores it, the photosystems which harvest light and use light energy in photosynthesis, the constituents of the respiratory chain, and many of the enzymes involved in the expression and transmission of genetic information. In studying coordination chemistry you are about to take your first steps into a vast and exciting world.

What is a Coordination Compound?

Coordination compounds consist of one or more metals bound to one or more Lewis base ligands. For example, hexaminechromium(III) ion is a coordination complex in which six ammonia ligands coordinate a Co³⁺ ion, as shown in Scheme \({\PageIndex{1}}\).

Scheme \(\PageIndex{1}\): Formation of hexaminechromium(III) ion from Co³⁺ and NH₃.

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Such complexes are called coordination compounds or complexes because the ligand-metal bond may be thought of as a coordinate covalent bond in which both of the bonding electrons come from the ligand, which is then said to coordinate the metal. This coordinate covalent model is a very useful formalism for understanding the basic features of coordination chemistry, although it does not always accurately reflect the actual details of the bonding in every coordination complex. Nevertheless, even in those cases where the simple coordinate covalent bond model breaks down the coordinate covalent bond concept supplies the language sophisticated models employ to describe the more complex bonding involved.

Additional Important Terms

Some of the common widely terms that follow from the coordinate covalent model of bonding in coordination complexes include

• Coordination compounds are also called coordination complexes, metal complexes, or just complexes. The term complex refers to coordination compounds' composite nature, in that they may be thought of as comprised of multiple ligand and metal ion parts that can be restored by breaking the coordinate covalent bonds holding the complex together. This is in contrast to inorganic or organic molecules which are more commonly thought about as whole molecules held together by the sharing of electrons contributed by all the atoms.
• Coordination complexes that are ions are called complex ions.
• Ligands bound to the coordination complex are said to reside in the primary or inner coordination sphere. These bound ligands are not readily exchangeable, in contrast to nearby counterions and solvent molecules, which are said to reside in the secondary or outer coordination sphere.
• The portion of the complex contributing the electron pairs is said to be the donor and the portion which receives them the acceptor. In conventional coordination compounds the ligand is the donor and the metal the acceptor. In these cases it would be equally convenient to refer to the ligand donor as the Lewis base and the metal acceptor as the Lewis acid. However, in more complex bonding scenarios there may be multiple electron pair donation and acceptance interactions taking place between each pair of atoms and donor-acceptor language will be more convenient.
• The number of ligand sites donating lone pairs to the central atom is referred to as the coordination number. For most complexes this will just be equal to the number of ligand atoms bound to the metal. In simple complexes it is just equal to the number of ligands. For instance, the cobalt in Scheme \(\sf{\PageIndex{1}}\) has a coordination number of six.
• Although technically compounds with metal-carbon bonds are coordination complexes term coordination complex is sometimes used to refer to complexes which do not possess metal-carbon bonds in their primary coordination sphere. Complexes which possess metal-carbon bonds are called organometallic compounds instead. The use of the terms organometallic and coordination to distinguish organometallic compounds from other types of coordination compounds is often convenient since many organometallic ligands engage in more than simple \(\sigma\) donor acceptor coordinate covalent bond formation with the metal center. However, this is true of some wholly inorganic ligands too so it should always be kept in mind that organometallic compounds are just a type of coordination compound and that inorganic ligands can in principle be tuned in interact with a metal center in much the same way an organic ligand does.

A summary of some of the concepts and terms used to describe coordination compounds is given in Scheme \(\PageIndex{2}\).

Scheme \(\PageIndex{2}\): Some terms used to describe coordination compounds. This work by Stephen Contakes is licensed under a Creative Commons Attribution 4.0 International License.

Contributors and Attributions

Stephen Contakes, Westmont College

Unless otherwise noted, all line drawings on this page are by Stephen Contakes and licensed under a Creative Commons Attribution 4.0 International License.

Thumbnail image is the structure of tris(oxalato)ferrate(III) from symmetry@otterbein