What do you know about complexes in chemistry?

Coordination complexes or compounds are compounds formed by a central atom or ion (usually a metal) that has free orbitals to accept pairs of electrons, surrounded by a group of ions or neutral molecules, capable of sharing one or more pairs of electrons, attached to the central ion by a coordinate bond and called ligands. The complex tends to maintain its identity, even in dissolution, but there is usually partial dissociation. The electrical charge of the complex depends on the charges of the central ion and of the ions or molecules that surround it, and can result in a cation, an anion or a neutral molecule.

These compounds play an essential role in many aspects of chemistry and biochemistry: for example, chlorophyll, which is vital for plant photosynthesis, is a magnesium complex and hemoglobin, which carries oxygen to the cells, is an iron complex. They can also play an important role in solubilization processes of different species.

The first theory that partially explained the existence of these compounds was that of Werner: “Werner’s coordination theory”. Its most important postulates are:

1. Most elements have two types of valence:

• primary: oxidation state
• secondary: number or coordination index

2. The secondary valences are directed towards fixed positions in space while the primaries are adirectional.

Esta teoría explica la existencia de los complejos, sus propiedades generales y su estereoquímica, sin embargo, esta teoría precede en más de 20 años al actual concepto electrónico del átomo. For this reason, although it continues to be the basis of the chemistry of coordination compounds, there are three theories to explain the nature of bond 2 of said compounds in current terms, many of whose aspects complement each other:

• Valence bond theory
• Electrostatic theory of the crystalline field
• Molecular orbital theory

Come with me to do the following practice and let’s apply the chemistry complexes!