Complex Compounds

Structure of Complex Compounds

The structure of such substances is fundamentally different from simple salts. The main criterion for classifying them into a separate group is precisely their spatial arrangement.

Main structural components:

• Central atom (complexing agent) — an atom or ion occupying the central position in the structural unit of the substance.
• Ligands — atoms or atomic groups directly bonded to the complexing agent (e.g., $CN$ groups, water, or ammonia molecules).
• Coordination number — the exact number of ligands that have formed bonds with the central atom.

Key rule: a reaction leads to the formation of a coordination compound only when the coordination number of the central atom exceeds its valence.

Consider potassium ferricyanide ($K_3[Fe(CN)_6]$). The central atom is trivalent iron $Fe(III)$, surrounded by six $CN$ ligands. The coordination number (6) is greater than the valence (III), confirming it is a true coordination complex.

For comparison, let us take iron(III) chloride ($FeCl_3$). In its crystal lattice, a single iron atom is also surrounded by six chlorine atoms. However, there are only three chlorines in the formula unit itself. Therefore, $FeCl_3$ is not classified as a coordination compound, as it lacks discrete complex species.

Nomenclature rules

Names of coordination compounds typically consist of two words denoting the outer and inner spheres. The inner sphere is named according to a strict algorithm: first, the number of ligands is indicated using Greek prefixes (di-, tri-, tetra-, penta-), followed by the ligand name (a water molecule $H_2O$ is called aqua, ammonia $NH_3$ is called ammine), and finally, the central atom with its valence in Roman numerals.

There are three types of complexes, each governed by its own rules:

1. Anionic complexes (the complex carries a negative charge).

The outer-sphere metal cation is written first in the formula. The name of the inner sphere receives the suffix -ate, and the central atom uses its Latin name. Example: $Na[CuCl_2]$ — sodium dichlorocuprate(I).

1. Cationic complexes (the complex carries a positive charge).

In the formula, the inner sphere is placed first. In naming, it comes after the anion (outer sphere) name. The central atom is named in Russian using the genitive case. Example: $[Cu(NH_3)_4]SO_4$ — tetraamminecopper(II) sulfate.

1. Neutral complexes.

They completely lack an outer sphere. The name is written as a single word, and the valence of the central atom is omitted. Example: $[Zn(NH_3)_2Cl_2]$ — dichlorodiamminezinc.

Chemical reactions and properties

Two main types of chemical transformations are characteristic of the chemistry of coordination compounds.

1. Outer-sphere ion exchange reactions These proceed in aqueous solutions without disrupting the complex species itself. Only the ions outside the square brackets are replaced. Adding barium chloride to tetraamminecopper(II) sulfate triggers a classic exchange reaction: $$[Cu(NH_3)_4]SO_4 + BaCl_2 = [Cu(NH_3)_4]Cl_2 + BaSO_4\downarrow$$ This precipitates white barium sulfate. The violet-blue color of the solution remains intact because it is due to the $[Cu(NH_3)_4]^{2+}$ complex ion, which remains undamaged.

2. Ligand substitution reactions This is the process of replacing inner-sphere particles with others capable of forming stronger bonds with the central atom. For instance, in an aqueous solution, copper ions are bound to water, forming blue aqua complexes. Adding ammonia replaces water molecules with $NH_3$ molecules: $$[Cu(H_2O)_4]SO_4 + 4NH_3 = [Cu(NH_3)_4]SO_4 + 4H_2O$$ The aqua complex turns into an ammine complex. Visually, this is accompanied by a dramatic color change from blue to deep violet-blue.

Nature and biological role

The word itself derives from the Latin complexus — connected or combined. Typically, they form from simpler complex substances (which is also characteristic of acids or salts).

Coordination compounds are widespread in nature. Many minerals belong to this class based on their chemical structure. They play a critically important role in maintaining normal vital activity in both plants and animals.

Due to their specific chemical properties and ability to exchange ligands, these substances have found extensive practical applications. In modern practice, they are actively used as medicinal drugs and applied as industrial and laboratory dyes.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Determination of the coordination number of metals in complex compounds
• Specific ligand names in nomenclature
• Algorithm for writing the formula of a complex compound from its name
• Synthesis of zinc coordination compounds with ethylenediamine

Other topics in section «Chapter 3. Classification of Substances»

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.