Complex compound architecture
Complex (or coordination) compounds have a characteristic two-tier structure. The basis of any such substance is the inner sphere (the complex part). In writing, it is traditionally enclosed in square brackets `[ ]`. This sphere includes the central atom and the ligands surrounding it. The inner sphere can carry its own electrical charge, in which case it is called a complex ion.
Outside the square brackets lies the outer sphere. It is formed by ions whose main task is to neutralize the charge of the inner part. In chemical processes, outer-sphere ions behave in the exact same way as the constituent parts of familiar simple salts, acids, or bases.
Classification of complexes by outer sphere
Depending on which specific ions form the outer sphere, coordination compounds belong to various classes of substances. If the complex itself is electroneutral (i.e., the inner sphere has no charge and is a molecule), such a compound has no outer sphere at all. A striking example of an electroneutral substance is the iron complex with carbon monoxide(II) — $[Fe(CO)_5]$.
In other cases, the type of substance is determined by the nature of the outer ions:
| Outer-sphere ions | Substance class | Complex characteristics |
|---|---|---|
| H⁺ | Acids | Complex acids |
| OH⁻ | Bases | Complex bases |
| Metal cations, ammonium | Salts | Anionic complexes (negative internal charge) |
| Acid residue anions | Salts | Cationic complexes (positive internal charge) |
Ligands and central atom
Metal or non-metal cations usually act as the central atom. Ligands are particles directly bonded to this center. They can be:
- Halogen ions ($F^-$, $Cl^-$, $Br^-$, $I^-$).
- Hydroxo groups ($OH^-$) and cyano groups ($CN^-$).
- Thiosulfate groups ($S_2O_3^{2-}$).
- Organic acid anions.
- Ammonia molecules ($NH_3$) and organic amines.
In complex, polyatomic ligands, bonding with the complexing agent occurs through specific donor atoms: nitrogen (N), oxygen (O), sulfur (S), or carbon (C).
Special stability is imparted to complexes by the ability of certain ligands to form multiple bonds with the central atom simultaneously. This phenomenon provides enhanced compound strength (the chelate effect). For example, an ammonia molecule forms a single bond (monodentate ligand). However, an ethylenediamine molecule ($NH_2CH_2CH_2NH_2$) contains two nitrogen atoms, each binding to the center, acting as a bidentate ligand. Therefore, given an equal total number of bonds, the copper complex with ethylenediamine $[Cu(NH_2CH_2CH_2NH_2)_2]SO_4$ will differ in structure from the ammonia complex $[Cu(NH_3)_4]SO_4$.
Difference from simple compounds
It is important to understand that merely having a central atom does not make a substance a coordination compound. The main criterion is the ratio of the coordination number to the valence.
For example, in a sulfuric acid molecule ($H_2SO_4$), the sulfur atom is in the center, surrounded by other atoms. However, this is an ordinary, not a complex compound. The reason is that sulfur's coordination number in this case is 4, which is less than its valence of 6.
Briefly: answers to topical questions
What is the coordination number of the central atom?
The coordination number of the central atom is the number of atoms or groups (ligands) directly bonded to it. This value is an important characteristic of a coordination compound, determining the number of chemical bonds between the central atom and the ligands in the molecule or crystal structure.
Common coordination numbers:
- Frequent — 4 and 6.
- Rare — 2, 3, 8, and others.
The formation of a coordination compound occurs if the coordination number of the central atom exceeds its valence.
What is the spatial geometry of complexes with a coordination number of 6?
The spatial geometry of complexes with a coordination number of 6 is an octahedron. In such compounds, ligands are located at the vertices of an imaginary octahedron around the central atom.
Features of octahedral complexes:
- d-block metals — form 6 identical orbitals due to $d^2sp^3$ hybridization, directed from the center to the vertices of the octahedron.
- Silicon — resides in an octahedral environment in hexafluorosilicates due to the presence of a vacant 3d sublevel.
This geometry is characteristic of numerous coordination compounds.
Square brackets are the «house» of the complex. What is inside the house is the inner sphere (host center and its guest ligands). What remains outside is the outer sphere (ions).
Class and exam questions
Is every substance with a central atom a complex compound?
No. For example, sulfuric acid has a central sulfur atom, but is not a complex because sulfur's coordination number (4) is less than its valence (6).
Can a coordination compound lack an outer sphere?
Yes, it can. This happens if the inner sphere has no electrical charge (is an electroneutral molecule), as in the case of $[Fe(CO)_5]$.
Why are some coordination compounds more stable than others?
Stability increases sharply if ligands form two or more bonds with the central atom. This phenomenon is called the chelate effect.
What else to study on this topic
- The ratio of the coordination number to the valence of the central atom.
- The nature of donor atoms in polyatomic ligands (N, O, S, C).
- The mechanism of the chelate effect and ligand denticity.
- The chemical behavior of outer-sphere ions in solutions.
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Other topics in section «Chapter 3. Classification of Substances»
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