Coordination sphere

cis-[CoCl₂(NH₃)₄]⁺
The colored atoms form a coordination sphere around the ion of cobalt

In coordination chemistry, the coordination sphere refers to a central atom or ion and an array of molecules or anions, the ligands, around. ^{[1] [2]} Molecules that are attached noncovalently to the ligands are called the second coordination sphere.

Second coordination sphere

The second coordination sphere refers to the molecules that are attached noncovalently to ligands that occupy the first coordination sphere. These molecules are typically solvent. The interactions between the first and second coordination spheres usually involve hydrogen-bonding. For charged complexes, ion-pairing is important. Solvent effects are pronounced in complexes where the ligands in the first coordination sphere are strong hydrogen-bond donors and acceptors, e.g. respectively [Co(NH₃)₆]³⁺ and [Fe(CN)₆]^3-. Crown-ethers bind to polyamine complexes through their second coordination sphere.^[3]

Example

Hexamminecobalt(III) chloride
([Co(NH₃)₆]Cl₃)

In hexamminecobalt(III) chloride ([Co(NH₃)₆]Cl₃), the cobalt cation plus the six ammonia ligands comprise the first coordination sphere. The coordination sphere of this ion thus consists of a central MN₆ core "decorated" by 24 N-H bonds that radiate outwards.

In biological and computational chemistry

Mechanisms of metalloproteins often invoke modulation of the second coordination sphere by the protein. In solution, metal ions can be described as consisting of series of two concentric coordination spheres, the first and second. More distant from the second coordination sphere, the solvent molecules behave more like "bulk solvent." Simulation of this solvation is of interest in computational chemistry.

Role in mechanistic inorganic chemistry

The rates at which ligands exchange between the first and the second coordination sphere is the first step in ligand substitution reactions. In associative ligand substitution, the entering nucleophile enters via the second coordination sphere. These rates vary over many orders of magnitude. The energetics of many inner sphere electron transfer reactions are discussed in terms of second coordination sphere. Some proton coupled electron transfer reactions involve atom transfer between the second coordination spheres of the reactants:

[Fe*(H₂O)₆]²⁺ + [Fe(H₂O)₅(OH)]²⁺ → [Fe(H₂O)₆]³⁺ + [Fe*(H₂O)₅(OH)]²⁺

See also

• Coordination number
• Ligand cone angle
• Coordination geometry

References

1. ^ chemistry-dictionary.com - Definition of coordination sphere
2. ^ What Is A Coordination Compound?
3. ^ Lehn, J. M. Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinhiem, 1995.