Coordination sphere

The central metal atom or ion in a coordination compound together with the ligands directly bonded to it. It is commonly written inside square brackets, distinguishing it from counterions outside the brackets.

Connect

Coordination complex: A chemical species consisting of a central atom or ion bonded to surrounding molecules or ions called ligands. The coordination sphere is the metal-and-ligand core of a coordination complex.
Crystal field theory: A model that explains metal-ion orbital energies by treating ligands as electrostatic fields. It connects the identities and arrangement of sphere ligands to metal d-orbital splitting.
Werner's theory: Alfred Werner’s model distinguishing primary ionizable valence from secondary coordination valence in metal complexes. It established the distinction between inner coordination and ions outside the sphere.
Counterion: An ion accompanying an oppositely charged ion to maintain electrical neutrality. Counterions can accompany a complex without belonging to its coordination sphere.
Ligand: An atom, ion, or molecule that donates electron density to a central atom or ion through a coordinate bond. Ligands directly bonded to the central metal are included in the sphere.
Ligand field theory: A bonding model that combines crystal-field ideas with covalent metal–ligand interactions. It explains how the sphere’s bonds affect electronic structure more fully than a purely electrostatic model.
Cis–trans isomerism: Geometric isomerism in which substituents occupy adjacent or opposite positions around a central atom. Different ligand arrangements within a sphere can produce distinct cis and trans compounds.
Outer-sphere complex: An association of reactants in which no direct bond forms between their central metal centers and incoming ligands. Outer-sphere interactions occur without changing the metal’s directly bonded ligand set.
Coordination number: The number of donor atoms directly bonded to a central atom in a coordination complex. It counts the metal–donor-atom contacts within the sphere.
Chelation: Binding of a metal ion by a ligand that attaches through multiple donor atoms. Multidentate ligands occupy several sites within one coordination sphere.