Coordination number of Coordination compounds

p-Block Elements: Coordination compounds

Coordination number

The coordination number for a metal ion in a complex is the number of ligands that are directly attached to it. Mostly coordination numbers of 4 and 6 are observed, and 5 is of considerable interest, though coordination numbers can range from 2 to 12 and beyond.

Coordination number 2

The stoichiometry of these compounds is ML₂. Examples: Cu(NH₃)₂⁺, AgCl₂^– , Au(CN)₂ ^– , HgCl₂

These are linear and the ligand-metal-ligand bond angel is 180° with minimum ligand-ligand repulsion; Cl – Ag – Cl. These metal ions have d10 configuration in the ground state. Even though the metal-ligand bond may be considered to be formed due to the overlap of a σ-orbital of the ligands with sp-hybridized metal orbital, actually some d-orbital contribution is also there in the bond formation.

Coordination number 3

It is a rare coordination number. In many crystalline compounds, the stoichiometry may be ML₃but the actual coordination number may be greater than 3. When the ligands are extremely bulky, this coordination number exists. In some of the d¹⁰ systems, this coordination number exists, even when the ligands are not bulky. Examples: KCu(CN)₂, Pt(PPh₃)₃ etc. In such complexes, the metal atom and the ligands directly connected to it lie on the same plane.

Coordination number 4

This is one of the most common and important coordination numbers. A metal ion can connect with four lone pair in two shapes:

Tetrahedral:

If the central metal atom is not having any lone pair of electrons and if it is not a transition metal, then it will form a tetrahedral complex. BeCl₄^2- , BF₄^– , SnCl₄, AlF₄^– etc.

On the other hand, if the central metal atom is a transition metal but it does not have a d8 configuration, then it forms a tetrahedral complex. e.g. the tetrachlorocopper(II) ion, [CuCl₄] ^2-,FeCl₄^– , CoCl₄^2- , MnO₄ ^– etc.

When the central metal atom hasd⁸ configuration, usually it will form a square planar complex but sometimes it can also choose to obtain a tetrahedral complex structure. Example: NiCl₄^2- , NiCl₃OPPh₃. When the ligands are bulky, Co(I) and Ni(II) form tetrahedral complexes.

Square planar:

Square planar,If the central metal ion has a d8 configuration, mostly it will form square planar complexes e.g. the tetrachlorogold(III) ion, [AuCl₄] ^– Ni(CN)₄^2-, PdCl₄^2-, PtCl₄ ^2- etc., where the internal bond angles are all 90°.

Coordination number 5

Two shapes are

1. Trigonal bipyramid, e.g. pentachloro-cuprate(II), [CuCl₅] ^3-
2. The square pyramid, e.g. pentacyano-nickelate(II), [Ni(CN)₅] ^3-

The distortion of a trigonal bipyramidal structure creates a square pyramidal structure. Examples: CdCl₅^3- has a trigonal bipyramidal structure. Ni(CN)₅^3- has got a square pyramidal structure.

Coordination number 6

Complexes of the type, MA₆ has octahedral coordination and the symmetry is Oh have a coordination number 6. Other complexes of the type, MA₅B, MA₄B₂, etc. are also known as octahedral complexes. There is only one shape available for a metal that is enclosed by six lone pairs – octahedral, e.g. the Hexa-aquocopper(II) ion, [Cu (H₂O)₆] ²⁺