The total number of points of attachment to the central element is termed the coordination number and this can vary from 2 to as many as 16, but is usually 6.

Coordination Number 2 sp=dd

This arrangement is not very common for first row transition metal ion complexes and some of the best known examples are for Silver(I).
For instance, a method often employed for the detection of chloride ions involves the formation of the linear diamminesilver(I) complex.
The first step is:

	Ag+    +    Cl-      ->   AgCl (white ppt)

and to ensure that the precipitate is really the chloride salt, two further tests must be done:

 AgCl     +   2NH3     ->  [Ag(NH3)2]+   +  Cl- and 

 Cl- + [Ag(NH3)2]+ + 2HNO3 -> AgCl(re-ppts) + 2NH4NO3 

The reaction of a bidentate ligand such as 1,2-diaminoethane with Ag+ does not lead to chelated ring systems, but instead to linear two coordinate complexes. One reason for this is that bidentate ligands can NOT exist in trans arrangements that is can NOT span 180 degrees.

Coordination Number 3

Once again, this is not very common for first row transition metal ions. Examples with three different geometries have been identified:

Trigonal planar - sp²

Well known for main group species like CO₃^2- etc, this geometry has the four atoms in a plane with the bond angles between the ligands at 120 degrees.

Trigonal pyramid - sp³ 

More common with main group ions.

T-shaped ddd

The first example of a T-shaped molecule was found in 1977.

Coordination Number 4

Two different geometries are possible. The tetrahedron is the more common while the square planar is found almost exclusively with metal ions having a d⁸ electronic configuration.

Tetrahedral sp³

The chemistry of molecules centred around a tetrahedral C atom is covered in organic courses. To be politically correct, please change all occurrences of C to Co. There are large numbers of tetrahedral Cobalt(II) complexes known.

Square Planar dddd

This is fairly rare and is included only because some extremely important molecules exist with this shape.

Coordination Number 5

Square pyramid ddddd

Trigonal Bipyramid sp²d²

The structure of [Cr(en)₃][Ni(CN)₅] 1.5 H₂O was reported in 1968 to be a remarkable example of a complex exhibiting both types of geometry in the same crystal.
The reaction of cyanide ion with Ni²⁺ proceeds via several steps:

	Ni2+         +   2 CN-    ->   Ni(CN)2 

        Ni(CN)2      +   2 CN-  ->   [Ni(CN)4]2- 
                                         orange-red 

                                      log(beta4) = 30.1 

        [Ni(CN)4]2-  +     CN-     ->   [Ni(CN)5]3- 
                                          deep red

Oxovanadium salts (Vanadyl, VO²⁺) often show square pyramidal geometry, for example, VO(acac)₂. Note that the Vanadium(IV) can be considered coordinatively unsaturated and addition of pyridine leads to the formation of an octahedral complex.

Coordination Number 6

Hexagonal planar

Unknown for first row transition metal ions, although the arrangement of six groups in a plane is found in some higher coordination number geometries.

Trigonal prism

Most trigonal prismatic compounds have three bidentate ligands such as dithiolates or oxalates and few are known for first row transition metal ions.

Octahedral  sp³d²

The most common geometry found for first row transition metal ions, including all aqua ions.
In some cases distortions are observed and these can sometimes be explained in terms of the Jahn-Teller Theorem.

Coordination Number 7

Three geometries are possible:

Capped octahedron

Capped trigonal prism

Pentagonal Bipyramid

Coordination Number 8

Dodecahedron

Cube

Square antiprism

Coordination Number 12

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Created March 1996. Last modified 13th March-99.
URL http://wwwchem.uwimona.edu.jm:1104/courses/IC10Kcn.html