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Coordination Number 2

As in the case of C.N.1 , compounds with the coordination number 2 are very common for main group elements but are less common for transition metal complexes. Two gometry types are possible: Linear and bent.

-Linear B-A-B

This first possibility includes examples in main group chemistry that have this geometry due to the electron pair configuration:


No lone pairs (e.g. BeF2 in the gasphase, CO2)
Total number of electron pairs of 5 and 3 lone pairs (e.g. I3-, KrF2)
In the case of the transition metals a coordination number of 2 occurs with small ligands for electronic reasons, e.g with a d10 electron configuration. Examples for these complexes include the Ag(I), Cu(I), Au(I) and Hg(II) complexes Ag[NH3]2+ (the silver diammine complex that is formed, when AgCl is dissolved in ammonia), [CuCl2]-, [AuClPR3] (phosphine gold(I) chloride complexes), and [Hg(CH3)2] (dimethylmercury)  
The other possibility to get two-coordinate complexes is to make the ligands very sterically demanding so that no other ligand can coordinate to the transition metal, as shown in the complex bis(bis(methyldiphenylsilyl)amido-N)iron(II) (Reference: Bartlett, R. A. Power, P.P. J. Am. Chem. Soc., 109, 1987, 7563) below.
N-Fe-N Formula  

The partial structure is shown here





- Bent B-A-B

Bent geometries can be a result of lone pairs in main group chemistry.As a result of the steric demand of nonbonding lone pair electrons, these need room but are not visible in the structure. For example in the case of water with 4 electron pairs, two of which are lone pairs, a bent molecular geometry is the result.

In transition metal chemistry just based on steric demand, most cases the ligand-metal-ligand angle should be close to 180°. However, there are several examples where this is not the case (see below).The only bonds that are "real" are the ones between Cr and N as seen in the picture below. Reference: Bartlett, R.A.; Chen, H.; Power, P.P. Angew. Chem. Int. Ed. Engl. 88, 1989, 316.

N-Cr-N Formula
N-Cr-N Picture Enlarged chromium geometry


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Updated: December 2005