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Coordination Number 3

The coordination number 3 is also relatively rare for transition metals. It occus in the case of d10 complexes and when bulky ligands are present. Most transition metal examples are trigonal planar, some with a slight tendancy for pyramidalization.

- Trigonal PlanarAB3

The following structures represent examples for trigonal planar geometries.Main group geometries with 3 bonding electron pairs are trigonal planar

BF3 tert-butyl cation
Depending on the ligands and the ligand arrangement, structures can be distorted and deviating from the ideal trogonal shape as can be seen in the following bipyridine gold complex

- Trigonal Pyramidal


Typically, main group elements from trigonal pyramidal geometries (e.g. ammonia, trialkylphosphines). But transition metal compounds can have pyramidalization as well. The following example is a gold compound (tris(triphenylphospine gold(I) cation) that has slightly pyramidal structure. This can be better seen in the structure on the right, where all the carbon and hydrogen atoms are left out.
NH3 tris(triphenylphospine gold(I) cation
Partial geometry showing the gold coordination


- T-Shaped AB3 T-shaped

The T-shaped geometry is only observed for hypervalent main group elements in compounds such as iodine trichloride, chlorine trifluoride etc.



©2005 Paul Kiprof, All Rights Reserved
Updated: December 2005