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.
The following structures represent examples for trigonal planar geometries.Main group geometries with 3 bonding electron pairs are trigonal planar
| BF3 | tert-butyl cation |
[HgI3]- |
CuBr(P(C6H5)3)2 |
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 |
|
| 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 | |
The T-shaped geometry is only observed for hypervalent main group elements in compounds such as iodine trichloride, chlorine trifluoride etc.