For coordination number 4 there are 2 basic geometries, tetrahedral and square planar, and distorted versions thereof. Some main group compounds also adopt the seesaw type structure. The tetrahedral geometry is also the the main geometry for carbon with C.N. 4.
| Typically, oxoanions like permanganate (MnO4-) and other compounds with metal-oxygen or metal nitrogen multiple bonds (e.g. OsO4) have tetrahedral geometries. Another class of compounds are the halo-anions of Ni(II), Co(II), Cu(II) and Fe(II)/Fe(III). | |
Methane |
FeCl4- |
Cu(bipy)4+ |
OsO2(NtBu)2 |
| The square planar rearrangement is preferred for d8 electron complexes of the second and third row and sometimes 1st row, e.g. Au(III), Pt(II), Ir(I). Sometimes the square planar eometry can be distorted, as in the Pd complex shown on the right below, due to steric demand of the ligands. | |
Cisplatin Pt(NH3)2Cl2 |
Pt(C6H12(NH2)2)Cl2 |
Ni(bipy)Cl2 |
Pd(bipy)2 Slightly distorted from square planar because of the steric interaction of the two bipy units |
SF4 is an example for this geometry.