Main Answer: The geometries commonly observed in coordination complexes of transition metal ions with a d8 electron configuration are square planar and octahedral.
Explanation: In a d8 electron configuration, there are eight electrons in the d-orbitals of the transition metal ion. Coordination complexes can have different geometries based on the arrangement of ligands around the metal center. For d8 transition metal ions, the two most common geometries are:
1. Square planar: This geometry consists of four ligands arranged in a square around the metal ion, with the metal ion at the center. This arrangement results in lower energy due to the absence of electron repulsion between the ligands and the d-orbitals. Examples of square planar complexes include [Ni(CN)4]2- and [Pt(NH3)2Cl2].
2. Octahedral: In this geometry, six ligands are arranged around the metal ion, forming an octahedron. This is another common geometry for d8 transition metal ions as it provides a stable arrangement of ligands. Examples of octahedral complexes include [Co(NH3)6]3+ and [Fe(CN)6]3-/4-.
Conclusion: The square planar and octahedral geometries are the most common arrangements observed in coordination complexes of transition metal ions with a d8 electron configuration due to their stability and minimal electron repulsion.