How does an increase in atomic radii affect coordination numbers of complexes?

An increase in atomic radii can lead to higher coordination numbers in coordination complexes due to the greater spatial availability for surrounding ligands to fit around the central metal ion. As the size of the central metal atom increases, it can accommodate more ligands because there is more surface area available for bonding interactions. This is particularly true for metal ions that are larger in size, allowing them to coordinate with more ligands simultaneously.

Additionally, as atomic radii increase, the weakening of ligand-metal ion interactions typically occurs; however, larger metal ions often still possess the ability to interact with multiple ligands effectively. This capacity contributes to forming complexes with higher coordination numbers, especially in certain geometries that promote such coordination, like octahedral or square planar arrangements.

In the case of smaller metals, the limited space might restrict the number of ligands that can approach closely to form stable bonds. Therefore, the trend is generally observed that as the atomic radius grows, the coordination number can increase, aligning with the versatility and spatial arrangement of the ligands available for bonding. This relationship highlights the significance of atomic size in determining the architecture of coordination complexes.