What is the favored geometry for four-coordinate complexes with small cations and large ligands?

In coordination chemistry, the favored geometry for four-coordinate complexes often depends on the size and charge of the central metal cation in relation to the ligands surrounding it. For small cations paired with large ligands, tetrahedral geometry is typically favored.

This is largely because tetrahedral coordination allows for optimal spatial arrangement where the large ligands can be accommodated without significant steric hindrance. The tetrahedral geometry also allows for the ligands to be positioned at angles of approximately 109.5 degrees from each other, which can minimize repulsive interactions among them, especially when the ligands are bulky.

In contrast, square planar geometry may lead to increased steric crowding or unfavorable interactions in cases where large ligands are involved, as that geometry would require the ligands to occupy a more crowded arrangement. Therefore, while it can occur in certain cases with larger metal cations, it's not generally favored for small cations with large ligands.

Overall, the tetrahedral arrangement in four-coordinate complexes is particularly effective in maximizing the efficiency of binding while maintaining stability and minimizing steric clashes, making it the most favorable geometry given the criteria specified.