The position of ligands in the spectrochemical series depends on a few important factors:

1. Electronegativity: This is a measure of how strongly an atom can attract electrons. Ligands that have higher electronegativity usually cause stronger field splitting. For example, the fluoride ion ($F^-$) is a weak field ligand, while the cyanide ion ($CN^-$) is a strong field ligand.

2. Size and Charge: Smaller ligands that are highly charged create stronger interactions. This means they have a greater effect on the field strength. For instance, ammonia ($NH_3$) is stronger than water ($H_2O$) even though both are neutral.

3. π-acceptor ability: Some ligands can accept electrons back from the metal they are attached to. Examples include carbon monoxide ($CO$) and phosphines ($PR_3$). These ligands show stronger effects because they overlap better with the metal's d-orbitals.

The way ligands are ranked affects something called crystal field splitting energy ($\Delta$). This is very important for understanding how electrons move in complex compounds.