Why do you start with the least polar solvent/solvent mixture and progress to increasing polar solvent/solvent mixtures when eluting the ferrocene compounds from the column rather than starting with more polar solvent system and progressing to less polar solvents

Generally in normal phase column, inside the column silica gel present is in normal phase and it strongly reacts or combine with more polar ferrocene compounds. Whereas, less polar ferrocene compounds weakly interact with this silica gel.

Hence, on starting with least polar solvent or solvent-mixture the less polar ferrocene compounds will tend to elute first of all because these will weakly interact with the silica gel. Therefore, readily able to move away from the gel.

On increasing the polarity of solvent or solvent mixture, more polar ferrocene compounds will start to elute first. As a result, on changing the solvent polarity we can (from least to more polar) it is possible to get pure products.

But if we start with more polar solvent at first then almost all of the ferrocene compounds will elute leading to poor separation from the column due to which you will end up having a mixture of ferrocene compounds.