Question

The cis and trans isomers of [Cr(CO)4(PPh3)2] are loaded onto a SiO2 column and eluted with ether, which isomer will elute first and why

Answer 1

Explanation:'

Cis and Trans metal carbonyls isomers will have a different type of ligands orientation. That means that the van der Waals dispersion forces (intermolecular attraction) between molecules will be different. In general, Cis isomer is more polar than Trans isomer (non-polar).

In the case of normal phase chromatography where the stationary phase is polar (SiO₂) and the mobile phase is non-polar (petroleum ethers), The non-polar compounds usually elutes first because of a strong association between analyte with mobile phase (ether).

In our problem, the Trans isomer is non-polar will get elute first using ether as mobile phase, and upon increasing mobile phase polarity (while adding MeOH solvent) the high polar cis compounds elutes last.