- Most of the members in the series are ligands where the donor atom is nitrogen, oxygen or carbon.
If we consider only those can a parallel bases on the HSAB principle be drawn i.e. carbon which is softer and more polarisable than nitrogen generally is the donor atom in strong field ligands like CO or CN–. Similarly, it is followed by ligands having nitrogen, which is comparatively softer than oxygen but harder than carbon.
- It may be argued that the halogens like iodine and bromine which are very soft are at the end but then they are down the group and also in different periods.
- Although the series is dominated by C, N and O donor atoms, they are in the same period. Similarly, PPh3 where phosphorus is the donor atom is a stronger field ligand than the comparatively harder sulpur atom (in ligands like SCN– or S2-) in the same period.
Can such a parallel be drawn?