Question

Dimethyl sulfoxide (DMSO) can act as a ligand in transition metal chemistry. For an Fe3+ complex, would you expect the DMSO ligand to bind through the sulfur atom or the oxygen atom. Rationalize using Hard Soft Acid Base Theory.

Answer 1

Fe^3+ is a hard acid since it has small ionic radius, high positive charge with empty d-orbitals

In DMSO, two donar sites are present one is Sulfur and another is oxygen. Since Oxygen is hard base due to small size and high electronegitivity and Sulfur is weak base due to large size and low electronegativity.

Thus according to HSAB principle, hard acids prefer binding to the hard bases to give ionic complexes, whereas the soft acids prefer binding to soft bases to give covalent complexes.

Hence Fe^3+ preferentially binds with DMSO through Oxygen atom.