Thiocyanate ion, SCN is an ambidentate ligand meaning it can bind to a metal ion

Question

Thiocyanate ion, SCN is an ambidentate ligand meaning it can bind to a metal ion through either the nitrogen end or the sulfur end. For example, thiocyante ion readily reacts with Pt(II) and Pt(IV) to form square planar complexes with four thiocyanate ligands attached to each metal ion. For each oxidation state of Pt, two equilibria can be written since SCN can coordinate through either the sulfur or nitrogen end. For Pt(II) Pt2+ + 4 SCN-_ Pt(SCN)42- Ks 2+ For Pt(IV) Pt 4 SCN Pt(SCN)4 Ks PU++ 4 SCN- Pt(NCS), KN where Ksand KN are the equilibrium constants for coordination through sulfur and nitrogen ends respectively. Explain why Ks > Ks for Pt(II), whereas Ks

Explanation / Answer

This can be explained on the basis of hard-soft acid-base (HSAB) principle. HSAB principle states that hard acids react with hard bases and soft acids react with soft bases to form stable compounds. According to HSAB theory, metals in their higher oxidation state are hard acids as compared to their lower oxidation states. So, in this case, the Pt(II) (soft acid) prefers to coordinate with sulfur atom which is a soft base compared to nitrogen. Conversly, the harder metal ion Pt(IV) will prefer to coordinate through the hard base nitrogen.

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