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 yante ion readily reacts with Pt(l) and the nitrogen end or the sulfur end. For example, thiocy 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. Pt2++ 4SCNT pe+ + 4 SCN Pt(SCN)42- Pt(NCS),2- For Pt(II) Ks KN For Pt(IV) Pt4+ 4 SCNPt(SCN)4 Ks Pta++ 4 SCN- Pt(NCS), KN where Ks and Kvare the equilibrium constants for coordination through sulfur and nitrogen en respectively. Explain why Ks > Kv for Pt(II), whereas Ks

Explanation / Answer

The binding of Pt with SCN- depends upon he oxidation state of metal.
This binding is governed by HSAB principle i.e. Hard acid binds strongly with hard base and vice versa.
Now Pt(II) is a soft acid and thus should bind strongly with soft base i.e. binding will be via S in SCN-.
whereas Pt(IV) is hard acid because by loss of 4 electrons, the tendency to form bond with electronegative ligand increases drastically and thus forms bond through N in SCN-.
This explains why Ks > Kn for Pt(II) and Ks < Kn for Pt (IV).

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