Question

OH^- and CN^- are both Brønsted bases, and both can form complexes with metal ions. Explain how OH^- can be a much stronger Brønsted base than CN^-, and at the same time much lower in the spectrochemical series.

Answer 1

Answer :

OH-can be a much stronger Brønsted base than CN-, and at the same time much lower in the spectrochemical series. This may be explained as below:-

You must know that basicity of a bronsted base is a measure of its affinity for protons, while the spectrochemical series is a measure of strength of its interaction with a transition metal ion in a metal-ligand complex. However, these are acid-base interactions, but still have some important differences.

The interaction between metal and ligand is sensitive to the degree of orbital overlap betweent metal d-orbitals and the electron pair on the ligand that forms the dative bond. The atom having high electronegativity such as oxygen have contracted orbital that overlap poorly with metal d-orbitals

.one more reason is in metal-cyanide complexes is π-backbonding, that involves electron donation from the metal d-orbital to the ligand π* orbital. The proton-base interaction is much less sensitive to the electronegativity of the ligand, since the proton is small and can share electrons even with a very electronegative atom. Thus oxygen-containing ligands such as OH- can be strong Brønsted bases but relatively weak ligands.

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