Q30 · CSIR-NET Chemistry, June 2014

Paper: CSIR-NET June 2014 · Subject: Inorganic Chemistry · Chapter: Coordination Chemistry · Topic: Electron Transfer Redox · Marks: 2 · Difficulty: Medium

Consider the second order rate constants for the following outer-sphere electron transfer
\[ \begin{array}{l} \left[\mathrm{Fe}(\mathrm{H}_2\mathrm{O})_6\right]^{3+} / \left[\mathrm{Fe}(\mathrm{H}_2\mathrm{O})_6\right]^{2+} \quad 4.0\ \mathrm{M^{-1}\,sec^{-1}} \\ \left[\mathrm{Fe(phen)_3}\right]^{3+} / \left[\mathrm{Fe(phen)_3}\right]^{3+} \quad 3.0\times10^{7}\ \mathrm{M^{-1}} \\ (\text{phen} = 1,10\text{-phenanthroline}) \end{array} \]
The enhanced rate constant for the second reaction is due to the fact that
(a)The 'phen' is a $\pi$-acceptor ligand that allows mixing of electron donor and acceptor orbitals that enhances the rate of electron transfer
(b)The phen' is a $\pi$-donor ligand that enhances the rate of electron transfer
(c)The 'phen' forms charge transfer complex with iron and facilitates the eletron transfer
(d)The 'phen' forms kinetically labile complex with iron and facilitates the electron transfer.
Answer
Answer: A ✓ checked by 4AB · confidence high
Explanation
The π-acceptor phenanthroline delocalises the metal t₂g electrons onto the ligand, giving good donor–acceptor orbital overlap between the two complexes and a small reorganisation energy — a fast outer-sphere exchange.

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