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Discussion

Consider the following pairs of complexes

[CoF(NH3)5]2+ and [Cr(OH2)6]2+
[Co(NH3)5 (OH2)]3+ and [Cr(OH2)6]2+
[Co(NH3)6]3+ and [Cr(OH2)6]2+
[CoI(NH3)5]2+ and [Cr(OH2)6]2+

The electron transfer rate will be fastest in the pair

  • A.[CoF(NH\(_{3}\))\(_{5}\)]\(^{2+}\) and [Cr(OH\(_{2}\))\(_{6}\)]\(^{2+}\)
  • B.[Co(NH\(_{3}\))\(_{5}\) (OH\(_{2}\))]\(^{3+}\) and [Cr(OH\(_{2}\))\(_{6}\)]\(^{2+}\)
  • C.[Co(NH\(_{3}\))\(_{6}\)]\(^{3+}\) and [Cr(OH\(_{2}\))\(_{6}\)]\(^{2+}\)
  • D.[CoI(NH\(_{3}\))\(_{5}\)]\(^{2+}\) and [Cr(OH\(_{2}\))\(_{6}\)]\(^{2+}\)

Answer: D

The fastest electron transfer is between [CoI(NH3)5]2+ and [Cr(OH2)6]2+.

Cr2+(aq) is labile, and it reduces Co(III) pentaammine complexes by Taube's inner-sphere mechanism: the ligand X on cobalt bridges the two metals in Co-X-Cr, the electron passes through it, and X ends up on chromium. The rate depends on how well the bridge transmits the electron, and for these halide complexes the measured rate constants rise in the order F- < Cl- < Br- < I-: the large, polarisable iodide is the best bridge.

[Co(NH3)6]3+ has no lone pair available to bridge, so it can react only by the much slower outer-sphere route. Coordinated water is a poor bridge too, and fluoride, though it bonds strongly to Cr(III), transmits the electron less efficiently than iodide.

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