The rate of reduction of [Co(NH_3)_5 (H_2 O)]^3+ by Cr(II) is seven orders of ma
ID: 1070213 • Letter: T
Question
The rate of reduction of [Co(NH_3)_5 (H_2 O)]^3+ by Cr(II) is seven orders of magnitude slower than the reduction of its conjugate base [Co(NH_3)_5 (H_2 OH)]^2+ by Cr(II). What do these observations suggest about tire mechanisms for these reactions? Your answer should include sketches or descriptions of proposed intermediates. When the same compounds are reduced by [Ru(NH_3)_6]^2+ instead of Cr(II) the rates differ by less than a factor of 10. What do these observations suggest about the mechanisms? Why do the Ru and Cr compounds behave differently? Consider the following data for a ligand exchange reaction: [Ti(H_2 O)_6]^3+ + Y^- rightarrow [Ti(H_2 O)_5 Y]^2+ + H_2 O What do the data suggest regarding the mechanism of ligand exchange for this reaction? Is this conclusion in agreement with Taube's rules? Explain. It has been observed that V(CO)_6 reacts with PPh_3 to give V(CO)_5 PPh_3 much more rapidly than the analogous reaction of Cr(CO)_6. Explain this difference inExplanation / Answer
The rate of reduction of [Co(NH3)5(H2O)]3+ is seven times slower than the rate of reduction of [Co(NH3)5(OH)]2+ with Cr(II).
(a) In case of [Co(NH3)5(OH)]2+ complex, the OH- ion is capable of forming a bridge between the complex it is present in and the approaching metal Cr(II). This is a type of inner sphere complex mechanism,
[Co(NH3)5(OH)]2+ + Cr(II) ---> [Co(NH3)5-OH-Cr(II)]4+
The electron transfer thus occurs much more easily in this complex and rate is much higher.
Such a bridging property does not exist for the [Co(NH3)5(H2O)]3+, H2O ligand and hence the rate of reduction is much slower in this case. The electron transfer occurs here by an outer sphere pathway.
(b) When reduction is done with [Ru(NH3)6]2+, with Ru2+ in d6 configuration, a low spin state, the resultant ligand field stabilization energy is very large which gets partially reduced by exchange of ligands. This thus makes the reducing complex of Ru2+ inert. An inert Ru2+ complex is unable to form a bridged complex structure as was observed in case of Cr(II). Electron transfer thus occurs through an outer sphere mechaism and hence the rate of reaction difference is not that large.
(c) The difference in reactivity of the Cr(II) and Ru(II) comes from the their electronic configuration. While Cr(II) is a high spin d4 ion which forms long and weak axial bonds which can easily be replaced with other liagnds, Ru(II) is a low spin d6 ion, which does not form weak bonds and therefore ligands cannot replace it quite easily.
(d) The drastic change in rate of reaction upon changing the ligand in the substitution reaction, shows that the mechanism of substitution followed here would be an associative SN2 type of substitution. The transition state is formed and is greatly affected by the approaching ligand in determining the energy of transition state for a favorable and unfaourable forwards reaction to occur.
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