Question

1. Put these complexes in order of increasing rate of substitution by H₂O: [Ni(NH₃)₆]²⁺, [Co(NH₃)₆]³⁺, [Rh(NH₃)₆]³⁺

2. State the effect of the following changes on the rate of reactions of octahedral Rh(III) complexes.

a) Increase in charge of the complex.

b) Changing the spectator ligands from NH₃ to PPh₃.

c) Changing the entering group from Cl^- to I^-.

Answer 1

1) As size of metal ion decreases, the strength of metal-ligand bond increases and it becomes difficult to replace the ligand and hence the rate of substituiton decreases with decrease in size of metal ion. Thus, the size metal ions are Rh^3+ > Ni^2+ > Co^3+

The increasing order of rate of substitution by H₂O:[Rh(NH₃)₆]³⁺ > [Ni(NH₃)₆]²⁺ > [Co(NH₃)₆]³⁺

2) a) Increasing the charge of complex, size of ion decreases and hence rate of reaction decreases

b) Changing the spectator ligand does not show any effect on rate of reaction

c) Changing the entering group from Cl^- to I^- increases the rate of reaction because I- is a better nucleophile than Cl-