Consider the following series of substitution reactions. [Co(NH_3)_6]^2+ + CN^-
ID: 976820 • Letter: C
Question
Consider the following series of substitution reactions. [Co(NH_3)_6]^2+ + CN^- rightarrow [Co (NH_2)_5(CN)]^- + NH_3 220 [Co(NH_3)_5(CN)]^+ + CN^- rightarrow [Ci(NH_3)_4(CN)_2] + NH_3 110 [Co(NH_3)_4(CN)_2 + CN^- rightarrow [Co(NH_2)_3(CN)_4]^2+ + NH_3 10 [Co(NH_3)_2(CN)_4]^2- + CN^- rightarrow [Co(NH_3)(CN)_5]^3- + NH_3 650 [Co(NH_3)(CN)_5]^3- + CN^- rightarrow [Co(CN)_6^4- + NH_3 370 Write out the overall net reaction. Explain the trend in the equilibrium constant data for reactions I through IV. Explain why the equilibrium constant changes the way it does from reaction IV to V.Explanation / Answer
[Co(NH3)6]2++ 6 CN- ------> [Co(CN-)6]4- + 6NH3
2. Co2+ has a d7 configuration and hence its octahedral complex is subjected to a Jahn- Teller distortion. [Co(NH3)6]2+ with a high spin d7 configuration exhibits a weak distortion and thus a moderately labile complex. The K values for the substitution reactions are thus moderate. Upon substitution of each NH3 with CN-, the number of NH3 groups left to be replaced, decreases. Thus statistically speaking, fewer NH3 ligands are available to be replaced and hence the reaction becomes slower for each subsequent substitution. This explains the decrease in the K values from I-IV.
3. Also note that we are replacing a weak field ligand (NH3) with a strong field ligand (CN-)
Upon completion of step IV, we get the complex [Co(NH3)2(CN-)4]2- , which has a centre of symmetry and has a d7 configuration in a strong field. Thus we have a low spin d7 configuration which exhibits a strong Jahn Teller distortion and hence is unstable and undergoes elongation of bonds along z-axis. This makes the M-NH3 bond weaker and hence it is easily substituted. Thus the K value increases in this case as compared to the previous step IV.