Question

time-10 mimdes) Cer Question 10 he mickel and copper Explain the general trends in the stepwise stability constants for t complexes indicated in Figure 1. Mention the specific changes in the respective values for log Ks. (a) 0 Cu" 6 Figure 1. Stepwise stability constants for the displacement of H:O by NHs [Ni(0H2%)2+ and [Cu(OH2%)2+. Suggest reasons to explain and rationalise the relative log K values for the following reaction where X is a halide: (b) For X - F, log K-0.7 For X- Br, log K--0.6 For X = 1, log K =-1.3

Answer 1

Part (a).

Here, NH₃ is a hard ligand.

Copper metal (Cu) is harder than nickel metal (Ni). Hence, the first four stability constants of Cu-NH₃ binding are higher than that of Ni-NH₃ binding.

The fact is that due to the smaller size of copper than nickel, copper can form up to 4-coordinate complexes with NH₃ with a steady decrease in stability constant (K), but it is difficult for copper to form 5-coordinate complexes, etc. Whereas due to the larger size of nickel than copper, nickel can also form a 5-coordinate complex with NH₃ with higher stability constant than that of Cu and Ni can also form 6-coordinate complexes with NH₃.

Log(K) value for the 5-coordinate [Ni(NH₃)₅]²⁺ ~ 1.0 and that for the 5-coordinate [Cu(NH₃)₅]²⁺ ~ -0.5

Part (b).

Zn²⁺ is hard metal ion.

The decreasing order for the hardness of halide ions can be written as follows.

F^- > Cl^- > Br^- > I^-

Due to the stable hard-hard interaction between Zn²⁺ and F^-, the stability constant (K) and hence Log(K) is the highest for X = F, and it gradually decreases for Zn²⁺-Cl^-, Zn²⁺-Br^-, Zn²⁺-I^- interactions as the hardness of halide ion decreases.