[itsmemario]

[{L}Ni-OH2]⁴⁺  +  NCS-           [{L}Ni-CNS]³⁺  +  H2O
[{L}Cr(OH2)3]⁴⁺  + NCS-           [{L}Cr(OH2)2NCS]]³⁺  +  H2O

For the above reactions, why is the rate of reaction for the Cr ligand substitution proportional to the [H+] concentration? yet the Ni reaction is independent of [H+]?

Any help at all would be appreciated.

Thanks

[kriggy]

I think that Cr comples has elimination-addition mechanism - the H2O must leave before the NCS can coordinate and it leaves by being protonated by H+. Ni comples undergoes addition-elimination mechanism where the rate determining step is addition of NCS to form comples with coordination number incresed by 1 and then water molecule is eliminated