For each of the following pairs of metal complexes, identify the one that will have the lowest co stretching frequency. Briefly and clearly discuss your reasoning for each case.
1. [Fe(H₂O)₆]₂ + vs. [Fe(CN)₆]⁴⁺
2. [Co(NH₃)₆]³⁺ + vs. [Co(CN)₆]³⁺
3. [Ni(CO)₄] vs. [Ni(CN)₄]²⁻
4. [Pt(NH₃)₄]2+ vs. [PtCl₄]²⁻
