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-