Final answer:
The coordination numbers for each metal ion in the given compounds are 4, 6, 6, and 4.
Step-by-step explanation:
The coordination number refers to the number of ligands attached to the central metal ion in a complex. Here are the coordination numbers for each metal ion in the given compounds:
a. [Ni(CN)₄]²⁻ - The coordination number is 4 because there are four CN- ligands attached to the Ni²⁺ ion.
b. [Co(H₂O)₆]³⁺ - The coordination number is 6 because there are six H₂O ligands attached to the Co³⁺ ion.
c. [Cr(OH)₆]³⁻ - The coordination number is 6 because there are six OH- ligands attached to the Cr³⁻ ion.
d. [PtCl₄]²⁻ - The coordination number is 4 because there are four Cl- ligands attached to the Pt²⁺ ion.