Textbook 9C9

[Caitlin_Tang_Dis3H]

Hello!
Can someone help me understand this problem?
With the help of Table 9C.1, determine the coordination number of the metal ion in each of the following complexes:
(a) [NiCl4]2-
(b) [Ag(NH3)2]+
(c) [PtCl2(en)2]2+
(d) [Cr(edta)]-
The answers would be, ni=4, Ag=2, Pt=6, Cr=6 but does the charge on the outside of the complex not matter? When I calculated it with the outside total charge I got a different number... Can someone help me understand why the charge on the outside was not used? Thank you in advance.
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[Charmaine Ho 2G]

The coordination number of is the number of points where ligands are attached to the central metal atom.

So for example, [NiCl4]2- , the ligand Cl- is monodenate and there are 4 of these attached to Ni, meaning there are 4 points where a Cl- is attached to Ni. In this case, because the ligand is monodentate, the number of ligands matches the coordination number.

However, in cases of polydentates, the number of ligands will not match the coordination compound. For example, in [PtCl2(en)2]2+, (en) is a bidentate and since there are two of these, and two of monodentate Cl-. there are 6 points where a ligand is attached to the Pt. So the coordination number is 6.

You only use the charge of the complex ion when calculating the oxidation number.