Achieve Wk 9 #2

[Annabelle Jeon 3K]

I just had a question about question 2 on week 9's achieve. So I get how to determine the coordination number for most of the compounds except K3[CoF6]. All the other compounds seem quite intuitive because there is an obvious central atom and the central atom happens to be the cation as it does for most the examples we've also had in class. So that brings me to two questions:

1) Is the central atom usually the cation, and if so why?
2) How do we go about determining the central atom in a very large compound? For example, why wouldn't K be the central atom in K3[CoF6]?

[Avantika MOhan_2B]

In this question, the coordination compound has formed a bond with a cation. The coordination compound is always the complex within the square brackets. So in this case, it would be Cobalt (Co). This is one of the cases where the overall charge on the coordination compound is negative allowing it to bind with the Potassium positive ion. So we have [CoF6], so the coordination compound has a coordination number of 6. The oxidation state of the Co cation in the compound, however, is going to be +3. The overall charge on the coord compound is -3 (because it binds to 3 K+ ions. We know that Flourine has a negative charge of -1, which means that the oxidation number of Co is +3.