[oxfordstudent]

Hi, I'm doing a first year university exam paper, and got stuck on this question:

Comment on the observation that Chromium(II) oxide does not exist although Chromium(VI) oxide does. However, Cobalt(II) oxide does although Cobalt(IX) oxides does not exist.

My answer is as follows:
In the hypothetical compound "CrO", chromium is in oxidation state +2: Cr²⁺, which is a d⁴ metal cation. High-spin d⁴ complexes are very prone to Jahn-Teller distortions comprised of elongations along the z-axis: this would make the complex unstable, and thus CrO does not exist.

CrO₃: Cr²⁺: d⁰.
 
In the hypothetical compound Co₂O₉, cobalt is in oxidation state +9, which is d⁰
In CoO: Co²⁺: d⁷.

I need some help with my argument, why is it that some of these oxides do not exist? I can only think of Jahn-Teller distortions....

Thanks!

[n4k]

CrO₃: Cr²⁺: d⁰.
 

Oxidation state Cr⁶⁺

[n4k]

Co₂O₉=CoO₃*CoO₆
CoO₆ - is a peroxide

[oxfordstudent]

Oh, sorry about that! I misspelt it
Thanks!

[Borek]

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http://www.chemicalforums.com/index.php?topic=28369

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