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# Oxidation using chromium trioxide

Oxidation of alcohol with $$\ce{CrO3}$$

I have come across different kinds of oxidation using ester formation (eg. Selenium/Osmium ester).This mechanism deals with the chromate ester. The structure of $$\ce{CrO3}$$ seems rather confusing to me:

I found claims that $$\ce{Cr}$$ atom has any one out of the following hybridisation.

1. $$\ce{sp^2}$$ (claimed by an instructor)
2. $$\ce{d^2s}$$ (claimed by a student on different forum)
3. $$\ce{pd^2}$$ (claimed by me)

I do not know which is correct description, but post computational analysis, I can be pretty confident for the 3rd.

My questions are as follows:

1. What is the correct (nearly) hybridisation (if it exists)?
2. Geometry of the mechanism (i.e. approach of electrons in a 3D model, can be provided as a thought as well)
3. Why isn’t the five membered ring completed with $$\ce{OH}$$ instead of pi bonded Oxygen.