[Squirrelz]

Whereas MoCl₅ and MoF₅ are known?

I've got 2 theories:
First I thought it had something to do with the IE and that only fluorine has a high enough EA to compensate for it but I couldn't find anything on the fifth ionisation energies for the transition metals…
But this would align more with the questions we had discussed in class.

Second one is that fluorine is the only halide that bonds with chromium because bigger halides would not find the space to (is repel a good term for that?) Whereas Mo and its radius is bigger so that larger halides e.g. chlorine can bond.
But that's just what I found out after reading some books and articles. I don't know if that explains the whole phenomenon.

Thanks!

[Enthalpy]

I won't follow you with the mental image of so-called ionic compounds.

In a so-called ionic crystal, the "ions" touch an other, so the electrons are very close to the positive ion. So close that it's impossible to tell to which atom they belong.

The ionisation energies and electron affinities, as opposed, characterise actions on electrons up to infinite distance. They don't fit that task at all.

In addition, one must consider partial charges. "+5" is only a simple image that helps guess which compounds are possible or not. Experiments tell something different.

The proper model to understand bonds is: molecular orbitals. Alas, for a crystal it's not handy at all.

You can find ionisation energies there
https://www.webelements.com/
https://www.webelements.com/chromium/atoms.html
https://www.webelements.com/molybdenum/atoms.html