[dscot]

Hi all,

This my first post so hopefully I'm doing it right

Ok, so I have the Monsanto Process and I'm trying to count the electrons --

http://content.screencast.com/users/apple741/folders/Jing/media/58bd654c-0733-458f-a0d8-a8b671cccd29/00000341.png

For example I know that answer for step 3 is  18e- but I always get 20e-

Rh -       9e-
3*I -      3e-
2*CO -   6e-
CH3CO - 2e-

= 20e- which I know is wrong?

Any thoughts on what I could be doing wrong?

I'm also a little confused about coordination numbers, are they just the number of ligands bound to a central atom? So in this case it would be 4,6,5,6 (Starting from Rh(CO)2I2).

Thanks!

[stewie griffin]

Looks to me like you are mixing up the covalent counting method with the ionic method. Either is appropriate to use, but you can NOT mix them and use them both at the same time. I would suggest you pick one method, master it, and stick with it. I always liked the ionic method b/c it made more intuitive sense to me, but others like the covalent.

I'll use the ionic method....
First of all, the compound you are asking about has a minus 1 charge which must be taken into account. You have four anionic ligands (3 I^-'s and an organic R^- group). Therefore, to have an overall compound charge of minus 1, we must have a Rh³⁺. That gives us:
Rh³⁺ = 6 electrons (Rh starts with 9 electrons, but since it's ³⁺ it only has 6 now)
3 I^- = 6 electrons (each anion contributes 2 electrons in the ionic method)
the organic R^- = 2 electrons
2 CO = 4 electrons
The grand total is 18 electrons. 

Yes, coordination number is simply the number of ligands on the central metal atom.

[dscot]

Hi Stewie,

Thanks very much, I didn't realize I was mixing the ionic and covalent methods

Everything now makes sense to me! One small issue I still have, you mentioned that the compound has an overall charge of -1, but I don't see that in the diagram?

Thanks
Dave

[stewie griffin]

You are correct that in the picture you linked to that there is no minus charge shown. Looks like it was cut off.
However, that compound does have a minus one charge. See the full cycle here http://en.wikipedia.org/wiki/Monsanto_process.
The compound you are interested in is on the left hand side. Notice the minus charge on the top right of the brackets.

[dscot]

Wow thanks so much Stewie, I've been stuck on this for ages!

I tried to give you more karma but have to wait 24hrs

Thanks again!