[atomicpedals]

I'm beginning to think that I can't count... I've got two examples which should both total 18 e-, but I'm just not seeing how the second one works out. The first is Ni(CO)4, 10 from the Ni and 2*4 = 8 from the (CO)4. The other is Re2(CO)10 assuming a Re-Re single bond, with the CO alone I'm counting 20; clearly I'm overlooking something basic but I've been looking at this too long to tell what.

Thanks for any nudges in the right direction!

[unsu]

(CO)5Re-Re(CO)5

Each Re atom has 7 own electrons
When Re-Re single bond is formed, one electron from one Re is shared with one electron from another Re:

Re(6e+1e) + Re(6e+1e) = (6e)Re(2e)Re(6e)

After a single Re-Re bond is formed, each Re atom is surrounded by 8 electrons:

 (6e)Re(2e)Re(6e) and  (6e)Re(2e)Re(6e)

Plus 10 electrons from five CO ligands.
Thus, each Re atom is surrounded by total 18 electrons

I hope this helps

[kelvinLTR]

18e rule is for complexes with one central metal.

Here there are two metal atoms.

The above reply explains the rest. Try drawing the structure and counting for one metal atom.