[browninm]

More of a theoretical question - What's the electron count on any individual Rh in Rh6(CO)16?  What I think I'm doing wrong is assigning the incorrect electron contribution from the mu-3 CO ligands (4e- distributed across three Rh atoms=1 1/3 e- per Rh?), but I'm not sure.  

Using the neutral ligand method, I'm getting:

Rh                    9e-
4(Rh-Rh)           4e-
(2) terminal CO  4e-  
(2) mu-3 CO      1 1/3 e-
                        _________

                       = 18 1/3e-

This doesn't sound right, since it seems the electron count should be below 18 e- per Rh in this octahedral config.  Anyone have a suggestion for me?  Thanks...
             

   

[AWK]

Are you sure that C in CO can share 4 electrons.
Note - I do not know structure of Rh6(CO)16 but in structure of closely related Ir6(CO)16 there are two different isomers

[green-goblin]

In electron counting i've never seen fractions used, such is in your 1 1/3 example.
It might be that i've never came accross an example like that.  

[green-goblin]

I just found my old inorgainc lecture notes. Its says that CO is a neutral 2 electron donating species and mu3 CO is a neutral ligand donating 1 electron to each metal.

[AWK]

Exclusions proves the rules. Second isomer of iridium is mu2

[browninm]

Yeah - I don't know about the fractional electron counting thing, either - I was thinking of it as more of a theoretical bookkeeping tool than any real partial electron.  Which led me to thinking each mu-3 CO ligand (which I had incorrectly assigned 2e-  each) was equally shared across the three atoms of Rh.  I was led to believe (by hints from an organometallic prof) that the theoretical electron count would be under 18 e- and possibly a funky number, so it helps a lot knowing that mu-3 carbonyls only donate 1 e- each as green-goblin suggested.

The problem pointed to a structure of Rh6(CO)16 to use for our reference - I'm not sure about the other isomer, but it brings up a good point.  This site has a good representation of the octahedral structure of the complex similar to the one in my text:

http://www.dur.ac.uk/a.k.hughes/clusters/rhodium/qugboj.html

Thanks a bunch for the replies....

[AWK]

Isomers of Ir6(CO)16
JACS 106 (1984) 6664-6667