I was wondering why is this equilibrium below established at all... Why can't Co(CO)4 exist on its own, as a monomer?
$\begingroup$
$\endgroup$
3
-
4$\begingroup$ Reference 5 in the corresponding Wikipedia article gives a lot of information if not the complete answer. $\endgroup$– andselisk ♦Commented Nov 3, 2022 at 18:08
-
$\begingroup$ Also note that the monomer you speak of is not a neutral molecule. It is actually charged, $\ce{[Co(CO)4]-}$. See: chemistry.stackexchange.com/questions/97571/… $\endgroup$– Nilay GhoshCommented Nov 4, 2022 at 2:51
-
1$\begingroup$ @NilayGhosh The monomer in this case is the (17 electron) neutral Co(CO)4 molecule; [Co(CO)4]− is a different entity. c.f. NO2, N2O4 and NO2- $\endgroup$– Ian BushCommented Nov 4, 2022 at 8:29
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
3
The reason for this difference in behavior between Ni(CO)4 and Co2(CO)8 lies in the electronic configuration and the atomic radius of the metal atoms.
Electronic Configuration:
- Ni(CO)4: Nickel has a [Ar] 3d8 4s2 electronic configuration. In Ni(CO)4, the nickel atom achieves a stable 18-electron configuration through the donation of 2 electrons from each of the 4 CO ligands.
- Co2(CO)8: Cobalt has a [Ar] 3d7 4s2 electronic configuration. In Co2(CO)8, each cobalt atom achieves a 17-electron configuration, which is not stable. To achieve a stable 18-electron configuration, the two cobalt atoms dimerize, forming a metal-metal bond.
Atomic Radius:
- Ni(CO)4: Nickel has a smaller atomic radius (1.24 Å) compared to cobalt (1.26 Å). This smaller radius allows the nickel atom to accommodate the 4 CO ligands in a tetrahedral arrangement without significant steric crowding.
- Co2(CO)8: Cobalt has a slightly larger atomic radius, which leads to increased steric crowding when attempting to accommodate 4 CO ligands in a tetrahedral arrangement.
-
$\begingroup$ Why you have mentioned about $\ce{Ni(CO)4}$? when its not asked in the question? $\endgroup$ Commented Nov 23 at 7:03
-
$\begingroup$ I suspect because the Nickel compound contrasts with the cobalt one in that one is monomeric, the other dimeric. This answer (why has it been voted down??) then shows the underlying rationalisation to this difference, the 18 electron rule, hence answering the original question. $\endgroup$– Ian BushCommented Nov 23 at 8:05
-
1$\begingroup$ I had downvoted it because it had just one line in the initial draft. I have removed it after it was edited @IanBush $\endgroup$ Commented Nov 23 at 11:00