5
$\begingroup$

When $\ce{\eta^1-C5H5-}$ acts as a ligand, does the donor carbon become $sp^3$ hybridized, and would this be a sigma donor through sigma orbitals? Or, is it $sp^2$ where the lone pair is donated through a pi orbital, making it a sigma donor through a pi orbital?

The second option seems much more likely as the ligand changes the $\ce{C}$ attached to metal randomly with time through rearrangements.

Also, does "sigma complex" and "pi complex" only differentiate the hapticity? as $\ce{\eta^1-C5H5-}$ is a sigma complex and $\eta^3$, $\eta^5$ are pi complex.

$\endgroup$
4
$\begingroup$

Even though depictions of $\eta^1$-coordination might suggest a $\mathrm{sp^3}$-carbon donating, there is little evidence, that it actually is. Taking the example you have presented in your answer from Casey et. al.[1] I have run a geometry optimisation to obtain molecular orbitals at the DF-BP86/def2-SVP level of theory. The following geometry parameters around the coordinating carbon already suggest only little pyramidalisation: \begin{array}{lrr}\hline \text{Parameter} & \text{Experimental [1]} & \text{DF-BP86/def2-SVP}\\\hline \mathbf{d}(\ce{Re-C^1}) & 236.0 & 240.2\\ \mathbf{d}(\ce{Re-H}) & 288.7 & 282.6\\ \mathbf{d}(\ce{Re-C^2}) & 317.8 & 321.3\\ \mathbf{d}(\ce{Re-C^5}) & 315.6 & 315.6\\ \angle(\ce{Re-C^1-H}) & 114.3 & 100.7\\ \angle(\ce{Re-C^1-C^2}) & 110.8 & 109.5\\ \angle(\ce{Re-C^1-C^5}) & 108.5 & 106.4\\ \angle(\triangle[\ce{C^1C^2C^5}]\ce{-H}) & 122.8 & 139.1\\ \text{RMSD} & 0.0 & 0.1\\\hline \end{array}

We know that crystal structures are not really that accurate with protons, hence I'd put a little more trust into the calculation at this point. The paper does not give a value for any protons.
Here is a picture of the calculated structure.

display

When we look at the canonical MO we will see that all (occupied) π-obitals of the Cp-ligand are still there. Obviously they are distorted, but clearly recognisable.

mo81mo87mo88

The second orbital is the HOMO-1, which is the main contribution for the coordination, and the last is the HOMO (click for large). The only s-contribution to the HOMO-1 is from the carbon of the $trans\text{-}\ce{CO}$-ligand (and some hydrogen).
In output numbers, missing from 1 are minor contributions (<0.05):

 Alpha occ 81 OE=-0.276 is C39-p=0.19 C42-p=0.12 C41-p=0.12 C43-p=0.11 C40-p=0.11
 Alpha occ 87 OE=-0.185 is C39-p=0.26 C42-p=0.23 C41-p=0.22 Re24-d=0.08 C27-s=0.05
 Alpha occ 88 OE=-0.153 is C43-p=0.32 C40-p=0.32 C41-p=0.13 C42-p=0.12

When we localise the orbitals with the Natural Bond Orbital (NBO) analysis, we find a lone-pair at $\ce{c^1}$ (blue/orange) which donates into the anti-bonding $\ce{Re-^{$trans$}CO}$ orbital (yellow/red): interaction of localised orbitals

The composition of the lone pair orbital is

     (Occupancy)   Bond orbital / Coefficients / Hybrids
 36. (1.27157) LP ( 1) C 39            s(  8.77%)p10.40( 91.23%)d 0.00(  0.00%)

The low occupancy is normal for donor-acceptor-complexes with some covalent character.

TL;DR Most $\eta^1$ complexes are in first approximation π-coordination complexes like the definition suggests[2]. This may change significantly when moieties are altered.


Further thoughts:
Wikipedia makes one important statement (it rarely does, but here it is true)3:

Molecules with polyhapto ligands are often fluxional, also known as stereochemically non-rigid.

You can expect that in $\eta^1$-ligands the metal can easily migrate from one carbon to another, as the MO show. I would expect some kind of pseudo-rotation.

Ref.

  1. Charles P. Casey, Joseph M. O'Connor, William D. Jones, Kenneth J. Haller. Organometallics 1983, 2 (4), 535–538. (link via doi, pdf via researchgate.net) Crystallographic data via CCDC number: 1118829.
  2. η (eta or hapto) in inorganic nomenclature
  3. Wikipedia: Hapticity and fluxionality
$\endgroup$
3
$\begingroup$

Casey and coworkers in Organometallics 1983, 2, 535:

enter image description here

http://www.ilpi.com/organomet/cp.html

It's $sp^3$

$\endgroup$
  • $\begingroup$ It is really not that easy. Just because you found a Lewis structure that displays it like an sp3 carbon, does not mean that this is actually true. There is a lot of flexibility in these complexes and I very much doubt that there is much s character in the co-ordination bond. Sorry, but I have to down-vote your answer. $\endgroup$ – Martin - マーチン Jan 27 '17 at 12:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.