3
$\begingroup$

I fully understand and recognise the manner and which the series works.

I cannot find any references for the change in sigma donor capability moving up the series.

For example, comparing $\ce{Cl-}$ and $\ce{NH3}$. Which of the two would be more a more willing sigma donor ?

Does this have something to do with how diffuse the orbitals are ?

$\endgroup$
1
$\begingroup$

The spectrochemical series is an ordering of relative ligand interaction strength with metal ions. It can be used to infer relative $\sigma$-donor strength, if $\pi$-interactions are not possible for those particular ligands. Examples of this type ligand with no $\pi$-interactions include $\ce{NH3}$, $\ce{CH3-}$, and $\ce{H-}$.

Most ligands will exhibit $\pi$-interactions, operating as $\pi$-donors or $\pi$-acceptors, and these interactions dominate the ordering of the spectrochemical series. The $\sigma$-donor interactions are obscured and heavily affected by the $\pi$-interactions. Experimentally all of these effects are important, and it is difficult to separate these interactions from one another.

For example, the $\sigma$-donor interaction of a $\pi$-acceptor ligand is drastically increased from some initial level. $\pi$-backbonding reduces electron density at the metal center, leading to increased $\sigma$-donation (which increases $\pi$-backbonding, and so on). This will make a given $\pi$-acceptor ligand act as a better $\sigma$-donor than would be expected. The converse effect is observed with $\pi$-donor ligands reducing $\sigma$-donor strength.

The complexity of these interactions tempers the question of gauging $\sigma$-donor interactions only. The combined bonding effects influence one another.

Theoretically separating out $\sigma$-donor interactions from all $\pi$-interactions could be possible computationally, but I am unaware of any research on this specific topic. From a very general standpoint, improved metal-ligand orbital overlap will increase $\sigma$-donor strength, which may help estimate these characteristics qualitatively.

Most inorganic textbooks should be able to provide further detail on this topic under sections on ligand field theory.

$\endgroup$
  • 1
    $\begingroup$ can relative Bronsted basicity be used gauge sigma donor capabilities? $\endgroup$ – Dissenter Jan 7 '15 at 20:28
  • 1
    $\begingroup$ I would say that Lewis basicity would be a better approach to evaluating sigma-donation as this is more accurate in how ligands coordinate to metals. Gauging Lewis basicity as only sigma-donation generalizes somewhat the complete electronic structure. Nevertheless, this seems like a good way to determine approximate sigma-donation strength. Below is a link to a book that has more information on this, where it appears they have computational and thermodynamic values for complexation enthalpies of various Lewis bases to Lewis acids such as BF3: books.google.com/books?id=KsF44nB9Ex8C $\endgroup$ – venture Jan 7 '15 at 21:20

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.