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Course knowledge

When considering the NCS- ion, I take all atoms to be sp hybridized. This means that there are 8 electrons that are accommodated in the $\pi$-system.

There are two orthogonal 3-orbital 4-electron pi systems: similar to the allyl anion, each made of an antibonding, non-bonding and bonding set. The bonding and non-bonding orbitals are occupied in this system. As the non-bonding orbitals are both full of electrons, NCS- therefore can act as a $\pi$ donor.

The above is what I have been taught, however I am confused with this rationalisation when comparing the positions of NCS- and SCN- in the spectrochemical series.

Spectrochemical series

SCN- is a weak field ligand, which comfortably suggests that it is a $\pi$-donor. However, NCS- is relatively high field, and is therefore a $\pi$-acceptor - which disagrees with what I've been taught. As sulfur is less electronegative than sulfur, it is higher in energy and therefore contributes more to the anti-bonding orbital.

Questions

Wouldn't this mean that SCN- would be the better $\pi$-acceptor rather than NCS-? Or is it the case that since sulfur is larger than nitrogen, the better orbital overlap with the transition metal favours donation?

Additionally, how can we say that the pi-donation from the non-bonding orbitals wins out over the pi-acceptance from the anti-bonding orbitals? Is it because the anti-bonding orbitals are involved in the ligand bonding whereas the non-bonding orbitals are not?

Also, why is NCS- a pi-donor (according to what I have been taught) even though it is relatively high field?

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  • $\begingroup$ Although the thiocyanate ligand acting through the nitrogen atom is higher field than water (a $\pi$ donor), it is still weaker than $\sigma$ donors, such as $\ce {NH3}$. Thus, it should not be classified as a $\pi$ acceptor. $\endgroup$ – Tan Yong Boon Jul 29 at 10:00

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