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I tried to figure out the MO-scheme of the tetragonal-bipyramidal complex trans-$\ce{[Co(en)2(NCS)2]SCN}$ in which the isothiocyanate ligands are bound to the $\ce{Co^3+}$-Ion in $\eta^{1}$-mode (en = ethylenediamine). While undertaking this task I noticed that I don't know the MO-scheme of the isothiocyanate ion, which I need to determine whether it acts as a $\sigma$ and/or $\pi$ acceptor or donor.

I tried to construct the MO via a symmetry-based treatment but ran into the problem that the isothiocyanate ion - belonging to thet $C_{\infty v}$ point group - lacks symmetry elements perpendicular to its principal axis. Thus, I wasn't able to construct the bonding and antibonding sigma-orbitals with this method.

I could construct those $\sigma$-interactions by hand or use $\ce{CO2}$ as a model but I don't know whether this will give me the right results. I haven't found the MO-scheme in the internet so I would be happy if someone could advise me on how to construct a qualitative MO-scheme of $\ce{SCN-}$ or simply show it to me. I would also be very grateful for getting some information on how exactly the isothiocyanate ion acts as a ligand (which $\pi$-interactions and how strong are they, is there also $\sigma$-donation as with carbonyl ligands, etc.).

Edit: I forgot to mention explicitly that the $\ce{SCN-}$ ion is bound to the metal center via its nitrogen atom.

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One issue with SCN-, is that it can bind either at the nitrogen or at the sulfur. The way you presented the formula, $\ce{[Co(en)2(NCS)2]SCN}$, suggested that the SCN- ligand is bound via the nitrogen. If it were bound by the sulfur, the formula might be written $\ce{[Co(en)2(SCN)2]SCN}$.

SCN- should have a similar MO diagram to OCN-, which is isoelectronic with CO2, so your idea to compare to CO2 is not a bad one. However, since SCN- has a different symmetry group, you cannot trust that comparison too much. Your comparison to CO is also a good one, since SCN- and CO have the same point group.

However, what you really need to do is determine the symmetry group for $\ce{[Co(en)2(NCS)2]SCN}$. Then, assign symmetry labels to the SCN- ligands. The MO that is most important for SCN- is, qualitatively, the "lone pair" on the atom pointing at the metal center, which can form a "$\sigma$-bond" with cobalt. I would only consider using those $\pi$ orbitals on SCN- if the complex needs more electrons to get to 18. When you assign symmetry labels to the SCN- ligands, consider them as a pair: i.e. assign a symmetry label to the in-phase pair and then assign a symmetry label to the out-of-phase pair. The in-phase pair should probably have the same symmetry label as the cobalt $d_{z^2}$ orbital.

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    $\begingroup$ Do you really think that the most important interaction of $\ce{SCN-}$ ligand with the metal center is a $\sigma$ interaction? I would expect that the HOMO and (maybe) also the LUMO of $\ce{SCN-}$ is comprised of $\pi$-MOs (non-bonding (-> lone pairs) for the HOMO and antibonding for the LUMO). The $\sigma$-interaction should only come about by an additional interaction of a $\sigma$-Orbital with the metal $d_{z^2}$. But since the $\sigma$-MOs should be rather low in energy I wouldn't expect the interaction to very strong. Am I missing some points or do you think my arguments are reasonable? $\endgroup$
    – Philipp
    Commented Jun 21, 2012 at 10:06
  • $\begingroup$ The HOMO of SCN<sup>-</sup> is probably a nonbonding orbital, ie one of the "lone pairs". $\endgroup$
    – Ben Norris
    Commented Jun 21, 2012 at 17:31
  • $\begingroup$ Sorry if it wasn't clear from my first comment, that I totally agree with you there: If $\ce{SCN-}$ is similar to $\ce{CO2}$ then I would expect the HOMO to be a pair of nonbonding $\pi$ orbitals with out-of-phase "p-orbitals" on $\ce{S}$ and $\ce{N}$ and a nodal plane through $\ce{C}$. Would this mean that $\ce{SCN-}$ will be mostly a $\pi$-donor ligand (maybe with some additional $\sigma$-donation from lower lying $\sigma$-MOs) or will the higher lying $\pi^{*}$-MOs (= LUMO) be more important for the interaction with the metal center so that $\ce{SCN-}$ is more of a $\pi$-acceptor ligand? $\endgroup$
    – Philipp
    Commented Jun 23, 2012 at 12:12
  • $\begingroup$ @Philipp Inspection of the spectrochemical series on Wikipedia seems to suggest that the thiocyanate ion seems to be more of a pi donor/sigma donor ligand since the ligand seems to be placed together with other such ligands. However, it is interesting to note that the pi donor character increases when coordination is done through the N atom while the sigma donor character increases when the coordination is done through the S atom. $\endgroup$
    – Tan Yong Boon
    Commented Apr 7, 2019 at 15:32
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    $\begingroup$ @Philipp I have misread the Wiki page regarding the last part of the previous comment. The pi donor character is higher when coordination is through S and is lower when the coordination is through N. This is easily rationalised by the fact that the lone pair orbitals on S are larger and more diffuse allowing for easier overlap with the metal d orbitals. $\endgroup$
    – Tan Yong Boon
    Commented Apr 7, 2019 at 15:49

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