# MO-Scheme of SCN- and its bonding properties when used as a ligand

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.

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 $\ce{[Co(en)2(SCN)2]SCN}$.
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.
• 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? – Philipp Jun 21 '12 at 10:06
• 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? – Philipp Jun 23 '12 at 12:12