I was reading a paper which built a series of trans-philicity (a term they coined to indicate both kinetic trans-effect and thermodynamic trans-influence) from extensive calculations. And I found that in the series, $\ce{:\!CN-}$ is shown to have a weaker trans effect than $\ce{MeC#N \!:}$ [1] (Note: this is for octahedral complexes)

However, this trend seems odd to me, because I cannot explain it in terms of electron donating and withdrawing capacities. I had heard that the trans-effect is strongest in ligands which were either strong $\sigma$-donor or strong $\pi$-acceptor or a combination of both. This is why $\ce{CN-}$, $\ce{CO}$ etc. have very strong trans-effects.

Now, in the experimental trans-effect series that is commonly found in inorganic textbooks, we have: $$\ce{H2O < NH3 < ... < CO < CN-}$$ $\ce{NH3}$ is near the very beginning of the series. $\ce{MeC#N}$ should be a worse $\sigma$-donor than $\ce{NH3}$ (as the lone pair is in an $\mathrm{sp}$-orbital). It also seems unlikely that $\ce{MeCN}$ will be a better $\pi$-acceptor than $\ce{CN-}$ because $\ce{MeCN\!:}$ binds via the more electronegative $\ce{N}$ while $\ce{:\!CN-}$ binds with the less electronegative $\ce{C}$ (which would mean the tendency to accept electron density in the $\pi$-orbitals should be lower for $\ce{MeCN}$).

So, I don't understand how it is possible to have the trans-effect order $\ce{CN- < MeCN}$. Is there any explanation for this?


[1]. A. C. Tsipis, "Building trans-philicity (trans-effect/trans-influence) ladders for octahedral complexes by using an NMR probe", Dalton Trans. 2019, 48, 1814-1822 (DOI: 10.1039/C8DT04562C).


According to the reference mentioned in the question (Ref.1):

The term ‘trans-influence’, being a long-established concept of broad relevance in the realm of inorganic chemistry, was defined first in 1966 by Pidcock et al. as the ability of ligand L in a complex to weaken the metal–ligand bond trans to itself. This ground-state phenomenon should be distinguished from the kinetic phenomenon called the ‘trans-effect’, which is the effect of coordinated ligand L upon the rate of substitution reactions of the ligand in trans-position to L.
Note: Pidcock et al. 1966: Ref.2

Considering the high sensitivity of the $\ce{^{13}C}$-$\mathrm{NMR}$ isotropic shielding tensor elements to small structural/electronic changes, the authors of Ref.1 have published a reliable trans-philicity ladder for octahedral $\ce{[Cr(CO)5L]^{−/0/+}}$ complexes using $\ce{^{13}C}$-$\mathrm{NMR}$ isotropic shielding tensor elements. In $\ce{[Cr(CO)5L]^{−/0/+}}$ complex, $\ce{L}$ represents a wide variety of ligands (50 ligands) commonly used in coordination and organometallic chemistry. Briefly, all $\ce{^{13}C}$-$\mathrm{NMR}$ isotropic shielding tensor elements and other parameters have been calculated using PBE0/Def2-TZVP(Cr)∪6-31G(d,p)(E)/PCM and PBE0/Def2-TZVP(Cr)∪6-311++G(d,p)(E)/PCM computational protocols set in dichloromethane solution where the latter protocol is more sophisticated than the former.

I think, major drawback in this publication is the lack of experimental date to support the calculations. For instance, the authors admit that to the best of their knowledge, experimental data for $\delta \ \ce{^{13}C}$-$\mathrm{NMR}$ chemical shifts of $\ce{[Cr(CO)5L]^{−/0/+}}$ complexes are available only for the $\ce{Cr(CO)6}$ complex and the “free” $\ce{CO}$ ligand, which are $212$ and $\pu{184.4 ppm}$, respectively.

When compared the calculations of $\delta \ \ce{^{13}C}$-$\mathrm{NMR}$ chemical shifts of the $\ce{Cr(CO)6}$ complex and the “free” $\ce{CO}$ ligand employing the two computational protocols, the PBE0/Def2-TZVP(Cr)∪6-31G(d,p)(E)/PCM predicted $\delta \ \ce{^{13}C}$-$\mathrm{NMR}$ chemical shifts of $210.2$ and $\pu{186.1 ppm}$, respectively for two compounds, while the protocol PBE0/Def2-TZVP(Cr)∪6-311++G(d,p)(E)/PCM) predicted $\delta \ \ce{^{13}C}$-$\mathrm{NMR}$ chemical shifts of $226.8$ and $\pu{197.6 ppm}$, respectively for the same two compounds:

$$\begin{array}{l|cc} \hline \text{Compound} & \ce{\delta \ ^{13}C} \text{ (calculated)}^1 & \ce{\delta \ ^{13}C} \text{ (calculated)}^2 & \ce{\delta \ ^{13}C} \text{ (experimental)} \\ \hline \ce{Cr(CO)6} \text{ (complex)} & \pu{210.2 ppm} & \pu{226.8 ppm} & \pu{212.0 ppm} \\ \ce{CO} \text{ ('free' ligand)} & \pu{186.1 ppm} & \pu{197.65 ppm} & \pu{184.4 ppm} \\ \hline \end{array}\\ ^1 \text{From protocol PBE0/Def2-TZVP(Cr)∪6-31G(d,p)(E)/PCM;} \\ ^2\text{ From protocol PBE0/Def2-TZVP(Cr)∪6-311++G(d,p)(E)/PCM.} $$

Evidently, the GIAO/PBE0/Def2-TZVP(Cr)∪6-31G(d,p)(E)/PCM computational protocol is a better performer in the calculation of the $\ce{^{13}C}$-$\mathrm{NMR}$ spectra of $\ce{[Cr(CO)5L]^{−/0/+}}$ complexes than that of PBE0/Def2-TZVP(Cr)∪6-311++G(d,p)(E)/PCM one. Nevertheless, the differences of the calculated $\Delta\sigma \ \ce{^{13}C}$-$\mathrm{NMR}$ descriptors of trans-philicity for the complexes using either protocol were minimal.

Yet the authors have mentioned that:

It can be seen that the $\mathrm{NMR}$ trans-philicity ladders constructed by the two computational protocols are similar with some minor local reversed orders in the trans-philicity series of similar ligands. The PBE0/Def2-TZVP(Cr)∪6-31G(d,p)(E)/PCM computational protocol predicts for the $\ce{NCR}$ ligands the order: $\ce{NCH \gt NCPh \gt NCMe}$, while the PBE0/Def2-TZVP(Cr)∪6-311++G(d,p)(E)/PCM computational protocol predicts the order: $\ce{NCMe \gt NCPh \gt NCH}$. Consideration of the $\sigma$-donor and $\pi$-acceptor abilities of the $\ce{NCR}$ ligands supports the order predicted by the PBE0/Def2-TZVP(Cr)∪6-31G(d,p)(E)/PCM computational protocol.

Thus, I could argue that the difference in two protocols make this difference than actual situation. Unless we have experimental data to support the finding, it is just speculation.


  1. A. C. Tsipis, "Building trans-philicity (trans-effect/trans-influence) ladders for octahedral complexes by using an NMR probe", Dalton Trans. 2019, 48, 1814-1822 (DOI: 10.1039/C8DT04562C).
  2. A. Pidcock, R. E. Richards, and L. M. Venanzi, "$\ce{^{195}Pt–^{31}P}$ nuclear spin coupling constants and the nature of the trans-effect in platinum complexes," J. Chem. Soc. A 1966, 1707–1710 (DOI: https://doi.org/10.1039/J19660001707).
  • $\begingroup$ I don't understand how this answers the question? I was asking about the difference between CN- and MeCN. MeCN has a higher trans-philicity than CN- with both basis sets. Is there any experimental data you know about the trans-effect of MeCN vs CN- that disproves the order from the calculation? Otherwise your answer is just explaining what's written in the paper. $\endgroup$ – S R Maiti Jun 4 at 7:51
  • $\begingroup$ @ Shoubhik R Maiti: Before you question, did you read the paper? It was stated that two protocols give opposite results. Thus, none is conclusive. That's what I said in my answer. Please read the paper first. $\endgroup$ – Mathew Mahindaratne Jun 4 at 16:13
  • $\begingroup$ Of course I read the paper, that's why I mentioned it in the question. The two protocols give different results for RCN groups, but I am not asking about the order of that. (And HCN is not the same as CN-, they are different ligands in the paper) In both protocols the order of CN- and MeCN is the same. Are you arguing that because the methods predict different order for RCN ligands, the whole series is questionable? $\endgroup$ – S R Maiti Jun 4 at 19:33
  • $\begingroup$ Did you have any other evidence that it is different than what the paper said? $\endgroup$ – Mathew Mahindaratne Jun 4 at 19:43
  • 1
    $\begingroup$ Well, seems like I misunderstood your question. I'll see some evidence to add. $\endgroup$ – Mathew Mahindaratne Jun 4 at 19:51

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