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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?


Reference:

[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).

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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.


Reference:

  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).
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  • $\begingroup$ 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
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    $\begingroup$ 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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