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I've observed several notations for the ligand in the literature in general, say, for the typical complexes like $\ce{MX_n}$ ($\ce{M}$ - metal) where ligand $\ce{X}$ is denoted as (according to my subjective frequency of occurrence):

  1. $\ce{L}$;
  2. $\ce{Lig}$;
  3. $\ce{Lg}$;
  4. $\ce{Lgnd}$.

I can imagine $\ce{Lg}$ being a poor choice as it can be easily confused with leaving group, and $\ce{Lgnd}$ which looks more like a company's NASDAQ code, but what about the first two options? And what normative document governs the proper notation anyway?

Regarding the $\mathrm{L}$ use for leucine (see comments section here and a thread on Biology.SE What is the preferred way to abbreviate amino acids?), I encountered more sources using $\mathrm{Leu}$ for that matter rather than $\mathrm{L}$. I guess 1-letter code for 20 amino acids is quite suitable and eliminates confusion between leucine ($\mathrm{L}$) and lysine ($\mathrm{K}$), but for me handling $\mathrm{Leu}$ and $\mathrm{Lys}$ is mnemonically easier.

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    $\begingroup$ (1) is out due to ambiguity - it is the one-letter code for leucine. I'd go with (2), as that is what I've seen in the literature. $\endgroup$ – Todd Minehardt Aug 4 '17 at 13:54
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    $\begingroup$ I often use Lig. for ligand. $\endgroup$ – Nilay Ghosh Aug 4 '17 at 16:16
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    $\begingroup$ @ToddMinehardt I think that is a useful distinction, but I would say it is context dependent. If you working with biological systems, it would be wise to use Lig to avoid ambiguity, but for purely inorganic systems, I think L should be used for the sake of brevity and the limited likelihood that it would cause confusion in that context. $\endgroup$ – Tyberius Aug 4 '17 at 17:04
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    $\begingroup$ @PrittBalagopal Following this logic one can say Let Tg be element tungsten, but people don't do that because there is a general agreement in a form of standardized periodic table. Same goes for classes such as lanthanides (Ln, not Ld or anything else), or coordination number (C.N. to avoid confusion with cyanide). I'm trying to find the same standard way for another entity, which this time happen to be ligand. $\endgroup$ – andselisk Aug 15 '17 at 15:05
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    $\begingroup$ @PrittBalagopal If a term ligand would have been that broad, it wouldn't exist in the first place as an unifying class. Morphologically there is no difference between isotopes of elements and denticity/hapticity of ligands. Also, any science is by definition implies search of generalization. If there is something that seems too vast for you, it doesn't mean it shouldn't/cann't be generalized. I also don't see the message you are trying to deliver. There are certain notations already established, and the only question is, which one fits better. $\endgroup$ – andselisk Aug 16 '17 at 0:25
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Going off Linear Christmas's comment, I will note that the notation from the referenced article has become relatively common for inorganic chemists. I was introduced to this notation from chapter 2 of The Organometallic Chemistry of the Transition Metals by Robert Crabtree, most of which is provided in the linked Google Book. You may also want to look at Application of the Covalent Bond Classification Method for the Teaching of Inorganic Chemistry (DOI 10.1021/ed400504f)
by M.L.H Green et al, who developed the Covalent Bond Classification scheme.

The notation largely boils down to describing metal complexes using three classes of ligands: $\ce{L}$ type ligands, which are neutral ligands that give $2\ce{e^-}$ to the complex (Lewis Base) and $\ce{X}$ type ligands which are either a radical which gives $1\ce{e^-}$ to a neutral metal. There are also $\ce{Z}$ type ligands, which aren't named as such in Crabtree, but are mentioned in regards to being $0\ce{e^-}$ ligands. An example used in Crabtree and also Green's paper is $\ce{BF3}$, which is like the other $\ce{Z}$ types is identified as a Lewis acid.

To more directly reference how Green's paper describes this notation, I include the passage below:

Adopting the view that the bonding in many covalent molecules can be represented in terms of 2-center-2-electron bonding interactions, there are three possible scenarios that describe the construction of these bonds in a molecular orbital sense, as illustrated in Figure 4. Thus, with respect to the central element of interest (M), the neutral ligand can contribute either two (L), one (X), or zero (Z) electrons to the bonding orbitals. The classification of ligands as L-, X-, or Z-type, as featured in a variety of textbooks,4 is now well established, and some simple examples of these ligands are provided in Table 1.

Some examples of classifications you can make are given in the table below or in the linked preview:

Table of common ligands and their electron counts in two different models of bonding

We can note some of the more complicated ligands we can simplify with this notation. $\eta^5$ cyclopentadienyl can be thought of as a $\ce{L_2X}$ type ligand based on the number of electrons contributed and it's charge. Again from Green:

Although many ligands coordinate to a metal center by a single covalent bond, a variety of multidentate ligands coordinate via more than one covalent bond. Such ligands are classified as $\ce{[L_lX_xZ_z]}$, where $l, x,$ and $z$ are the respective number of $\ce{L, X,}$ and $\ce{Z}$ functionalities that are associated with the frontier orbitals of the ligand in the geometry that corresponds to its binding mode.

This notation provides an easy means of electron counting, which is useful for determing whether a compound follows the 18 electron rule (a useful heuristic for determine the stability of metal complexes).

In regards to your main question, as far as I can gather from this, the reason $\ce{L}$ has become widely used as a generic ligand (or in this case, a generic $2\ce{e^-}$ donating ligand) is that, at least within the realm of inorganic chemistry, it is short and unambiguous when used to write a formula.

In addition, along with your answer that the ACS uses $\ce{L}$ for ligand, Green mentions that a number of textbooks have adopted the convention since their original paper in 1995, which means that the current generation of teachers and students is thinking of the material with this notation. So in addition to researchers that simply liked the scheme and chose to use it in their writing, students who learned this scheme from the beginning are starting to publish and make use of it because its what they know.

Some reasons why I expect there isn't a lot of confusion with leucine are:

  • Even now, authors will still specify what they mean when they throw in an extra L.

  • Leucine is uncommon outside biochemistry, but the term ligand is used in essentially every sub-discipline of chemistry.

  • I would imagine that the three letter amino acid codes are used more commonly anyway because they are clearer in their meaning (besides situations where space is at a premium and the context makes it clear, as is the case when writing out amino acid sequences).

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  • $\begingroup$ The following is not a directive, more an advice. I guess it's better either to refer to the original paper from Green, or it's more recent adaptation (Green, M. L. H.; Parkin, G. Journal of Chemical Education 2014, 91 (6), 807–816. DOI 10.1021/ed400504f). Also, according to the CBC method Crabtree is citing, there are 3 types of ligands based on how they contribute electrons to a bonding orbital: 2 (L), 1 (X), or 0 (Z). It also would be great if you could draw some sort of conclusion why $\mathrm{L}$ is preferred over other abbreviations (also, situation with leucine). $\endgroup$ – andselisk Aug 14 '17 at 11:39
  • $\begingroup$ My main reason for referring to the book was the Google preview allowed anyone to see at least a brief source of my information (I also didn't have access to the paper myself over the weekend). I will try to add in some additional information. $\endgroup$ – Tyberius Aug 14 '17 at 15:51
  • $\begingroup$ GoogleBooks also don't guarantee 100% accessibility to the given pages over time, and there is a queue for pages being visible to certain geolocations at certain periods of time. I think quoting the primary source is the most robust way to go. $\endgroup$ – andselisk Aug 14 '17 at 15:55
  • $\begingroup$ @andselisk anything else you are looking for out of answer? $\endgroup$ – Tyberius Aug 16 '17 at 1:53
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    $\begingroup$ In my limited experience, three-letter-codes are used for all applications except for amino acid sequences which tend to use one-letter-codes to save space. $\endgroup$ – Jan Sep 20 '17 at 2:00
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The ACS Style Guide suggests using $\ce{L}$ [1, p. 186] in APPENDIX 10-2 Abbreviations, Acronyms, and Symbols:

\begin{array}{ll} [\ldots] & \\ \small{L} & \text{absolute configuration} \\ \mathrm{L} & \text{leucine} \\ & \text{ligand} \\ & \text{liter} \\ \mathrm{L_I} & \text {spectral line} \\ [\ldots] & \end{array}


IUPAC "Green Book" Quantities, units, and symbols in physical chemistry also defines $\ce{L}$ [2, p. 160] in chapter 9 ABBREVIATIONS AND ACRONYMS:

\begin{array}{ll} [\ldots] & \\ \mathrm{KS} & \text{Kohn-Sham} \\ \\ \mathrm{L} & \text{ligand} \\ \mathrm{L2TOFMS} & \text {laser desorption laser photoionization time-of-flight mass spectroscopy} \\ [\ldots] & \end{array}


Glossary of Class Names of Organic Compounds and Reactivity Intermediates Based on Structure (IUPAC Recommendations 1995) uses $\ce{L}$ [3, p. 1349]. For example, see this entry:

metal-carbyne complexes:
Metal complexes of the type $\ce{RCML_n}$, ($\ce{M}$ = metal, $\ce{L}$ = ligand) in which formally a carbyne is coordinated to a metal. E.g.
enter image description here


$\mathrm{Lig}$ and other notations are not mentioned in the above sources as suitable ligand notations.

Bibliography

  1. The ACS Style Guide: Effective Communication of Scientific Information, 3rd ed.; Coghill, A. M., Garson, L. R.; American Chemical Society; Oxford University Press: Washington, DC; Oxford; New York, 2006. DOI 10.1021/bk-2006-STYG.
  2. IUPAC “Green Book” Quantities, units, and symbols in physical chemistry, 3rd ed.; Mills, I., Eds.; IUPAC Recommendations; RSC Pub: Cambridge, UK, 2007. ISBN 978-0-85404-433-7.
  3. Moss, G. P.; Smith, P. A. S.; Tavernier, D. Pure and Applied Chemistry 1995, 67 (8–9), 1307–1375 DOI: 10.1351/pac199567081307.
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    $\begingroup$ A paper by M. L. H. Green, 'A new approach to the formal classification of covalent compounds of the elements' (J. Organomet. Chem, $1995$, vol. 500, issues 1–2, pp 127–148, DOI: 10.1016/0022-328X(95)00508-N), might be of interest. It denotes (or classifies) general ligands mostly by letters $\mathrm{L,\ X,\ Z}$. This is based on the sometimes ambiguous number of electrons donated by the ligand. Green himself calls it the covalent bond classification. $\endgroup$ – Linear Christmas Aug 10 '17 at 21:28
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    $\begingroup$ @LinearChristmas Wow, that's a great find, and quite a lengthy reading. If you have some time, please do feel free to post this comment as an answer (maybe you can add a few crucial details/examples). Even though it seems like the paper doesn't touch biochemistry (the leucine case), it is still an interesting concept (and a new one to me). $\endgroup$ – andselisk Aug 10 '17 at 21:52
  • $\begingroup$ Thanks for the encouragement to turn the comment into an answer! I'll hold off for now since I have not read the whole thing. I might have time to do so in the second week of September; if it's still relevant by then... :) Sorry! $\endgroup$ – Linear Christmas Aug 10 '17 at 22:06
  • $\begingroup$ @LinearChristmas Aside from the bounty expiring in 6 days there is no hurry, I already started to read this thing. Maybe you can just post it now as is, and expand later if necessary. If nobody else answers this question in 6 days, I'm going to loose 100 xp for nothing (for the second time, first time I lost 50:)). $\endgroup$ – andselisk Aug 10 '17 at 22:16
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    $\begingroup$ Haha, I didn't notice there's a bounty (honestly). Unfortunately, I wasn't lying about not having time, in that I'm not just being lazy ;). If I don't make it before the bounty expires, and there's a good chance of that happening, it's my fault. I wouldn't be worthy... $\endgroup$ – Linear Christmas Aug 10 '17 at 22:24

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