Is there a mnemonic to study the spectrochemical series?

Question

Mnemonics help to by-heart things which are difficult to do so and which are important. I found mnemonics for studying the periodic table, electromagnetic series etc which were of great help. So, is there a mnemonic to study the spectrochemical series since knowing it will be of great help to find the geometry of a given co-ordination compound?

The spectrochemical series is a list of common ligands for transition metals in order of increasing ligand-field splitting parameter $\Delta$, which is the energy difference between the sets of $\ce{d}$ orbitals:

$$\ce{I-} < \ce{Br-} < \ce{S^2-} < \ce{SCN-} < \ce{Cl-} < \ce{NO3-} < \ce{N3-} < \ce{F-} < \ce{OH-} < \ce{H2O} \approx \ce{C2O4^2-} \text{ (oxalate)} < \ce{NCS-} < \ce{CH3CN} < \ce{py}\text{ (pyridine)} < \ce{NH3} < \ce{en}\text{ (ethylenediamine)} < \ce{bpy}\text{ (2,2'-bipydiridyl)} < \ce{phen}\text{ (1,10-phenanthroline)} < \ce{NO2-} < \ce{PPh3} < \ce{CN-} \approx \ce{CO}$$

Is there a mnemonic device for remembering this series?

Answer 1

I would not recommend a mnemonic for the spectrochemical series for a number of reasons:

1. Often, it depends on both metal and ligand and coordination geometry how big a certain splitting is.

2. The splitting of some ligands ($\ce{NH3, H2O}$) is even dependent on the pH — even though that could sort-of be overcome by including their corresponding anions in the list.

3. Any mnemonic is bound to be incomplete. N-heterocyclic carbenes (NHCs) are very important ligands especially in organometallic chemistry, but they aren’t on your list.

4. Some ligands (e.g. $\ce{NO}$) can display very complex redox chemistry that defies them being ordered in a simple list.

5. Rather than learning some list off by heart and rote memorising it later, it is much better to know why a ligand is strong- or weak-field and to deduce that property from the ligand’s structure.

The last point is most important. Rather than learning or attempting to remember that chloride is a weak-field ligand, ask yourself why. It is a π donor ligand that will split the $\mathrm{t_{2g}}$ orbitals of the central metal in a bonding–antibonding way, lifting the metal’s $\mathrm{t_{2g}}$ orbitals and lowering the energy difference. That way, I immediately know what to expect of (say) selenide $\ce{Se^2-}$ even if it is not present in the list. The strength of a certain ligand in this discussion is equivalent to the usual trends in the periodic table. Quickly memorised, no long mnemonic and applicable even to new ligands.

Answer 2

The following mnemonic could be used to remember the spectrochemical series:

I brought some colourful sweets from office containing water. Nisha ate nine in canteen corridor

$\ce{I- < Br- < SCN- < Cl- < S^{2-} < F- < OH- < C2O4^{2-} < H2O < NCS- < edta^{4-} < NH3 < en < CN- < CO}$

Answer 3

I use the following to remember the spectrochemical series;

I brought some cloth, nylon net from office, collect water near cyanomethane place, among each big pants, place no proper concentration or concern.

($\ce{I, Br, S, Cl, NO_3, N_2, F, OH, CrO_2, H_2O, CH_3CN, py, NH_3, en, bpy, phen, NO_2, PPh3, CN, CO}$ respectively.)