Is there a mnemonic to study the spectrochemical series? 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:

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


  Is there a mnemonic device for remembering this series?
I would not recommend a mnemonic for the spectrochemical series for a number of reasons:


Often, it depends on both metal and ligand and coordination geometry how big a certain splitting is.
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.
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.
Some ligands (e.g. $\ce{NO}$) can display very complex redox chemistry that defies them being ordered in a simple list.
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 &#x3c0; 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.