# What is the meaning of '(en)'?

What is the meaning of $\ce{(en)}$ on this two coordination compounds:

$\ce{[Co(en)_2(NO_2)_2]Cl}$
$\ce{[Cd(en)_2(CN)_2]}$

Is it a type of compound? If yes, what is the meaning?

It's an ethylenediamine ligand.

Edit - elaborating as per your request: Unfortunately, you don't provide any information as to how much you already know about complex chemistry, so I'm going back to the roots here. If you already know what a complex is, skip this paragraph. As you probably know, atoms and molecules may bind to each other, forming new compounds. We all know covalent bonds in which two atoms move close together and mutually share their electrons, and ionic bonds wherein one of the partaking partners is more egocentric in terms of electrons, "stealing" it from the other but staying in the vicinity nontheless. What these types have in common is that each atom or molecule provides an electron for the formation of the bond. However, it may also happen that both electrons come fron one partner alone; an electron-rich molecule or ion approaches another atom or ion, "donating" his electrons without really giving them up. This is the situation in coordination chemistry: A "central atom" or ion -- usually, but not necessarily a transition metal -- is surrounded by a varying number of "ligands" which donate electron pairs to the central atom, all while these electron pairs remain "located" at their respective ligands (very dangerous expression, I know ...). The compound that is formed is called a complex.

So ligands are atoms or molecules which can provide lone pairs for coordination bonds, i.e. electrons in non-bonding, doubly filled orbitals. In the above example of ethylenediamine, the binding centers of the molecule are the nitrogen atoms. The en ligand is an example of a multidentate (or chelate) ligand which may form more than one coordinating bond per molecule, while the $\ce{NO2^-}$ and $\ce{CN-}$ are monodentate ligands. The chloride anions in your first compound does not bind to the copper ion, but rather stay outside its "coordination sphere" to balance the positive charge of the cobalt complex (which you might then interpret as an ionic bonding between the complex cation and the chlorine anion).

Note that there are, of course, many quirks to this. Complexes with multiple central atoms, different theories about bonding, bridging or radical ligands, charge-transfer complexes, etc. This is a very wide and important field of chemistry, after all; nothing you could exhaustively explain in one or two paragraphs.

• Could you elaborate? Maybe explain a bit about what a ligand is etc. Your answer is really too small in its current form. Make it interesting :) Oct 21, 2012 at 10:06
• Oh, I thought it would be just about the nomenclature :) okay, editing ... Oct 21, 2012 at 12:16
• Ah, muuch better :) This sort of answer lets people learn more without having to look elsewhere.. Makes Stack Exchange a valuable internet resource. Oct 22, 2012 at 8:11

$\ce{(en)}$ serves as a ligand that binds to the metal ($\ce{Co}$) in the complex dinitro bis(ethylenediammine) cobalt (II) chloride.

en is $\ce{H2NCH2CH2NH2}$, a bidentate ligand coordinating through its nitrogen lone pairs, one pair/nitrogen. Compare with oxalate.