(thing) by Iguanaonastick Mon Apr 26 2004 at 20:08:59

What's in a name?

Working with a compound without knowing it's name is like making love to a beautiful man/woman without knowing his/her name: If it came down to it you could, but it would be awkward. For your pleasure and your lab partners', know your names. Names tell you the ions, structure, and even isometries of a compound. If only our names were that descriptive.

A rose by any other name would smell as sweet

Every compound has a unique name and a structural formula. Use the name when talking and use the formula when writing: It makes everything a whole lot easier. Trying to say "[Cu(CN)4(H2O)2]Cl2" is an exercise in futility, while writing "Diaquatetracyanocopper (VI) chloride" is a waste of paper.

Let's get this party started!

This whole process will be much more enjoyable if you put on some music. I recommend smooth jazz, minimal techno or porno music.

We start just like we're naming regular ol' compounds:

(Cation)(Anion)

Peachy. Next we add the ligands. Ligands go before the metal:

(Ligands-Metal)(Anion) or (Cation)(Ligands-Metal) or (Ligands-Metal)(Ligands-Metal)

Anionic ligands get an -o where there was an -ide or an -ato where there was an -ate. Look:

You get the idea. The neutral ligands keep their original names, with a couple exceptions: I shall call him... Mini-me

You're probably gonna have more than one ligand, so we need prefixes.

  1. mono- Note: usually omitted
  2. di-
  3. tri-
  4. tetra-
  5. penta-
  6. hexa-
  7. hepta-
  8. octa-
You'll never see more than 8 ligands on an atom, unless you're some tweaked out research chemist with no life and a bunch of lanthanides. It feels so good!!!!

Don't get too excited. If the ligand already has a number prefix, like ethylenediamine, then we have to do something else to tell how many ions we have. Watch closely:

  1. DOES NOT EXIST
  2. bis
  3. tris
  4. tetrakis
  5. pentakis
  6. hexakis
  7. septakis
  8. octakis
Now we can name compounds like [Co(en)3]Cl3: it's tris(ethylenediamine)cobalt(III) chloride.

Easy as one two three...

If you have more than one type of ion, put them in alphabetical order. Just like kindergarden:

  • [Cu(CN)4(H2O)2]Cl2 is diaquatetracyanocopper(VI) chloride
  • [Cr(NH3)5Br]SO4 is pentaamminebromocromium(III) sulfate
So now that we've got the ligands down, let's talk about the metal. Just like with regular ol' compounds, you have to put the oxidation number of the metal in Roman numerals in parentheses after the ion. If it's 0, then put 0. If we're dealing with a cation, the name of the metal stays the same, but if we're dealing with an anion, we've got some work to do. Take the -um (or whatever other suffix, if there is one) at the end, and stick on an -ate. If the element has a Latin name, use that (except mercury).
Witness!
Genius!!!

Oh, in case you were wondering, you can thank Alfred Werner for coming up with these rules in 1893 when he figured out how this whole ligand thing worked (for which he got a Nobel Prize in 1913). You can also thank IUPAC for keeping them, with a few modifications.

But Wait!! There's More!!

So far our description covers the ions and the basic structure, but not the isometries. There are all sorts of isometries:

Coordination isometries
Ions can coordinate either directly to the metal or bond to the ligands. Ex: [Cr(NH3)5(OSO3)]Br and [Cr(NH3)5Br]SO4
Linkage isometries
Some ligands can bond to the metal in more than one atom. Ex: SCN- (the S bonds) and NCS- (the N bonds)
Geometric isometries
Ligands can attach to the metal in many different configurations, some of which are not identical.
Optical isometries
Different geometric isometries polarize light differently.
Oh boy...

Don't worry, we can deal with all of them. Coordination isometries are already taken care of in our naming system, and the rest aren't too hard. Linkage isomers just get an iso-:

  • SCN- is thiocyanate
  • NCS- is isothiocyanate

Two down, two to go

Geometric isometries are a little trickier. See the stereoisomerism node for a good explanation. Depending on the relative positions of the ligands, the names get either a cis- or a trans- (Note: the italics are not for effect):

  • cis-Pt(NH3)2Cl2 is cis-diammineplatinum(II) chloride
  • trans-Pt(NH3)2Cl2 is trans-diammineplatinum(II) chloride

Optical isomers (aka enantiomers) are funky, too. You should check out the enantiomer and optical isomerism nodes. Suffice it to say that when polarized light passes through different enantiomers, the light is rotated differently. Clockwise rotation (when looking towards the light source) is denoted by d-, for dextrorotatory. Counterclockwise rotation is denoted by l-, for levorotatory.
  • dextrorotatory cis-[CoCl2(en)2]Cl is d-cis-dichlorobis(ethylenediamine)cobalt(III) chloride.
  • levorotatory cis-[CoCl2(en)2]Cl is l-cis-dichlorobis(ethylenediamine)cobalt(III) chloride.

That's all, folks!

Congratulations! Now you can impress your family and friends with your vast knowledge of chemistry nomenclature.


Sources:
http://www.chem.purdue.edu/gchelp/cchem/index.html
Ebbing & Gammon General Chemistry 7th ed. © 2002.

I like it! 2 C!s

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