# Why is dimethylglyoximate a bidentate ligand and not a tetradentate ligand?

Why can't the 2 oxygen atoms donate a lone pair and hence become binding sites? Is it because of the angle made by the oxygen and Nitrogen atoms?

You are right. I took this two pictures from Wikipedia to compare EDTA (a tetradentate) to dmg (a bidentate).

For EDTA you got this:

For dmg, on the other hand, you got this structure:

You can notice that for both of the cases, the metal complex is connected to the ligands through five-membered rings. This observation is general in chemistry: to have stable rings, you need five (sometimes six) members for those rings.

Now it is clear to see a general rule of thumb for multidentate ligands: the two donating atoms must be connected through 2 (sometimes 3) atoms. For dmg, you can see that the two nitrogens satisfy this rule, while the oxygen-nitrogen pairs do not.

For EDTA, you can see that going through the molecule, you can find an oxygen, a nitrogen, a second nitrogen and a second oxygen, all bridged by 2 atoms to each other (while the two oxygens of the carboxyl group are not connected by 2 bridge atoms, otherwise you might even have a hexadentate ligand).

• Would it it be possible to scale the structures so that the font on the illustrations approximately matches the one used in main text? Two structures look too tiny, and two are tremendously large and take too much of screen real estate for no good reason. Oct 29 '19 at 13:48
• I'm currently in the process of trying, but it looks like I am not skilled enough!
– user23638
Oct 29 '19 at 13:48
• @andelisk would manually scaling the pictures using a 3rd party software and then uploading it help? Oct 29 '19 at 13:52
• @TheJadeReaper Sure you can, but it will only give a satisfactory result for the larger ones. Upscaling a raster image will result in horrible quality, so someone has to either find appropriately scaled images, or just draw them from scratch. Oct 29 '19 at 14:01
• Suggestion for scaling: if you replace the Markdown image syntax with <img src="<imgur link>" width="<number>" alt="alternate text">, you can play around with the width to scale. Note: the arguments have to be in exactly that order.
– Jan
Oct 29 '19 at 14:53