there was this one question about complexes. It asked which would most likely form ligands in a Co+2 complex: CO, O, or Iodide, and something else. Well, how do you determine which will most likely form the ligand or on?

CO-ligands prefer low-oxidation state metal centers. This is unlikely to form a complex with Co(2+).

 

O-ligands prefer high-oxidation state metal centers, think of Mn(VII) or Cr(VI) in the permanganate ion and chromate ion.

 

I(-) is intermediate, it indeed forms a complex with Co(2+). I've tried this and Co(2+) forms a deep green complex with I(-), but it requires some heating and a really high concentration of iodide.

 

As for redox reactions and for acids, there also is a table for ligands. They can be ordered from affinity for low-oxidation state metal centers to very high oxidation metal centers. With this table at hand, you can make reasonable predictions on which ligands form a complex with a certain metal. This table, however, is not as predictive as a table for redox reactions or for acids.

 

Hard lewis bases prefer hard lewis acids and soft lewis bases prefer soft lewis acids. The ligands are the lewis bases and the metal centers are the lewis acids.

 

Metals can be classified as hard, intermediate or soft lewis acids and so can ligands be classified. By using these tables you can predict to some extent, whether a complex is stable or not.

 

If you google for the four words hard lewis base complex then you'll find a lot of documents, with explanations about this stuff. A nice one is the following:

 

http://www.meta-synthesis.com/webbook/43_hsab/HSAB.html