[BilloRani2012]

A compound contains a transition metal coordination complex. This coordination complex consists of one cobalt(III) ion with five ammonia molecules and one cyanide ion attached as ligands. The formula of the compound, including an appropriate anion or cation to satisfy the ionic charge, could properly be written as

a) [Co(NH3)5CN]SO4
b) K4[Co(NH3)5CN]
c) K2[Co(NH3)5CN]
d) [Co(NH3)5CN](SO4)2

I am tossing between a) and d)..but i'm pretty sure its a)...would that be right?

[Honclbrif]

What charge does Co bring? What charge does ammonia bring? What charge does cyanide bring? When you add them up according to the formula, what is the total? How do you balance it?

[BluesClues]

There are two methods you can use to figure this out: (1) Ionic  or (2) Neutral counting. There are benefits and cons to both, but I prefer ionic so I'll teach you that. This is what was meant by Honcibrif (above) when he is asking what charges the other ligands bring to the metal. So here we go. To go about Ionic method, go ahead and draw the complex (with the Metal at the center of course, and the ligands connected by appropriate bonding). Now break them apart heterolytically by moving the electrons(2 per single bond) towards the ligands (so imagine without the bonds connecting the ligands to the metal). Look at your ligands, and using your common organic knowledge, see if they are either charged or neutral (for example: halides are charged... why? make sure you know this). Now for each ligand that is charged, add the opposite charge to the metal - so for example, if you had a halide which is negatively charged, you would add +1 to the metal. do this for all the ligands until you finish. Add up all the charges on the metal, and that should be your oxidation state.

Basically, what you are doing is figuring out the number of electrons donated from ligand to metal.

Heres an example:
M-Cl  -->  M  + :Cl(-1) --> therefore the metal must have had an oxidation of +1.