[Ciara]

Why can you not use a Co(III) ion as starting material for methlycobaloxime (model for vitamin (B12))? Im guessing its something to do with the ligand field splitting energies of cobalt complexes but im still not getting it please help this is driving me mad because I know its probably something simple.

[Grundalizer]

No idea, but the all knowing google gave me this.  It looks like in the intro it talks about starting with Co(I) or Co(II) because ligands bind to it quicker or something.  Didn't read it in its entirety, but looks like something you were looking for.

http://www.chm.bris.ac.uk/motm/vitaminb12/cobaloximes/expt4.htm

[Ligander]

Why can you not use a Co(III) ion as starting material for methlycobaloxime (model for vitamin (B12))? Im guessing its something to do with the ligand field splitting energies of cobalt complexes but im still not getting it please help this is driving me mad because I know its probably something simple.

IMHO, it's because low spin d⁶ such as Co(III) complexes are very inert.