When do cyanide ligands form a tetrahedral geometry with Co2+ ?

[adf89812]

TL;DR

cyanide ligands form a tetrahedral geometry when?

Is there something about the repulsion of the lone pair on thenitrogen that prevents the cyanides being all on the same plane?

 

[mjc123]

When do cyanide ligands form a tetrahedral geometry with Co2+ ?​

Do they? Wikipedia (yes, I know) suggests that Co²⁺ forms Co(CN)₅^3-:
https://en.wikipedia.org/wiki/Cyanometalate
Ni(CN)₄^2- is square planar, so there's nothing preventing 4 cyanides being in the same plane.