[akalovexx]

1) Explain why cobalt(III) complexes are almost all six coordinate, while  complexes
of high-spin cobalt(II) can be both four and six coordinate.  What  are the expected
geometries for these coordination numbers? (6 marks)

i understand four co-ordiante being tetrahedral and 6 co-ordinate being octehedral and that Co3+ --> 3d6
Co2+ --> 3d7

but i dont know what actually makes it go tetrahedral or octehdral... please help ive been stressing over this particular question for some time.

Another similar question to do with shape preferences:

Nickel(II) can form complexes of octahedral, square-planar, and tetrahedral geometry.
Predict the geometry of the complexes formed with each of the ligands below and
justify your choice.
Chloride  Cyanide  Water.

Please help with the factors for octehedral, square planar or tetrahedral site preference.

Any help would be greatly appreciated, thank you very much in advance.