For example in the following compounds

  1. CoCl3.6H2O
  2. CoCl3.6NH3
  3. CoCl3.5NH3
  4. CoCl3.4NH3

in all of these complexes, we took the

a)oxidation state of Cobalt as +3 ( since there are 3 Cl- ions and NH3 and H2O are neutral ligands )

b) Co-ordination number of Cobalt as 6

and the answer to writing the compounds was:

1.[Co(H2O)6]Cl3 or [Co(H2O) 5Cl]Cl2.H2O

  1. [Co(NH3)6]Cl3

  2. [Co(NH3)5Cl]Cl2

  3. [Co(NH3)4Cl2]Cl

My question is this: Why do we take the co-ordination number of Cobalt always six?

Can't Co take other co-ordination numbers like 4? or 5?

So for the following compound


Can't we write it as [Co(NH3)4]Cl3 instead of


Also how to predetermine the C.N of the central metal atom just by looking at anions and ligands? (cases when no coordination sphere is marked like in my above examples)

  • $\begingroup$ This link includes a tetrahedral coordiation complex of cobalt, where the coordination number is 4. $\endgroup$
    – Lynn Gu
    Mar 13, 2021 at 6:12
  • $\begingroup$ So how do we pre-determine the C.N of the central metal atom ? $\endgroup$ Mar 13, 2021 at 6:21
  • 1
    $\begingroup$ @TanyaGupta We need to know the structure (for example, crystal structure). Even then there might be issues with the determination of C.N. — that's why a concept of fractional coordination numbers exists. $\endgroup$
    – andselisk
    Mar 13, 2021 at 8:53
  • $\begingroup$ Could you give an example that uses the concept of fractional co-ordination number for a compound? $\endgroup$ Mar 13, 2021 at 12:05

1 Answer 1


With these compounds, you have coordination number $6$, but like other transition metals cobalt can have different coordination numbers with the right ligands. In this answer the $\ce{CoO4^{3-}}$ ion is discussed as an example of a tetrahedral complex with the rare case of high spin despite the central ion having an oxidation state of $+5$. (The tetrahedral geometry and strongly pi-donating ligands favor the high-spin electron confuguration.) Five-coordination is seen in Co(II) complexes with pi-accepting organic ligands such as $\ce{o-(C6H4)(SCH3)(P(C6H5)2)}$ [1].

  1. Geoffrey Dyer and Devon W. Meek (1967). "Five-coordination. VI. Low-spin cobalt(II) complexes with with bidentate ligands". J. Am. Chem. Soc. 89, 16, 3983–3987. https://doi.org/10.1021/ja00992a006

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