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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

CoCl3.4NH3

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

[Co(NH3)4Cl2]Cl

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)

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  • $\begingroup$ 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
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    $\begingroup$ @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

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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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