Can we say that the coordination number of a particular element is fixed? If not please give an example where an element exhibits variable coordination numbers.
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2$\ce{Fe3O4}$ would do: some $\ce{Fe}$ have the coordination number of 4 and some have 6. – Ivan Neretin Jan 12 '16 at 10:48
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@Ivan that's the beginning of a very good answer. – M.A.R. Jan 12 '16 at 11:01
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2Heh, better to ask if there's any element which doesn't. – Mithoron Jan 12 '16 at 11:31
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No. Coordination numbers of no element are fixed. For example, if you add chloride ions to a pink solution of octahedral $\ce{[Co(H2O)6]^3+}$, a blue, tetrahedral complex is generated according to the following equation:
$$\ce{[Co(H2O)6]^3+ + 4 Cl- <=>> [CoCl4]- + 6 H2O}$$
In fact, if you study the mechanisms of different metal-catalysed organic reactions, you will notice that different coordination numbers of the central metal and specific switching between them is the key to understand these mechanisms.
Finally, for some complexes even establishing their coordination is hard. Zinc(II) forms aquacomplexes like any other cation. But while those complexes of transition metals with any number of d-orbitals unequal to zero or ten are often well-defined and established, that is not the case for zinc. Zinc(II) aquacomplexes rapidly equilibrate from tetracoordinated and pentacoordinated to hexacoordinated species.
Jan
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Can we say that for a particular ligand, the C.N. of a certain central metal ion is fixed? e.g., $\ce{[CoCl4]^2-}$ exist but does $\ce{[CoCl6]^4-}$ exist (for the same $\ce{Co^2+}$ metal ion)? – Apurvium Oct 21 '21 at 13:12
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1@Apurvium No. In organic chemistry, a number of $\ce{[Pd_x(dba)_y]}$ complexes are known and characterised including $\ce{[Pd2(dba)3]}$ (three ligands per palladium centre) and $\ce{[Pd(dba)2]}$ (two chelating ligands per palladium centre). – Jan Oct 21 '21 at 15:41