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The coordination number refers to the number of binders that interact with the central metal ion. So, in the complex $\ce{[Cr(C2O4)3]^3-}$ chromium is bound to three oxalate ions.
Why its coordination number is considered 6? So, is it an incorrect definition of coordination number?
Can you help me?
This seems to be the case of "show, don't tell".
IUPAC's definition of coordination number (C.N.) which applies to inorganic complexes:
In an inorganic coordination entity, the number of σ-bonds between ligands and the central atom. π-bonds are not considered in determining the coordination number.
Let's have a look at the one of the first determined structures of trioxalatochromate(III) inorganic salts [1]:
Figure 1. Crystal structure of $\ce{[Cr(C2O4)3]^3-}$ anion (ICSD-109607). Color scheme: $\color{#909090}{\Large\bullet}~\ce{C}$; $\color{#FF0D0D}{\Large\bullet}~\ce{O}$; $\color{#8A99C7}{\Large\bullet}~\ce{Cr}$.
Here, there are three oxalate $\ce{C2O4^2-}$ ligands arranged in distorted octahedral environment (depicted with semi-transparent coordination polyhedron). Each oxalate is a bidentate chelating ligand, resulting in C.N. 6 of chromium center.
As Ivan Neretin told it, each group oxalate has two groups $\ce{-COO^-}$. So it may be bound to a central metal by two oxygen atoms, one per group $\ce{-COO^-}$.
