Polydentate ligands can vary in the number of coordination positions they occupy. In each of the following, identify the polydentate ligand present and indicate the probable number of coordination positions it occupies: (a) \(\operatorname{Cr}(\mathrm{EDTA})^{-}\) (b) $\left[\mathrm{Ni}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right] \mathrm{Br}_{2}$ (c) $\left[\mathrm{Ru}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Cl}_{3}$ (d) \(\mathrm{K}_{2}\left[\mathrm{Fe}(\mathrm{O}\) -phen \()(\mathrm{CN})_{4}\right]\)

(a) In the complex \(\operatorname{Cr}(\mathrm{EDTA})^{-}\), the polydentate ligand is EDTA (ethylenediaminetetraacetic acid) which is a hexadentate ligand, hence it occupies six coordination positions. (b) In the complex \(\left[\mathrm{Ni}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]\mathrm{Br}_{2}\), the polydentate ligand is oxalate (\(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\)), which is a bidentate ligand, so it occupies two coordination positions. (c) In the complex \(\left[\mathrm{Ru}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{4}\right]\mathrm{Cl}_{3}\), the polydentate ligand is ethylenediamine (en), which is a bidentate ligand, and thus it occupies two coordination positions. (d) In the complex \(\mathrm{K}_{2}\left[\mathrm{Fe}(\mathrm{O}\)-phen\((\mathrm{CN})_{4}\right]\), the polydentate ligand is O-phenanthroline (O-phen). This ligand is a bidentate ligand and therefore occupies two coordination positions.