Question

For each of the following polydentate ligands, determine (i) the maximum number of coordination sites that the ligand can occupy on a single metal ion and (ii) the number and type of donor atoms in the ligand: (a) acetylacetonate ion (acac), (b) phenanthroline (phen), (c) diethylenetriamine, (d) carbonate ion, (e) triphosphate ion.

Short answer

For the given polydentate ligands, we have: (a) Acetylacetonate ion (acac): (i) Maximum number of coordination sites: 2 (ii) Number and type of donor atoms: 2 (oxygen) (b) Phenanthroline (phen): (i) Maximum number of coordination sites: 2 (ii) Number and type of donor atoms: 2 (nitrogen) (c) Diethylenetriamine: (i) Maximum number of coordination sites: 3 (ii) Number and type of donor atoms: 3 (nitrogen) (d) Carbonate ion: (i) Maximum number of coordination sites: 1 (ii) Number and type of donor atoms: 1 (oxygen) (e) Triphosphate ion: (i) Maximum number of coordination sites: 2 (ii) Number and type of donor atoms: 2 (oxygen)

Step-by-step solution

(a) Acetylacetonate ion (acac)

Acetylacetonate ion (acac) has the following structure: \( \hspace{0.5cm} \text{O} = \text{C} - \text{C} - \text{O} - \text{C} \hspace{0.5cm} \) In this ion, there are two oxygen atoms that can act as donor atoms: the ones that are double-bonded to the carbon atoms. Thus, acac can occupy two coordination sites on a single metal ion, and each donor oxygen atom can form a single bond with the metal ion. (i) Maximum number of coordination sites: 2 (ii) Number and type of donor atoms: 2 (oxygen)

(b) Phenanthroline (phen)

Phenanthroline (phen) has the following structure: \hspace{1cm} N \hspace{0.5cm} \text{C} = \text{C} - \text{C} - \text{C} = \text{N} \hspace{0.5cm} In this molecule, there are two nitrogen atoms that can act as donor atoms. Thus, phen can occupy two coordination sites on a single metal ion, with each donor nitrogen atom forming a single bond with the metal ion. (i) Maximum number of coordination sites: 2 (ii) Number and type of donor atoms: 2 (nitrogen)

(c) Diethylenetriamine

Diethylenetriamine has the following structure: \(\hspace{1cm} \text{N} - \text{C} - \text{C} - \text{N} - \text{C} - \text{C} - \text{N} \hspace{0.5cm} \) In this molecule, there are three nitrogen atoms that can act as donor atoms. Thus, diethylenetriamine can occupy three coordination sites on a single metal ion, with each donor nitrogen atom forming a single bond with the metal ion. (i) Maximum number of coordination sites: 3 (ii) Number and type of donor atoms: 3 (nitrogen)

(d) Carbonate ion

The carbonate ion has the following structure: \hspace{1cm} \text{O} = \text{C} - \text{O}^{-} \hspace{0.5cm} The oxygen that is single-bonded to the central carbon and has a negative charge can act as a donor atom. Thus, the carbonate ion can occupy one coordination site on a single metal ion, with the donor oxygen atom forming a single bond with the metal ion. (i) Maximum number of coordination sites: 1 (ii) Number and type of donor atoms: 1 (oxygen)

(e) Triphosphate ion

The triphosphate ion has the following structure: \hspace{1cm} \text{O}^{-} - \text{P} - \text{O} - \text{P} - \text{O} - \text{P} - \text{O}^{-} \hspace{0.5cm} In this ion, the single-bonded oxygens with negative charges on each end of the chain can act as donor atoms. Thus, the triphosphate ion can occupy two coordination sites on a single metal ion, with each donor oxygen atom forming a single bond with the metal ion. (i) Maximum number of coordination sites: 2 (ii) Number and type of donor atoms: 2 (oxygen)