Question

The cobalt(III) ion, \(\mathrm{Co}^{3+},\) forms a 1: 1 complex with \(\mathrm{EDTA}^{4-}\). What is the net charge, if any, on this complex, and what would be a suitable formula for it (using the symbol EDTA)?

Short answer

The net charge on the \(\mathrm{Co}^{3+}\):\(\mathrm{EDTA}^{4-}\) complex is -1, and the suitable formula for the complex is \(\mathrm{[Co(EDTA)]^-}\).

Step-by-step solution

Understanding the cobalt(III) ion

The cobalt(III) ion, denoted as \(\mathrm{Co}^{3+}\), carries a charge of +3. It is important to know the charge of the metal ion before determining the charge of the entire complex.

Understanding the EDTA ion

EDTA, which stands for ethylenediaminetetraacetic acid, in its fully deprotonated form, is denoted as \(\mathrm{EDTA}^{4-}\) and carries a charge of -4. This will be the ligand that coordinates with the cobalt(III) ion.

Determining the net charge of the complex

In a 1:1 complex between \(\mathrm{Co}^{3+}\) and \(\mathrm{EDTA}^{4-}\), the charges will combine to give the net charge. The cobalt ion contributes +3 and EDTA contributes -4. Therefore, the net charge is \(3+(-4)=-1\).

Writing the formula for the complex

The formula for the complex will include both the metal ion and the EDTA ligand. Since the complex has a 1:1 ratio, the formula will be \(\mathrm{[Co(EDTA)]^-}\), with the overall net charge of -1 indicated as superscript outside the square brackets.

Key concepts

Coordination Chemistry

Coordination chemistry is a branch of inorganic chemistry focusing on coordination compounds where central metal atoms are bonded to one or more ligands. These molecules are called coordination compounds or complexes. A ligand is an ion or molecule that donates a pair of electrons to the metal atom, forming a coordination bond. Unlike simple ionic or covalent bonds, coordination bonds form a distinctive structure often represented in square brackets in chemical formulas.

The central metal ion in a coordination complex can have different oxidation states, which plays a critical role in determining the properties and reactivity of the complex. In our example, the cobalt(III) ion exhibits a +3 oxidation state, a fact that substantially influences how it interacts with ligands.

Net Charge of Complexes

The net charge of a coordination complex is the algebraic sum of the oxidation state of the metal ion plus the charges on the ligands. It is essential to balance the charges when constructing the complex. For the cobalt(III) EDTA complex, the cobalt ion has a +3 charge, while the EDTA ligand carries a -4 charge. By combining these, the net charge of the resulting complex is calculated to be -1, indicating that it is an anionic complex. Understanding and calculating the net charge is critical when predicting the physical and chemical behavior of the complex, including its solubility, color, and reactivity.

Writing Formulas for Complexes

Writing formulas for coordination complexes involves clear rules that link the structure of the complex with its chemical formula. The central metal ion is written first, followed by the ligands in alphabetical order. The entire coordination sphere, comprising the metal and its attached ligands, is enclosed in square brackets. The charge, if any, appears as a superscript outside the right bracket. For instance, the formula for the cobalt(III) EDTA complex is correctly written as \[\mathrm{[Co(EDTA)]^-}\], where EDTA is bound to cobalt in a 1:1 ratio, and the negative superscript represents the net charge of the complex. This standardized notation allows chemists to communicate complex structures unambiguously.

Ligand-Metal Ion Coordination

Ligand-metal ion coordination refers to the process where ligands bond to a central metal ion, forming a complex. A ligand must have at least one pair of electrons to donate, and the metal ion generally has empty orbitals to accept these electrons. Coordination number is a term used to describe the number of ligand donor atoms that are bonded to the central metal ion. EDTA, a hexadentate ligand, has six donor atoms and thus can form several coordination bonds with a single metal ion, such as cobalt(III).

In the cobalt(III) EDTA complex, the EDTA ligand wraps around the cobalt ion and donates its electron pairs, resulting in an octahedral structure typical for such complexes. The metal-ligand coordination not only defines the geometry of the complex but also its stability and properties in solution.