Question

Consider the complex ion \(\left[\mathrm{Co}(\mathrm{en})_{2}(\mathrm{SCN}) \mathrm{Cl}\right]^{+} .\) (a) Identify the ligands and their charges. (b) What is the oxidation number of cobalt? (c) What is the formula for the sulfide salt of this ion?

Short answer

Question: Identify the ligands in the complex ion [Co(en)₂(SCN)Cl]⁺, determine the oxidation number of cobalt, and find the formula for the sulfide salt of this ion. Answer: The ligands present in the complex ion are ethylenediamine (en, charge 0), thiocyanate (SCN, charge -1), and chloride (Cl, charge -1). The oxidation number of cobalt in the complex ion is +3. The formula for the sulfide salt of this ion is [Co(en)₂(SCN)Cl]₂S.

Step-by-step solution

(a) Identifying Ligands and Their Charges

First, we have to identify the ligands within the complex ion. Ligands are atoms, ions or molecules that bind to a central metal atom to form a complex ion. The ligands in the complex ion \(\left[\mathrm{Co}(\mathrm{en})_{2}(\mathrm{SCN})\mathrm{Cl}\right]^{+}\) are: 1. Ethylenediamine (en): It is a neutral bidentate ligand that forms two coordinate bonds with the central metal atom. The charge on ethylenediamine is \(0\). 2. Thiocyanate (SCN): It is a monodentate ligand that forms a single coordinate bond with the central metal atom. The charge on thiocyanate is \(-1\). 3. Chloride (Cl): It is also a monodentate ligand that forms a single coordinate bond with the central metal atom. The charge on chloride is \(-1\).

(b) Oxidation Number of Cobalt

To find the oxidation number (ON) of cobalt, we need to balance the charges in the complex ion. The overall charge of the complex ion is \(+1\). Let's denote the oxidation number of cobalt (Co) as x. The sum of the charges of the ligands must equal the oxidation number of cobalt minus one, since the complex ion has a charge of +1: \(x + (0) + (0) + (-1) + (-1) = +1\) Solving for x, we get the oxidation number of cobalt (Co): \(x - 2 = +1\) \(x = +3\) The oxidation number of cobalt in the complex ion is \(+3\).

(c) Formula for the Sulfide Salt of the Ion

The sulfide salt of this ion would form when the positive ion associates with the sulfide ion (S\(^{2-}\)). To find the formula for the sulfide salt, we need to make sure that the charges are balanced. The complex ion has a charge of +1, while the sulfide ion has a charge of \(-2\). To balance the charges, we need two moles of complex ions for each mole of sulfide ions: \(2([\mathrm{Co}(\mathrm{en})_{2}(\mathrm{SCN})\mathrm{Cl}]^{+}) + S^{2-} \rightarrow [\mathrm{Co}(\mathrm{en})_{2}(\mathrm{SCN})\mathrm{Cl}]_{2}S\) Therefore, the formula for the sulfide salt of the complex ion is \([\mathrm{Co}(\mathrm{en})_{2}(\mathrm{SCN})\mathrm{Cl}]_{2}S\).

Key concepts

Ligands and Their Charges

Understanding ligands and their charges is critical for analyzing complex ions in chemistry. Ligands are molecules or ions that can donate a pair of electrons to a central metal atom or ion to form a coordination bond. In the exercise, the complex ion \(\left[\mathrm{Co}(\mathrm{en})_{2}(\mathrm{SCN})\mathrm{Cl}\right]^{+}\) is examined, revealing varieties of these ligands.

Ligands can be neutral molecules like ethylenediamine (\(en\)), which in the given example has no charge, or negatively charged ions such as thiocyanate (\(SCN^-\)) and chloride (\(Cl^-\)). The charges of these ligands assist in determining the overall charge of the complex.

• \(en\): neutral charge (0).
• \(SCN^-\): charge (-1).
• \(Cl^-\): charge (-1).

It's important for students to identify that while ethylenediamine forms two bonds with the metal (thus termed 'bidentate'), the charge remains zero, and the unit charge count per ligand comes into play when we balance the charge of the entire complex.

Oxidation Number Determination

The oxidation number of the central metal in a coordination compound is key to understanding the compound's properties and reactivity. It is the hypothetical charge that the central atom would have if all the ligands were removed along with the electrons they shared with the metal. To calculate the oxidation number in our complex ion, we need to consider the sum of the charges of all the ligands and compare that to the overall charge of the ion.

For \(\left[\mathrm{Co}(\mathrm{en})_{2}(\mathrm{SCN})\mathrm{Cl}\right]^{+}\), the sum of the charges from the thiocyanate and chloride ligands is -2. Uncharged ligands like ethylenediamine do not count towards this sum. As the overall ion carries a charge of +1, the oxidation number of cobalt (Co) must then balance the charge difference, leading us to an oxidation number of +3 for cobalt. The balancing act can be denoted by the equation \( x - 2 = +1 \) pointing clearly to the sought oxidation number \( x = +3 \).

This skill of balancing charges and determining oxidation states of transition metals is foundational in the study of inorganic chemistry.

Coordination Compounds

Coordination compounds, such as the complex ion discussed, encompass a wide array of substances that feature metal atoms or ions surrounded by ligands. The distinctive arrangement within these compounds is called a coordination sphere, which includes the central metal and its surrounding ligands linked through coordinate covalent bonds.

Their formulas often seem overwhelming, but they provide a plethora of information once unraveled. The complex ion in question gives us insight into this type of compound. It showcases a central cobalt ion bound to ethylenediamine and accompanied by monoanions like thiocyanate and chloride. The overall positive charge is a clue to the kind of counterions or opposite charges needed to balance the ionic formula, as seen when determining the chemical formula for the sulfide salt of this complex: \([\mathrm{Co}(\mathrm{en})_{2}(\mathrm{SCN})\mathrm{Cl}]_{2}S\).

Students should recognize that coordination compounds can have vast applications, from medicinal biochemistry to materials science, due to the various possible combinations of metal centers and ligands.