Question

Indicate the coordination number of the metal and the oxidation number of the metal in each of the following complexes: (a) \(\mathrm{K}_{3}\left[\mathrm{Co}(\mathrm{CN})_{6}\right]\) (b) \(\mathrm{Na}_{2}\left[\mathrm{CdBr}_{4}\right]\) (c) \(\left[\mathrm{Pt}(\mathrm{en})_{3}\right]\left(\mathrm{ClO}_{4}\right)_{4}\) (d) \(\left[\mathrm{Co}(\mathrm{en})_{2}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\right]^{+}\) (e) \(\mathrm{NH}_{4}\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{2}(\mathrm{NCS})_{4}\right]\) (f) \(\left[\mathrm{Cu}(\mathrm{bipy})_{2} \mathrm{III}\right.\)

Short answer

(a) Co: coordination number = 6; oxidation number = +3 (b) Cd: coordination number = 4; oxidation number = +2 (c) Pt: coordination number = 6; oxidation number = +4 (d) Co: coordination number = 6; oxidation number = +1 (e) Cr: coordination number = 6; oxidation number = +3 (f) Cu: coordination number = 4; oxidation number = +3

Step-by-step solution

(a) Finding the coordination number and oxidation number for \(\mathrm{K}_{3}\left[\mathrm{Co}(\mathrm{CN})_{6}\right]\)

The coordination number refers to the number of ligands attached to the central metal ion. In this case, the metal ion is Co. There are six cyanide (CN) ligands surrounding the Co ion, therefore the coordination number is 6. To find the oxidation number of Co, we must analyze the charges of the whole complex. The complex has an overall charge of -3, because there are three potassium ions (K) each with a +1 charge. Each cyanide ligand has a -1 charge, thus: \(\mathrm{Co}\) + 6*(-1) = -3 \(\mathrm{Co}\) - 6 = -3 The oxidation number of Co is +3. So, the coordination number is 6 and the oxidation number is +3.

(b) Finding the coordination number and oxidation number for \(\mathrm{Na}_{2}\left[\mathrm{CdBr}_{4}\right]\)

The metal ion is Cd, and there are four bromide (Br) ligands surrounding it. Therefore, the coordination number is 4. The complex has an overall charge of -2, due to two sodium ions (Na) each with a +1 charge. Each bromide ligand has a -1 charge. We can write the equation: \(\mathrm{Cd}\) + 4*(-1) = -2 \(\mathrm{Cd}\) - 4 = -2 The oxidation number of Cd is +2. So, the coordination number is 4 and the oxidation number is +2.

(c) Finding the coordination number and oxidation number for \(\left[\mathrm{Pt}(\mathrm{en})_{3}\right]\left(\mathrm{ClO}_{4}\right)_{4}\)

The metal ion is Pt, and there are three ethylenediamine (en) ligands surrounding it. Therefore, the coordination number is 6 since each en ligand has two donating sites. The complex has an overall charge of +4, due to four perchlorate (ClO4) ions each with a -1 charge. We can write the equation: \(\mathrm{Pt}\) = +4 The oxidation number of Pt is +4. So, the coordination number is 6 and the oxidation number is +4.

(d)Finding the coordination number and oxidation number for \(\left[\mathrm{Co}(\mathrm{en})_{2}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)\right]^{+}\)

The metal ion is Co, and there are two ethylenediamine (en) ligands and one oxalate (C2O4) ligand surrounding it. Therefore, the coordination number is 2*2 + 2 = 6. The complex has an overall charge of +1. We can write the equation: \(\mathrm{Co}\) = +1 The oxidation number of Co is +1. So, the coordination number is 6 and the oxidation number is +1.

(e) Finding the coordination number and oxidation number for \(\mathrm{NH}_{4}\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{2}(\mathrm{NCS})_{4}\right]\)

The metal ion is Cr, and there are two ammonia (NH3) and four isothiocyanate (NCS) ligands surrounding it. Therefore, the coordination number is 2 + 4 = 6. The complex has an overall charge of -1. We can write the equation: \(\mathrm{Cr}\) + 2*0 + 4*(-1) = -1 \(\mathrm{Cr}\) - 4 = -1 The oxidation number of Cr is +3. So, the coordination number is 6 and the oxidation number is +3.

(f) Finding the coordination number and oxidation number for \(\mathrm{Cu}(\mathrm{bipy})_{2}\)

The metal ion is Cu, and there are two bipyridyl (bipy) ligands surrounding it. Each bipy ligand has two donating sites, so the coordination number is 2*2 = 4. The oxidation number for this copper complex can be found because it's indicated with the Roman numeral III. Therefore, the oxidation number of Cu is +3. So, the coordination number is 4 and the oxidation number is +3.

Key concepts

Coordination Number

In coordination chemistry, understanding the concept of coordination number is key. The coordination number is the total count of atoms, ions, or molecules that a central metal atom or ion holds as its nearest neighbors in a complex. To determine it, simply count how many ligand atoms are directly bonded to the metal center.

• For example, in \([\text{Co}\left(\text{CN}\right)_{6}\right]^{3-}\), the Co metal is coordinated to six cyanide ligands, giving it a coordination number of 6.
• Similarly, in \([\text{CdBr}_{4}]^{2-}\), four bromide ions surround the Cd metal, so the coordination number is 4.
• The nature of the ligand may also influence the coordination number. In complexes with ethylenediamine (\text{en}), which is a bidentate ligand, each completes two coordination sites. Thus, three ethylenediamine ligands yield a coordination number of 6 for \([\text{Pt(en)}_{3}]\).

This measure helps predict the geometry of the complex, which could be tetrahedral, square planar, octahedral, etc., depending on the coordination number and the nature of the ligands.

Oxidation Number

The oxidation number of a metal in a complex refers to the charge the metal would have if all the ligands were removed from it, and the electron pairs that were shared with the metal were given back to the ligands. Calculating the oxidation number involves considering the overall charge of the complex and the known charges of the ligands.

• In \([\text{Co}\left(\text{CN}\right)_{6}]^{3-}\), the total charge is derived from the metal and the \text{CN}^{-} ligands. Balancing these, the Co oxidation number is found to be +3.
• For \([\text{CdBr}_{4}]^{2-}\), considering the charges of bromide ions leads to an oxidation number of +2 for Cd.
• In the neutral \([\text{Cu(bipy)}_{2}]\), the given Roman numeral III indicates Cu’s oxidation state as +3.

Recognizing the correct oxidation state helps understand the reactivity and properties of the metal complexes in different chemical environments.

Metal Complexes

Metal complexes form when metal ions bond with multiple ligands. These complexes play important roles in both biological systems and industrial applications. Understanding their structure is key to controlling their chemical behavior. A metal complex consists of a central metal ion surrounded by ligands that are attracted to the metal due to donor atom electron pairs.

• For instance, in \([\text{Co}\left(\text{CN}\right)_{6}]^{3-}\), Co is the central metal ion forming a stable complex with six CN ligands.
• In \([\text{Pt(en)}_{3}]\), Pt is surrounded by three bidentate ethylenediamine ligands, resulting in a robust coordination structure.
• The variety of ligands, ranging from small ions like \text{CN}^{-} and \text{Br}^{-} to larger molecules like \text{en}, contributes to the versatility of these complexes.
• Coordination compounds are particularly intriguing for their unique properties, such as their geometric shapes and magnetic characteristics, which can be explored in various chemical and technological applications.

Metal complexes are extensively studied to enhance areas like catalysis, material science, and medicine.