Question

Indicate the coordination number and the oxidation number of the metal for 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) \(\left[\mathrm{Cu}(\mathrm{bipy})_{2} \mathrm{I}\right] \mathrm{I}\)

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 = 5, Oxidation number = +3 (e) Cr: Coordination number = 6, Oxidation number = +3 (f) Cu: Coordination number = 5, Oxidation number = +2

Step-by-step solution

(a) K3[Co(CN)6] Exercise

First, we identify the metal atom and ligands in the complex. In this case, cobalt (Co) is the central metal atom with six cyanide (CN) ligands surrounding it.

(a) Coordination Number

The coordination number is the number of ligand atoms bonded to the central metal atom. Since there are six cyanide (CN) ligands surrounding cobalt, the coordination number of Co is 6.

(a) Oxidation Number

The overall charge of the complex ion is -3 since there are three potassium (K) counterions with a +1 charge each. Therefore, Co must have an oxidation number that allows the complex ion to have a -3 charge. Since each of the six cyanide ligands has a -1 charge: \[Oxidation \,number\, of\, Co + 6(-1) = -3\] The oxidation number of cobalt (Co) is +3.

(b) Na2[CdBr4] Exercise

In this complex, cadmium (Cd) is the central metal atom, with four bromide (Br) ligands surrounding it.

(b) Coordination Number

There are four bromide ligands bonded to the central metal atom. Hence the coordination number of Cd is 4.

(b) Oxidation Number

The overall charge of the complex ion is -2 due to the two sodium (Na) counterions with a +1 charge each. Each of the four bromide ligands has a -1 charge. Thus: \[Oxidation\, number\, of\, Cd + 4(-1) = -2\] The oxidation number of cadmium (Cd) is +2.

(c) [Pt(en)3](ClO4)4 Exercise

In this complex, platinum (Pt) is the central metal atom, surrounded by three ethylenediamine (en) ligands.

(c) Coordination Number

Each ethylenediamine (en) ligand has two donor atoms (nitrogen), so the coordination number of Pt is 3 × 2 = 6.

(c) Oxidation Number

The overall charge of the complex ion is +4 because there are four perchlorate (ClO4) counterions with a -1 charge each. Assuming all ligands are removed: \[Oxidation\, number\, of\, Pt = +4\] The oxidation number of platinum (Pt) is +4. Consider the following sections for the remaining complexes: (d) [Co(en)2(C2O4)]+: Coordination Number: 5 (2 en ligands with two donor atoms each, and 1 oxalate ligand with two donor atoms) Oxidation Number: +3 (assuming all ligands are removed, the overall charge of the complex ion is +1) (e) NH4[Cr(NH3)2(NCS)4]: Coordination Number: 6 (2 ammonia ligands with one donor atom each, and 4 thiocyanate ligands with one donor atom each) Oxidation Number: +3 (assuming all ligands are removed, the overall charge of the complex ion is -1 and balancing with +1 ammonium counterion) (f) [Cu(bipy)2 I]I: Coordination Number: 5 (2 bipyridine ligands with two donor atoms each, and 1 iodide ligand with one donor atom) Oxidation Number: +2 (assuming all ligands are removed, the overall charge of the complex ion is 0 and balancing with -1 iodide counterion)

Key concepts

Coordination Number

In coordination chemistry, the coordination number is a key concept used to understand the structure of metal coordination complexes. It refers to the number of ligand atoms that are attached to the central metal ion in a complex. Ligands are molecules or ions that donate a pair of electrons to the metal atom to form a coordinate covalent bond. This number gives us a sense of the geometry and arrangement of the ligands around the metal center. For instance, if a metal like cobalt in the complex \([\text{Co}(\text{CN})_6]^{3-}\) is surrounded by six cyanide ligands, the coordination number is 6. This often implies an octahedral geometry around the metal. Coordination numbers can vary, with common numbers being 4, 5, or 6, leading to various geometries such as tetrahedral, square planar, or octahedral. Understanding the coordination number is crucial as it affects both the physical and chemical properties of the metal complexes.

• A coordination number of 4, as seen in \([\text{CdBr}_4]^{2-}\), often leads to tetrahedral or square planar geometries.
• A coordination number of 6, like in the \([\text{Cr}(\text{NH}_3)_2(\text{NCS})_4]^{-}\) complex, usually leads to an octahedral geometry.

Oxidation Number

The oxidation number of a metal in a complex ion is a measure of the degree of oxidation of the metal atom. It indicates the charge that the metal ion would have if all the ligands (and associated electrons) were removed, with ionic bonding assumed. Calculating the oxidation number involves considering the charge contributions from each ligand and then determining what charge on the metal would balance the overall charge of the complex. For example, in the complex \([\text{Co}(\text{CN})_6]^{3-}\), the six cyanide ligands contribute a total charge of -6. Given the complex has an overall charge of -3, the cobalt must be in a +3 oxidation state to balance to -3:\[ \text{Oxidation number of } \text{Co} + 6(-1) = -3 \]This calculation is essential for understanding the reactivity and stability of coordination compounds.

• For \([\text{CdBr}_4]^{2-}\), with each bromide contributing -1, the oxidation number of cadmium is +2.
• In the complex \([\text{Pt}(\text{en})_3]^{4+}\), the oxidation state of platinum is +4.

Complex Ions

Complex ions form when a central metal atom or ion is surrounded by molecules or ions called ligands that donate a pair of electrons, resulting in a complex with a net charge. For example, in \([\text{Pt}(\text{en})_3]^{4+}\), the central platinum ion forms multiple coordinate covalent bonds with the ligands surrounding it. Understanding these structures is critical to grasping how metal ions behave in different chemical environments. Complex ions are crucial in various applications, from catalysis to medicine. They can exist in different oxidation states, influencing their reactivity and stability.

• The geometry of a complex ion like \([\text{Co}(\text{en})_2(\text{C}_2\text{O}_4)]^{+}\) helps dictate its chemical properties.
• Knowing the charge of a complex ion allows for predicting its behavior in reactions.

Ligands

Ligands are species that donate electron pairs to a central metal atom to form complexes. They are crucial because they determine the structure and properties of the overall metal complex. Ligands can be neutral molecules like water or ammonia, or they can be anions such as chloride or cyanide. The nature of the ligands significantly affects the chemical behavior of the metal coordination complexes. They can influence the color, magnetism, and stability of the complex. Some ligands, like ethylenediamine (en), are bidentate, meaning they can form two bonds with the central metal atom.

• Bidentate ligands like \(\text{bipy} \) in the \([\text{Cu}(\text{bipy})_2 \text{I}]\) complex help maximize the coordination number while stabilizing the complex.
• Monodentate ligands, such as bromide in \([\text{CdBr}_4]^{2-}\), form only one bond with the central atom.

Understanding the role of ligands is key to predicting the geometry and reactivity of complexes.

Metal Coordination Complexes

Metal coordination complexes consist of a central metal atom or ion bonded to one or more ligands. These complexes play a crucial role in many chemical processes, from industrial catalysts to biological systems. The properties of these complexes depend on the metal, the nature, and arrangement of the ligands, and the coordination number. Each unique combination provides specific chemical properties, which can be tweaked for desired applications. For example:- The central metal determines the electronic structure and reactivity.- The ligands affect the stability and solubility of the complex.- The geometry, determined by the coordination number, influences the complex's physical and chemical behavior.In complex such as \([\text{Co} (\text{en})_2(\text{C}_2\text{O}_4)]^{+}\), cobalt is the central metal, and it forms a stable structure by coordinating with various ligands. Such structures provide insights into metals' behavior in different environments and can be models for designing new materials with specific properties.