Problem 14 - Chemistry: The Central Science

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) \(\left[\mathrm{Cu}(\mathrm{bipy})_{2} \mathrm{I}\right] \mathrm{I}\)

Step-by-Step Solution

Answer

(a) Co: Coordination number = 6, Oxidation number = +3. (b) Cd: Coordination number = 4, Oxidation number = +2. (c) Pt: Coordination number = 3, Oxidation number = +4. (d) Co: Coordination number = 3, Oxidation number = +3. (e) Cr: Coordination number = 6, Oxidation number = +3. (f) Cu: Coordination number = 3, Oxidation number = +1.

Step 1: Identify the metal and ligands

In this complex, the metal is Co and the ligands are six cyanide (CN) ions.

Step 2: Determine the coordination number

The coordination number is the number of ligands attached to the metal, which in this case is 6.

Step 3: Determine the oxidation number

The overall charge of the complex is -3, and the charge on each CN ligand is -1. Therefore, the oxidation number of Co is +3. Answer for (a): The coordination number of Co is 6, and the oxidation number is +3. (b)

Step 1: Identify the metal and ligands

In this complex, the metal is Cd and the ligands are four bromide (Br) ions.

Step 2: Determine the coordination number

The coordination number is the number of ligands attached to the metal, which in this case is 4.

Step 3: Determine the oxidation number

The overall charge of the complex is -2, and the charge on each Br ligand is -1. Therefore, the oxidation number of Cd is +2. Answer for (b): The coordination number of Cd is 4, and the oxidation number is +2. (c)

Step 1: Identify the metal and ligands

In this complex, the metal is Pt and the ligands are three ethylenediamine (en) molecules.

Step 2: Determine the coordination number

The coordination number is the number of ligands attached to the metal, which in this case is 3.

Step 3: Determine the oxidation number

The overall charge of the complex is +4, and the charge on each en ligand is neutral (0). Therefore, the oxidation number of Pt is +4. Answer for (c): The coordination number of Pt is 3, and the oxidation number is +4. (d)

Step 1: Identify the metal and ligands

In this complex, the metal is Co and the ligands are two ethylenediamine (en) molecules and one oxalate (C2O4) ion.

Step 2: Determine the coordination number

The coordination number is the number of ligands attached to the metal, which in this case is 3.

Step 3: Determine the oxidation number

The overall charge of the complex is +1, the charge on each en ligand is neutral (0), and the charge on oxalate ligand is -2. Therefore, the oxidation number of Co is +3. Answer for (d): The coordination number of Co is 3, and the oxidation number is +3. (e)

Step 1: Identify the metal and ligands

In this complex, the metal is Cr and the ligands are two ammonia (NH3) molecules and four isothiocyanate (NCS) ions.

Step 2: Determine the coordination number

The coordination number is the number of ligands attached to the metal, which in this case is 6.

Step 3: Determine the oxidation number

The overall charge of the complex is -1, the charge on each NH3 ligand is neutral (0), and the charge on each NCS ligand is -1. Therefore, the oxidation number of Cr is +3. Answer for (e): The coordination number of Cr is 6, and the oxidation number is +3. (f)

Step 1: Identify the metal and ligands

In this complex, the metal is Cu and the ligands are two bipyridine (bipy) molecules and one iodide (I) ion.

Step 2: Determine the coordination number

The coordination number is the number of ligands attached to the metal, which in this case is 3.

Step 3: Determine the oxidation number

The overall charge of the complex is neutral (0), the charge on each bipy ligand is neutral (0), and the charge on the I ligand is -1. Therefore, the oxidation number of Cu is +1. Answer for (f): The coordination number of Cu is 3, and the oxidation number is +1.

Key Concepts

Coordination Number

Understanding the coordination number of a metal ion in a complex is fundamental in coordination chemistry. It refers to the total number of points of attachment between the metal ion at the center of the complex and the surrounding ligands. For instance, in a complex like \( \mathrm{K}_{3}[\mathrm{Co}(\mathrm{CN})_{6}] \), the metal ion cobalt (Co) is surrounded by six cyanide (CN) ligands. This means the coordination number of Co is 6. This number is not arbitrary; it depends on both the geometry of the molecule and the ability of the ligands to satisfy the valence of the metal ion.

Where confusion might arise, such as with polydentate ligands (ligands that can attach at multiple points), remember that the coordination number counts the total number of attachment sites. For example, ethylenediamine (en) is a bidentate ligand, which can bind through two nitrogen atoms. Hence, even though there may be fewer en molecules than other types of ligands, the overall coordination number might still be high due to these multiple binding sites.

Oxidation Number

Another crucial concept in coordination chemistry is the oxidation number (also known as oxidation state). It can be thought of as the hypothetical charge that an atom would have if all the compounds were to consist entirely of ions. For example, in the complex \( \mathrm{K}_{3}[\mathrm{Co}(\mathrm{CN})_{6}] \), the oxidation number of cobalt (Co) is +3. This is deduced by taking into account the overall charge of the complex and the known charge of the ligands—each cyanide ion has a charge of -1. Seeing as there are six ligands, their total charge is -6, and since the complex is neutral, the metal must have a charge of +3 to balance the negative charge of the ligands.

A careful balance equation is established considering the charge of the complex, the known charge of the ligands, and often the charge balance of the entire compound, including all ions present, to calculate the oxidation number. This process utilizes the principle that the sum of the oxidation numbers in a neutral compound must be zero, or equal the overall net charge for charged species.

Metal Complexes

Metal complexes, also referred to as coordination complexes, are compounds where a central metal atom or ion is bound to a set of molecules or anions, known as ligands. Delving into examples, complexes such as \( \mathrm{Na}_{2}[\mathrm{CdBr}_{4}] \) and \( [\mathrm{Pt}(\mathrm{en})_{3}](\mathrm{ClO}_{4})_{4} \) exhibit diverse geometries and properties that hinge on the specific metal and ligands involved.

Key to the study of metal complexes is their structural variety, which can range from simple square planar or tetrahedral geometries to more complex octahedral arrangements. This diversity is the result of differences in metal ion sizes, oxidation states, ligand types, and coordination numbers. The study of these complexes involves understanding their formation, properties, and roles in various chemical processes, including industrial catalysts and biological systems like hemoglobin and chlorophyll.

Ligands

Diving into the world of ligands, these are ions or molecules that donate one or more pairs of electrons to a central metal atom or ion to form a coordination complex. The nature of ligands influences the features of metal complexes, including their color, geometry, reactivity, and magnetic properties. Ligands can be simple ions like chloride (Cl-) in \( \mathrm{Na}_{2}[\mathrm{CdBr}_{4}] \) or more complex organic molecules like ethylenediamine (en) in \( [\mathrm{Co}(\mathrm{en})_{2}(\mathrm{C}_{2}\mathrm{O}_{4})]^{+} \).

Ligands are often categorized by their ‘denticity,’ or the number of electron pairs they can donate to the metal. Monodentate ligands, such as water or ammonia, can only donate one electron pair, thus forming one bond with the metal. Conversely, bidentates like ethylenediamine can form two bonds, and polydentates can form multiple bonds, which leads to the formation of very stable complexes. Ligands play a pivotal role in defining the properties of a complex and hence are a key study area in coordination chemistry.