Question

Some metal complexes have a coordination number of \(5 .\) One such complex is \(\mathrm{Fe}(\mathrm{CO})_{5}\), which adopts a trigonal bipyramidal geometry (see Figure 9.8). (a) Write the name for \(\mathrm{Fe}(\mathrm{CO})_{5}\), using the nomenclature rules for coordination compounds. (b) What is the oxidation state of Fe in this compound? (c) Suppose one of the CO ligands is replaced with a CN \(^{-}\) ligand, forming \(\left[\mathrm{Fe}(\mathrm{CO})_{4}(\mathrm{CN})\right]^{-}\). How many geometric isomers would you predict this complex could have?

Short answer

The complex Fe(CO)5 is named as pentacarbonyliron. In this complex, the oxidation state of Fe is 0. The complex [Fe(CO)4(CN)]− can have 2 geometric isomers, with the CN- ligand occupying either an equatorial or axial position in the trigonal bipyramidal geometry.

Step-by-step solution

(a) Writing the name for Fe(CO)5

To name the Fe(CO)5 complex, we first need to name the metal (Fe) and the ligands (CO). CO is a neutral, monodentate ligand and is called carbonyl. The metal is iron. Since there is no charge on the complex, iron will be in its neutral state. Now, we can name the complex: pentacarbonyliron

(b) Oxidation state of Fe

To determine the oxidation state of Fe, we assume a charge on the metal and balance it with the charges on the ligands. Since carbonyl is neutral ligand (having a zero charge), the overall charge on the metal should be zero for a neutral complex. So, Fe has an oxidation state of 0 in Fe(CO)5.

(c) Geometric isomers for [Fe(CO)4(CN)]−

The [Fe(CO)4(CN)]− complex has 5 ligands around Fe, so it will adopt a trigonal bipyramidal geometry. When the CN- ligand replaces one of the CO ligands, it could take one of two positions: either an equatorial position or an axial position. If the CN- ligand occupies an equatorial position, there will be one geometric isomer. If the CN- ligand occupies an axial position, there will be a different geometric isomer. Thus, we predict that the [Fe(CO)4(CN)]− complex can have 2 geometric isomers.

Key concepts

Coordination Number

In coordination chemistry, the coordination number refers to the number of ligand atoms that are directly bonded to the central metal atom or ion within a coordination complex. It essentially gives us the count of interactions around the metal center. For instance, in the case of the metal complex \( ext{Fe(CO)}_5\), the coordination number is 5. This means five carbon monoxide ligands, each with one lone pair, are attached to the iron atom, forming a stable structure.
Understanding the coordination number helps in predicting the geometry and reactivity of coordination compounds. Different metals and ligands can lead to varying coordination numbers, often influenced by factors such as the size of the central metal ion and the electronic properties of the ligands.

Trigonal Bipyramidal Geometry

Trigonal bipyramidal geometry is a specific shape that occurs when a metal complex has a coordination number of 5. This geometry is characterized by having three atoms or ligands in a plane (equatorial positions) and two more above and below this plane (axial positions).
In the example of \( ext{Fe(CO)}_5\), this geometry allows for optimal spatial arrangement and minimizes steric hindrance between the carbonyl ligands.

• Three ligands are positioned equatorially around the central iron atom.
• Two ligands are positioned axially, at opposite ends of the iron atom.

Such an arrangement contributes to the stability and unique properties of the complex, influencing how it interacts with other molecules or ions.

Coordination Compounds Nomenclature

Naming coordination compounds involves systematic rules established by the IUPAC. It starts by naming the ligands attached to the central metal first, followed by the name of the metal itself.
In the case of \( ext{Fe(CO)}_5\), which is a neutral complex, the name is constructed as 'pentacarbonyliron'. Here is how it's broken down:

• 'penta' indicates five ligands are present.
• 'carbonyl' refers to the CO ligands.
• 'iron' is the name of the central metal atom.

For anionic ligands, suffixes such as '-o' are added, and the metal name sometimes changes if the entire complex carries a negative charge. Understanding these rules is crucial for writing and interpreting the names of complex compounds effectively.

Oxidation State

The oxidation state of a metal in a coordination complex is determined by the overall charge of the complex and the charges on the ligands. For example, in \( ext{Fe(CO)}_5\), the complex is neutral, and CO is a neutral ligand.
Therefore, the oxidation state of iron in this particular complex is 0. By assigning this zero charge, it suggests that iron neither loses nor gains electrons when forming its bonds with CO.
Understanding the oxidation state helps in deducing the electron configuration of the metal and the type of bonding in the complex, ultimately affecting its chemical behavior and reactivity.

Geometric Isomers

Geometric isomers are different arrangements of ligands around a central metal atom that result in distinct spatial orientations, even though the components of the complex remain unchanged.
For a complex like \([ ext{Fe(CO)}_4( ext{CN})]^−\), which has a trigonal bipyramidal geometry, two distinct geometric isomers are possible.

• If the CN ligand takes an equatorial position, one geometric isomer forms.
• If the CN ligand takes an axial position, a second isomer arises.

These variations affect the physical and chemical properties of the compound. Recognizing geometric isomers is essential for understanding the reactivity and potential applications of coordination compounds in various fields, such as catalysis and material science.