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 \(\mathrm{CN}^{-}\) ligand, forming \(\left[\mathrm{Fe}(\mathrm{CO})_{4}(\mathrm{CN})\right]^{-}\) How many geometric isomers would you predict this complex could have?

Short answer

(a) The name for the complex Fe(CO)\(_5\) is Carbonylpentairon(0). (b) The oxidation state of Fe in Fe(CO)\(_5\) is 0. (c) The complex \(\left[\mathrm{Fe}(\mathrm{CO})_{4}(\mathrm{CN})\right]^{-}\) can have 2 possible geometric isomers.

Step-by-step solution

(a) Name of Fe(CO)\(_5\) complex

In order to name this complex, let's follow these steps: 1. Name the ligands in alphabetical order (regardless of their charge) - In this case, carbonyl (CO) is both the only and first ligand. 2. The metal ion is then named, followed by the oxidation state in Roman numerals enclosed by parentheses - In this case, we have iron (Fe) as the central metal, which we'll determine its oxidation state in part (b). Given that we still do not know the oxidation state for Fe, the partial name of the complex is Carbonylpentairon.

(b) Oxidation state of Fe in Fe(CO)\(_5\)

The overall charge of the complex is neutral (0) and we know that carbonyl ligands (CO) are neutral. Initially, the sum of charges of the ligands and the central atom should equal the overall charge of the complex. Since we have 5 carbonyl ligands, their cumulative charge is 5 * 0 = 0. So in this case, the oxidation state of Fe should also be neutral (0) to make the whole complex neutral. Thus, the oxidation state of Fe in Fe(CO)\(_5\) is 0. Now we can complete the name for the complex: Carbonylpentairon(0).

(c) Geometric isomers of \(\left[\mathrm{Fe}(\mathrm{CO})_{4}(\mathrm{CN})\right]^{-}\)

We know the complex adopts a trigonal bipyramidal geometry, which involves a central metal atom surrounded by 5 ligands. In this case, one of the CO ligands is replaced with a CN\(^-\) ligand, forming a new complex \(\left[\mathrm{Fe}(\mathrm{CO})_{4}(\mathrm{CN})\right]^{-}\). To determine the geometric isomers, let's label the positions in the trigonal bipyramidal structure as follows: 1. Equatorial positions (E): 3 ligands that form a triangle in the xy plane around the central metal atom. 2. Axial positions (A): 2 ligands that are above and below the central metal atom, along the z-axis. To find the possible isomers, we should determine possible positions for the CN\(^-\) ligand among the 5 positions provided in the trigonal bipyramidal geometry. First, the CN\(^-\) ligand could occupy any of the three equatorial positions, leading to 1 isomer. Alternatively, the CN\(^-\) ligand could occupy an axial position, producing another isomer. Thus, we have a total of 2 possible geometric isomers for the complex \(\left[\mathrm{Fe}(\mathrm{CO})_{4}(\mathrm{CN})\right]^{-}\).

Key concepts

Trigonal Bipyramidal Geometry

Trigonal bipyramidal geometry is a fascinating structure often found in coordination compounds with a coordination number of 5. Imagine a central atom surrounded by five ligands. This geometry consists of two types of positions: equatorial and axial.

- **Equatorial Positions**: Three ligands that lie in the same plane at 120-degree angles to each other, forming a trigonal plane around the central atom.- **Axial Positions**: Two ligands that extend above and below the equatorial plane at a 90-degree angle.

For example, in the compound \( ext{Fe(CO)}_5\), the iron atom is at the center, with five carbonyl ligands positioned in a trigonal bipyramidal structure. This arrangement helps minimize repulsion between the ligands, making it energetically favorable. Understanding this structure is crucial for predicting how ligands can be replaced and the resulting geometrical changes in complex compounds.

Oxidation State

The oxidation state of an element in a compound is a useful concept to determine the electron count or loss in bonding. In coordination chemistry, it reveals how a metal interacts with its ligands. To find the oxidation state of the metal, the charges of the ligands are subtracted from the overall charge of the complex.

For \( ext{Fe(CO)}_5\), the carbonyl ligands are neutral, meaning they do not contribute to the charge. The neutral overall charge suggests that the oxidation state of iron must also be neutral. Thus, the oxidation state of Fe in \( ext{Fe(CO)}_5\) is 0.

Knowing the oxidation state is critical. It influences the naming of compounds and affects their chemical behavior. In cases where the metal forms a negatively or positively charged complex, different oxidation states come into play, altering reactivity and properties.

Geometric Isomers

Geometric isomers are different spatial arrangements of ligands around a central atom in a coordination complex. These isomers exhibit different physical and chemical properties even though they have the same chemical formula.

In the complex \([ ext{Fe(CO)}_4( ext{CN})]^{-}\), understanding geometric isomers requires a look at its trigonal bipyramidal geometry. When one \( ext{CO}\) ligand is replaced by \( ext{CN}^{-}\), this new ligand can occupy different positions:

• If \(\text{CN}^{-}\) occupies an equatorial position, it's different from when it occupies an axial position.
• Thus, there are two possible geometric isomers. One where \(\text{CN}^{-}\) is equatorial and one where it is axial.

Isomerism is important because it can lead to compounds with significantly different properties, even if they have identical connectivity. This has practical implications, especially in catalysis and materials science, where specific properties are desired.