Problem 12 - Chemistry in 30 days

Question

What is the shape of \(\mathrm{Fe}(\mathrm{CO})_{5}\) molecule? (a) Tetrahedral (b) Octahedral (c) Trigonal bipyramidal (d) Square pyramidal

Step-by-Step Solution

Answer

The shape of \( \mathrm{Fe}(\mathrm{CO})_{5} \) is trigonal bipyramidal.

Step 1: Identify the central atom

In the given molecule \( \mathrm{Fe}(\mathrm{CO})_{5} \), the central atom is \( \mathrm{Fe} \) (Iron). This is because iron is typically the central atom due to its metal nature, around which the carbonyl (CO) ligands are arranged.

Step 2: Determine the number of ligands

Count the number of ligands attached to \( \mathrm{Fe} \). In \( \mathrm{Fe}(\mathrm{CO})_{5} \), there are five carbonyl ligands (CO) bonded to the \( \mathrm{Fe} \) atom.

Step 3: Use VSEPR theory to predict the shape

According to VSEPR (Valence Shell Electron Pair Repulsion) theory, when there are five ligands or electron pairs around a central atom, the geometry is typically **trigonal bipyramidal.** This maximizes the distance between electron pairs, minimizing repulsion.

Key Concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. This arrangement is determined by the repulsion between electron pairs in the valence shell of the central atom. VSEPR theory, or Valence Shell Electron Pair Repulsion theory, helps to predict the shapes of molecules based on this principle.

Generally, molecules arrange their electron pairs and atoms in a way that minimizes repulsion, maximizing the distance between them. For instance, linear geometry occurs when there are two bonding pairs, resulting in a bond angle of 180 degrees.

When there are three pairs, the geometry is trigonal planar with angles of 120 degrees. Four pairs create a tetrahedral shape with 109.5-degree angles. As in the case of our given molecule, when there are five ligands, as seen in \(\mathrm{Fe}\mathrm{(CO)}_5\), a trigonal bipyramidal shape emerges. This shape includes a combination of equatorial and axial positions that decrease repulsive forces.

Iron Carbonyls

Iron carbonyls are a class of metal carbonyl compounds, with Iron attached to carbon monoxide (CO) ligands. They are a significant class of organometallic chemistry, where carbonyl ligands bond to a central metal atom like iron through coordination bonds.

The bonding in iron carbonyls involves the donation of electron pairs from the filled lone pairs of the carbonyl ligands into empty metal orbitals. Conversely, there is also a back-bonding in which electrons from filled metal d orbitals are donated back into the antibonding \pi\*-orbitals of the CO ligands.

This interplay of donation and back-bonding strengthens the metal-ligand bond and stabilizes the complex. The most stable and notable of these complexes is \(\mathrm{Fe}\mathrm{(CO)}_5\), which is often used as a model to study such interactions.

Ligand Arrangement

The arrangement of ligands around a central metal atom is crucial in determining the molecule's geometry. In VSEPR theory, the number of ligands, or substituents, dictates the geometric configuration.

For \(\mathrm{Fe}\mathrm{(CO)}_5\), there are five carbonyl ligands surrounding the iron atom. This leads to a trigonal bipyramidal shape, where three ligands lie in an equatorial plane, and two occupy axial positions.

The equatorial ligands are separated by an angle of 120 degrees from each other, minimizing repulsive forces. Axial ligands are positioned above and below this plane, aligning perpendicular to it. This sophisticated and symmetrical arrangement ensures that the molecule remains stable by keeping repulsion at a minimum.