Carbon monoxide (CO) forms bonds to a variety of metals and metal ions. Its ability to bond to iron in hemoglobin is the reason that \(\mathrm{CO}\) is so toxic. The bond carbon monoxide forms to metals is through the carbon atom: $$ \mathrm{M}-\mathrm{C} \equiv \mathrm{O} $$ a. On the basis of electronegativities, would you expect the carbon atom or the oxygen atom to form bonds to metals? b. Assign formal charges to the atoms in CO. Which atom would you expect to bond to a metal on this basis? c. In the MO model, bonding MOs place more electron density near the more electronegative atom. (See the HF molecule in Figs. 9.43 and 9.44.) Antibonding MOs place more electron density near the less electronegative atom in the diatomic molecule. Use the MO model to predict which atom of carbon monoxide should form bonds to metals.

Step by step solution

01

Calculate electronegativities of carbon and oxygen

Use the Pauling electronegativity scale to find the electronegativities of carbon and oxygen. Carbon has an electronegativity of 2.5, while oxygen has an electronegativity of 3.5.

02

Compare electronegativities

Since the oxygen atom has a higher electronegativity than carbon, it attracts the electron density more strongly, making it more negative in nature. Typically, metals bond to more electropositive atoms. In this case, the carbon atom, being less electronegative, is more likely to bond to the metal. #b. Assign formal charges to the atoms in CO#

03

Determine formal charges

To calculate the formal charges of carbon and oxygen in CO, we need to compare the number of valence electrons in their neutral states to the electrons assigned to them in the molecule.

04

Formal charge of carbon

Carbon has 4 valence electrons in its neutral state. In CO, there is a triple bond between carbon and oxygen, and a lone pair on carbon. This means that 2 electrons are assigned to carbon as part of the 2 bonding pairs (1 electron from each pair) and 2 electrons from the lone pair. Therefore, the formal charge on carbon is 4 - 4 = 0.

05

Formal charge of oxygen

Oxygen has 6 valence electrons in its neutral state. In CO, two of these electrons are assigned to oxygen as part of the two bonding pairs (1 electron from each pair) and 4 electrons from the two lone pairs. Therefore, the formal charge on oxygen is 6 - 6 = 0.

06

Determine which atom bonds to metal

Based on formal charges, both carbon and oxygen atoms have a formal charge of 0, so it is not conclusive which atom would bond to the metal using this method alone. #c. Use the MO model to predict the bonding atom#

07

Analyze bonding molecular orbitals

In the MO model, bonding molecular orbitals place more electron density near the more electronegative atom. In CO, the more electronegative atom is oxygen. Thus, in bonding molecular orbitals, we would expect more electron density around oxygen.

08

Analyze antibonding molecular orbitals

Alternatively, antibonding molecular orbitals place more electron density near the less electronegative atom in the diatomic molecule. In CO, the less electronegative atom is carbon. Thus, we would expect more electron density around carbon in antibonding molecular orbitals.

09

Predict which atom bonds to metals

Since metals typically bond to atoms with more electron density available for bonding, the MO model suggests that the atom in CO with greater electron density available for bonding is carbon. Therefore, we predict that the carbon atom of carbon monoxide should form bonds to metals.

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