Antibonding molecular orbitals can be used to make bonds to...

Antibonding molecular orbitals can be used to make bonds to other atoms in a molecule. For example, metal atoms can use appropriate $d$ orbitals to overlap with the $\pi_{2 p}^{\star}$ orbitals of the carbon monoxide molecule. This is called $d-\pi$ backbonding. (a) Draw a coordinate axis system in which the $y$ -axis is vertical in the plane of the paper and the $x$ -axis horizontal. Write $" M^{\prime \prime}$ at the origin to denote a metal atom. (b) Now, on the $x$ axis to the right of $\mathrm{M},$ draw the Lewis structure of a $\mathrm{CO}$ molecule, with the carbon nearest the $\mathrm{M}$. The CO bond axis should be on the $x$ -axis. (c) Draw the CO $\pi_{2 p}^{*}$ orbital, with phases (see the "Closer Look" box on phases) in the plane of the paper. Two lobes should be pointing toward $\mathrm{M}$. (d) Now draw the $d_{x y}$ orbital of $\mathrm{M},$ with phases. Can you see how they (e) What kind of bond will overlap with the $\pi_{2 p}^{\star}$ orbital of $\mathrm{CO} ?$ is being made with the orbitals between $\mathrm{M}$ and $\mathrm{C}, \sigma$ or $\pi ?(\mathbf{f})$ Predict what will happen to the strength of the CO bond in a metal-CO complex compared to CO alone.

Antibonding molecular orbitals can be used to make bonds to other atoms in a molecule. For example, metal atoms can use appropriate $d$ orbitals to overlap with the $\pi_{2 p}^{\star}$ orbitals of the carbon monoxide molecule. This is called $d-\pi$ backbonding.
(a) Draw a coordinate axis system in which the $y$ -axis is vertical in the plane of the paper and the $x$ -axis horizontal. Write $" M^{\prime \prime}$ at the origin to denote a metal atom.
(b) Now, on the $x$ axis to the right of $\mathrm{M},$ draw the Lewis structure of a $\mathrm{CO}$ molecule, with the carbon nearest the $\mathrm{M}$. The CO bond axis should be on the $x$ -axis.
(c) Draw the CO $\pi_{2 p}^{*}$ orbital, with phases (see the "Closer Look" box on phases) in the plane of the paper. Two lobes should be pointing toward $\mathrm{M}$.
(d) Now draw the $d_{x y}$ orbital of $\mathrm{M},$ with phases. Can you see how they
(e) What kind of bond will overlap with the $\pi_{2 p}^{\star}$ orbital of $\mathrm{CO} ?$ is being made with the orbitals between $\mathrm{M}$ and $\mathrm{C}, \sigma$ or $\pi ?(\mathbf{f})$ Predict what will happen to the strength of the CO bond in a metal-CO complex compared to CO alone.

Key Concepts

Bond Strength Modulation in Ligand Complexes

Interactions such as backbonding significantly influence the intrinsic strength of bonds within a ligand. When a metal donates electron density into a ligand’s antibonding orbital, it reduces the bond order of the ligand, leading to bond weakening. This effect is important in rationalizing reactivity, vibrational spectra, and overall stability of complexes in coordination chemistry.

Orbital Symmetry and Overlap

Effective bonding between atoms requires that their interacting orbitals have compatible symmetry and spatial orientation. The phase relationships of the orbitals determine whether the overlap is constructive (leading to bonding interactions) or destructive (leading to antibonding interactions). This concept is crucial in understanding how metal d orbitals can mix with ligand ? or ?* orbitals in complex formation.

Backbonding

Backbonding involves the donation of electron density from filled metal orbitals, typically d orbitals, into empty or partially filled antibonding orbitals on a ligand. This interaction weakens bonds within the ligand while simultaneously strengthening the metal-ligand bond. It is a key concept in coordination chemistry that explains many observed structural and spectroscopic properties of metal complexes.

Molecular Orbital Theory

This theory describes the formation of molecular orbitals from the combination of atomic orbitals. It explains how electrons are delocalized in a molecule, occupying bonding and antibonding orbitals. Understanding the energy and phase relationships between these orbitals is fundamental to predicting chemical bonding, reactivity, and molecular properties.