Question

Give reasons for (a) CO stretching frequencies are higher in \(\left[\mathrm{F}_{3} \mathrm{PNi}(\mathrm{CO})_{3}\right]\) than in \(\left[\mathrm{Me}_{3} \mathrm{PNi}(\mathrm{CO})_{3}\right]\). (b) Extent of \(\pi\) -bonding in \(\left[\mathrm{Ag}(\mathrm{CNR})_{4}\right.\) is higher than in \(\left[\mathrm{Fe}(\mathrm{CNR})_{6}\right]^{2+}\).

Short answer

The CO stretching frequencies are higher in \([\mathrm{F}_{3}\mathrm{PNi}(\mathrm{CO})_{3}]\) than in \([\mathrm{Me}_{3}\mathrm{PNi}(\mathrm{CO})_{3}]\) due to the higher electronegativity of F compared to CH3, which strengthens the CO bond. The extent of \(\pi\)-bonding is higher in \(\left[\mathrm{Fe}(\mathrm{CNR})_{6}\right]^{2+}\) than \(\left[\mathrm{Ag}(\mathrm{CNR})_{4}\right.\) due to greater availability of d orbitals for \(\pi\)-bonding in Fe2+.

Step-by-step solution

Analyzing the CO stretching frequencies

The stretching frequency of a bond is directly related to the strength of the bond. A stronger bond vibrates or 'stretches' at a higher frequency. Here, it has been observed that the CO stretching frequencies in \([\mathrm{F}_{3}\mathrm{PNi}(\mathrm{CO})_{3}]\) are higher than in \([\mathrm{Me}_{3}\mathrm{PNi}(\mathrm{CO})_{3}]\). This suggests that CO bonds in the first compound are stronger than the latter.

Comparing the molecular structures

The two compounds differ only in the ligands attached to the P atom, F (fluorine) in one case and Me (methyl group) in the other. Fluorine is more electronegative than carbon (central atom in a methyl group). Therefore, \([\mathrm{F}_{3}\mathrm{PNi}(\mathrm{CO})_{3}]\) is able to pull electron density away from the Ni atom more effectively than \([\mathrm{Me}_{3}\mathrm{PNi}(\mathrm{CO})_{3}]\). This increases the positive charge on Ni in the first compound which then strengthens its bonds with CO, resulting in higher stretching frequencies.

Analyzing the \(\pi\)-bonding extent

The second part of the problem deals with the extent of \(\pi\)-bonding in \(\left[\mathrm{Ag}(\mathrm{CNR})_{4}\right.\) versus \(\left[\mathrm{Fe}(\mathrm{CNR})_{6}\right]^{2+}\). The extent of \(\pi\)-bonding depends on how many orbitals are available for overlapping and forming \(\pi\)-bonds.

Comparing the d orbitals

The Fe in \(\left[\mathrm{Fe}(\mathrm{CNR})_{6}\right]^{2+}\)has five d orbitals while the Ag in \(\left[\mathrm{Ag}(\mathrm{CNR})_{4}\right.\) has only one free d orbital due to its electronic configuration ([Kr] 4d10 5s1). Therefore, Fe2+ can participate in \(\pi\)-bonding with six CNR groups while Ag can only do so with one CNR group. This leads to higher extent of \(\pi\)-bonding in \(\left[\mathrm{Fe}(\mathrm{CNR})_{6}\right]^{2+}\) than in \(\left[\mathrm{Ag}(\mathrm{CNR})_{4}\right.\).

Key concepts

CO Stretching Frequencies

When considering CO stretching frequencies, we look at how molecules vibrate. The stretching frequency of a bond is an essential indicator of bond strength. It directly correlates with the strength of the bond; a stronger bond means higher stretching frequency. In the context of \([\mathrm{F}_{3}\mathrm{PNi}(\mathrm{CO})_{3}]\) and \([\mathrm{Me}_{3}\mathrm{PNi}(\mathrm{CO})_{3}]\), it is crucial to observe that the former exhibits higher CO stretching frequencies. This phenomenon suggests that the CO bonds in \([\mathrm{F}_{3}\mathrm{PNi}(\mathrm{CO})_{3}]\) are stronger than those in \([\mathrm{Me}_{3}\mathrm{PNi}(\mathrm{CO})_{3}]\).

This difference can be attributed to the nature of the ligands. Fluorine (F) is more electronegative than the carbon atom in a methyl group (Me). Therefore, \([\mathrm{F}_{3}\mathrm{PNi}(\mathrm{CO})_{3}]\) pulls electron density away from the nickel (Ni) atom more effectively than \([\mathrm{Me}_{3}\mathrm{PNi}(\mathrm{CO})_{3}]\). This increased electron-pulling effect makes the nickel atom more positively charged, which in turn strengthens its bonding with the CO ligands. As a result, the CO bonds share fewer electrons, enhancing the stretching frequencies.

Molecular Structures

Molecular structure plays a crucial role in dictating the properties of a compound. In \([\mathrm{F}_{3}\mathrm{PNi}(\mathrm{CO})_{3}]\) compared to \([\mathrm{Me}_{3}\mathrm{PNi}(\mathrm{CO})_{3}]\), the structural difference arises from the substitution of fluorine for methyl groups at the phosphorus center. This small difference significantly impacts the electron distribution around the nickel atom.

Fluorine, being much more electronegative than the methyl group, draws electrons from the nickel atom, leading to an electron-deficient center. This shift enhances the bond strength of Ni with CO, as the CO molecules respond to this electronic change by tightening their bond, thus leading to higher stretching frequencies.

Understanding these structural nuances helps in predicting chemical behavior. Electronegativity and atom size, among other factors, influence these interactions, which can be analyzed through various computational models or spectroscopic methods to provide insights into the compound's behavior.

Pi-Bonding Extent

The extent of \(\pi\)-bonding is a critical factor in understanding molecular stability and reactivity. In the study of \[\mathrm{Ag}(\mathrm{CNR})_{4}\right.\] compared to \[\mathrm{Fe}(\mathrm{CNR})_{6}\right.^{2+}\], the difference in the degree of \(\pi\)-bonding can be attributed to the availability of d orbitals for overlapping with ligand orbitals.

Iron (Fe) in \[\mathrm{Fe}(\mathrm{CNR})_{6}\right.^{2+}\] has five d orbitals available, allowing it to form extensive \(\pi\)-bonds with its ligands. This means when Fe interacts with six CNR groups, the extent of \(\pi\)-bonding is significant, leading to enhanced stability and different reactivity.

Conversely, silver (Ag) in \[\mathrm{Ag}(\mathrm{CNR})_{4}\right.\] has a limited ability to participate in \(\pi\)-bonding. Due to its electronic configuration, with only one free d orbital, Ag cannot form as many \(\pi\)-bonds compared to iron. Consequently, the \(\pi\)-bonding extent is limited in silver complexes, resulting in less overlap and weaker interaction with CNR ligands. Understanding these differences is essential in designing molecules with desired properties for catalysis or material science applications.