Question

The molecule dimethylphosphinoethane \(\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{PCH}_{2}-\mathrm{CH}_{2} \mathrm{P}\left(\mathrm{CH}_{3}\right)_{2}\right.\), which is abbreviated \(\left.\mathrm{dmpe}\right]\) is used as a ligand for some complexes that serve as catalysts. A complex that contains this ligand is \(\mathrm{Mo}(\mathrm{CO})_{4}(\) dmpe \()\). (a) Draw the Lewis structure for dmpe, and compare it with ethylenediammine as a coordinating ligand. (b) What is the oxidation state of Mo in \(\mathrm{Na}_{2}\left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}(\mathrm{dmpe})\right] ?\) (c) Sketch the structure of the \(\left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}(\mathrm{dmpe})\right]^{2-}\) ion, including all the possible isomers.

Short answer

(a) The Lewis structure for dmpe is: ``` H | H - C - P - CH2 - CH2 - P - CH3 | H ``` Dmpe and ethylenediammine both act as bidentate ligands, coordinating through their two phosphorus and nitrogen atoms, respectively. (b) The oxidation state of Mo in Na2[Mo(CN)2(CO)2(dmpe)] is +4. (c) Two main isomers are possible for the [Mo(CN)2(CO)2(dmpe)]2- ion: 1. Fac-isomer: ``` CN | CN - Mo - CO | dmpe ``` 2. Mer-isomer: ``` dmpe | CN - Mo - CO | CN ```

Step-by-step solution

To draw the Lewis structure, we need to count the number of valence electrons present, arrange the atoms correctly, and fulfill the octet rule. Find the total number of valence electrons in the dmpe molecule: - P has 5 valence electrons, two P atoms contribute a total of 10 electrons - C has 4 valence electrons, four C atoms contribute a total of 16 electrons - H has 1 valence electron, twelve H atoms contribute a total of 12 electrons The total number of valence electrons is 10 + 16 + 12 = 38 electrons. Now, draw the Lewis structure for dmpe. #Step 2: Comparing dmpe with ethylenediammine as coordinating ligands#

Discuss the difference in the coordination process and the coordination sites of the two ligands. #Step 3: Finding the oxidation state of Mo in Na2[Mo(CN)2(CO)2(dmpe)]#

Determine the oxidation state of Mo by considering the charges of the coordinated ligands and the overall charge of the complex. #Step 4: Drawing the structures for the [Mo(CN)2(CO)2(dmpe)]2- ion, including all possible isomers#

List the possible positions of the ligands in the complex and sketch the possible isomers, considering coordination sites and geometric arrangements. Now let's perform the steps to find the answers. #Step 1: Drawing the Lewis structure for dmpe# Based on the total number of valence electrons (38), the structure can be drawn as follows: ``` H | H - C - P - CH2 - CH2 - P - CH3 | H ``` #Step 2: Comparing dmpe with ethylenediammine as coordinating ligands# Dmpe can act as a bidentate ligand by coordinating through its two phosphorus atoms. Similarly, ethylenediammine (which has its formula as NH2-CH2-CH2-NH2), has two nitrogen atoms that can bind to the metal ion, making it also a bidentate ligand. Thus, both ligands can form two-coordinate bonds with the metal ion. #Step 3: Finding the oxidation state of Mo in Na2[Mo(CN)2(CO)2(dmpe)]# To determine the oxidation state of Mo, we need to account for the charges of the coordinated ligands: - Two CN ligands, each with charge -1, contribute a total of -2 - Two CO ligands are neutral, contributing 0 - dmpe is also a neutral ligand, contributing 0 The complex has a charge of 2- on the outside, and since we have two Na+ ions countering these charges, we can write: Mo oxidation state + (-2) + 0 + 0 = -2 So the oxidation state of Mo is +4. #Step 4: Drawing the structures for the [Mo(CN)2(CO)2(dmpe)]2- ion, including all possible isomers# The complex is an octahedral complex with six coordination sites (4 from ligands and 2 from the dmpe), and there are multiple possible isomers. Nevertheless, if we consider the fac-and mer-isomer concept based on the arrangement of individual ligands: 1. Fac-isomer, where the three ligand types (CN, CO, and dmpe) occupy adjacent positions (face): ``` CN | CN - Mo - CO | dmpe ``` 2. Mer-isomer, where the three ligand types (CN, CO, and dmpe) occupy a plane passing through the metal ion (meridian): ``` dmpe | CN - Mo - CO | CN ``` These are the two main isomers one should consider in this case.

Key concepts

Oxidation State

When working with coordination compounds, determining the oxidation state of the central metal atom is crucial. The oxidation state indicates the charge of the metal and how it bonds with different ligands. In our exercise, we focus on the molybdenum (Mo) complex:

• The compound given is \(\text{Na}_2[\text{Mo}(\text{CN})_2(\text{CO})_2(\text{dmpe})]\)
• This aggregate has a net charge of -2, balanced by two Na+ ions.
• We have to deduce the charge of Mo using the charges of the ligands.

Most ligands have distinct charges:

• CN, a cyanide ion, carries a -1 charge.
• CO, or carbon monoxide, is a neutral ligand with a charge of 0.
• Dmpe, as a neutral bidentate ligand, also carries a charge of 0.

By setting up the equation from the overall charge:\[\text{Mo oxidation state} + (-2) + 0 + 0 = -2\]So, the oxidation state of Mo is calculated to be +4. This estimation helps predict the stability and reactivity of the compound.

Bidentate Ligand

A bidentate ligand is capable of forming two bonds with a central metal atom. Bidentate ligands have two "teeth" or donor atoms that can simultaneously bind to the same metal center. This dual bonding helps create stable metal-ligand complexes. Let's look closer into the properties of bidentate ligands:

• In our molecule, dimethylphosphinoethane (dmpe) acts as a bidentate ligand.
• Dmpe coordinates with molybdenum using its two phosphorus atoms.
• Each phosphorus atom donates a pair of electrons to the metal, forming two coordination sites.

This behavior is akin to ethylenediamine, the typical bidentate ligand with two nitrogen donor atoms. Both these ligands increase complex stability by effectively encircling and anchoring to the metal center. The use of bidentate ligands is highly beneficial in creating more robust and often catalytically active complexes.

Lewis Structure

Lewis structures are diagrams representing the valence electron arrangement within molecules. They are essential for predicting molecular geometry, polarity, and reactivity. For dimethylphosphinoethane (dmpe), a key step is constructing its Lewis structure:

• Start by calculating the total number of valence electrons. Dmpe includes phosphorus (P), carbon (C), and hydrogen (H) atoms.
• P has 5 valence electrons. With two P atoms, the total is 10 electrons.
• C has 4 valence electrons. With four C atoms, it totals 16 electrons.
• H has 1 valence electron. With twelve H atoms, it totals 12 electrons.

Adding these gives us a total of 38 valence electrons. The Lewis structure shows how atoms are bonded, ensuring that each adheres to the octet rule when applicable. For organic compounds like dmpe, note:

• The electrons are shared to form stable bonds, depicted as lines connecting the atoms.
• The structure aligns to the formation of single or double bonds needed to satisfy the individual element's electron requirements, especially for nonmetals.

Lewis structures provide a foundational understanding of molecular interactions and help predict how molecules engage in chemical reactions.