Problem 71 - Chemistry: The Central Science

Question

The molecule dimethylphosphinoethane \(\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{PCH}_{2} \mathrm{CH}_{2}\right.\) \(\mathrm{P}\left(\mathrm{CH}_{3}\right)_{2}\), which is abbreviated dmpel 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 ethylenediamine as a coordinating ligand. (b) What is the oxidation state of Mo in \(\mathrm{Na}_{2}\left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}\right.\) (dmpe)]? (c) Sketch the structure of the \(\left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}(\text { dmpe })\right]^{2-}\) ion, including all the possible isomers.

Step-by-Step Solution

Answer

(a) The Lewis structure of dmpe is ((CH3)2PCH2CH2P(CH3)2), with no lone pairs on P atoms. In ethylenediamine (NH2CH2CH2NH2), N atoms have one lone pair each. As ligands, dmpe has P donating, while ethylenediamine has N donating. (b) The oxidation state of Mo in Na2[Mo(CN)2(CO)2(dmpe)] is 0. (c) The possible isomers for [Mo(CN)2(CO)2(dmpe)]^2- are cis, trans, and mer, based on the arrangement of the CN and CO ligands relative to the dmpe ligand.

Step 1: (a) Drawing the Lewis Structure for dmpe and comparing it with ethylenediamine as a coordinating ligand

Firstly, draw the Lewis structure for both dimethylphosphinoethane (dmpe) and ethylenediamine: For dmpe: ((CH3)2PCH2CH2P(CH3)2) - Each C atom in CH3 has 3 bonds (one to P and two to H), so it has no lone pairs. - Each P atom has 1 bond to C (connected to CH3), 1 bond to CH2, and 2 bonds to CH3, so it also has no lone pairs. - Each CH2 is connected to two P atoms (two bonds) and two H atoms (two bonds), so it has no lone pairs. For ethylenediamine (NH2CH2CH2NH2): - Each N atom in NH2 has two bonds to H and one bond to CH2, so it has one lone pair remaining. - Each CH2 group is similar to the one in dmpe, so it also has no lone pairs. As ligands, dmpe has phosphorus donating 1 electron pair to the central metal while ethylenediamine has nitrogen donating 1 electron pair to the central metal. Each donor atom in ethylenediamine possesses a non-bonding electron pair, while each donor atom in dmpe possesses no lone pair.

Step 2: (b) Finding the oxidation state of Mo in Na2[Mo(CN)2(CO)2(dmpe)]

Here, we need to find the oxidation state of Mo in the given compound. Let's denote Mo's oxidation state as x. Na2[Mo(CN)2(CO)2(dmpe)]: 2 Na+ ions balance the charge of the [Mo(CN)2(CO)2(dmpe)]^2- ion. Now, consider the [Mo(CN)2(CO)2(dmpe)]^2- ion: - CN has -1 charge, and there are two CN groups. - CO is considered neutral in charge. - dmpe is a neutral ligand, so it doesn't contribute to the charge. - Total charge on complex = 2-. Considering the above, we have: x + 2(-1) + 0 + 0 = -2 => x = 0 Thus, the oxidation state of Mo in Na2[Mo(CN)2(CO)2(dmpe)] is 0.

Step 3: (c) Sketching the structure of the [Mo(CN)2(CO)2(dmpe)]^2- ion (including all possible isomers)

To sketch the structure of the [Mo(CN)2(CO)2(dmpe)]^2- ion and all possible isomers, let's assign positions to the ligands: 1. Place dmpe at the equatorial positions (axially at 180°) with P atoms coordinating to Mo. 2. Place CN and CO ligands either at two axial (180°) or two equatorial (90°) positions. There are three possible isomers: cis, trans, and mer. - cis: The CN and CO ligands are at adjacent equatorial positions. In this configuration, the CO and CN ligands are closer to each other, specifying the cis form. - trans: The CN and CO ligands are at opposite equatorial positions. In this configuration, the CO and CN ligands are away from each other, specifying the trans form. - mer: The CN and CO ligands are at axial positions. In this configuration, the CO and CN ligands are farther from each other when compared to cis, specifying the mer form.

Key Concepts

Ligand Structures

A ligand is a molecule that can donate a pair of electrons to a metal center to form a coordination compound. Understanding ligand structures is crucial in inorganic chemistry, as it helps in predicting how the ligand will interact with the metal ion.

• Dimethylphosphinoethane (dmpe): This ligand has two phosphorus atoms, each attached to two methyl groups (CH₃). The phosphorus atoms are connected by an ethylene bridge (CH₂-CH₂).
• Ethylenediamine: Known as en, this ligand consists of two amino groups (NH₂) connected by the same ethylene bridge. Each amino group can donate a lone pair because of nitrogen's lone pair availability.

These ligands differ in their electron-donating abilities and spatial arrangements. In dmpe, phosphorus atoms coordinate to the central metal ion, while in ethylenediamine, nitrogen atoms do. The difference in electron lone pairs on the donor atoms leads to various bonding and structural scenarios in complexes formed by these ligands.

Oxidation States

The oxidation state of an element in a compound indicates the degree of oxidation or electron removal that element has undergone. It is important for determining the electron count of a metal in a coordination complex.
In the compound Na₂[Mo(CN)₂(CO)₂(dmpe)], the oxidation state of molybdenum (Mo) needs to be determined for understanding the electron flow in the complex.

• Charges of Ligands:
  • Cyanide (CN) has a charge of -1, supplying two ligands that total to -2.
  • Carbonyl (CO) and dmpe are neutral ligands, contributing no charge.

Taking into account the total negative charge of the complex, -2, the oxidation state of Mo in this compound is 0, because the positive oxidation state of the metal needs to balance out the negative charges from the ligands.

Molybdenum Complexes

Molybdenum (Mo) complexes play a vital role in catalysis and various industrial applications. These complexes typically form with a diverse range of ligands, including carbonyls, phosphines, and cyanides. In Na₂[Mo(CN)₂(CO)₂(dmpe)], the molybdenum center is coordinated by multiple ligands forming a complex that is key in many catalytic cycles.

• Coordination Geometry: Molybdenum often shows a coordination number of 6, forming an octahedral geometry, which can lead to various ligand arrangements.
• Electronic Interaction: Mo's ability to accept electron density from ligands makes it a versatile center in catalysis, enhancing reaction rates or altering product distribution.

Comprehending the geometry and electron interactions of molybdenum complexes aids in designing innovative catalysts and understanding their mechanisms.

Isomers in Coordination Chemistry

In coordination chemistry, isomers are compounds with the same chemical formula but different structural arrangements. Molybdenum complexes, such as [Mo(CN)₂(CO)₂(dmpe)]^2-, exhibit several isomeric forms based on ligand positioning.

• Cis and Trans Isomers:
  • The cis isomer has ligands positioned adjacent to each other; this arrangement often affects the compound's reactivity and interaction with other molecules.
  • In the trans isomer, ligands are located opposite each other, which can influence the compound's symmetry and properties.
• Meridional Isomers (Mer): These isomers have ligands arranged in such a way that three ligands form a T-shape around the metal center, typically occurring in complexes with a higher number of similar ligands.

Identifying these isomers helps in understanding the stereochemistry and chemical behavior of coordination complexes, which is fundamental for their application in molecular synthesis and industrial catalysis.