Question

The molecule dimethylphosphinoethane \(\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{PCH}_{2}^{-}\right.\) \(\mathrm{CH}_{2} \mathrm{P}\left(\mathrm{CH}_{3}\right)_{2},\) which is abbreviated dmpe \(]\) 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 \(\mathrm{Na}_{2}\left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}(\) dmpe \()\right] ?(\mathrm{c})\) Sketch the structure of the \(\left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}(\mathrm{dmpe})\right]^{2-}\) ion, including all the pos- sible isomers.

Short answer

(a) The Lewis structure of dmpe (\(\left(\mathrm{CH}_{3}\right)_{2}\mathrm{PCH}_{2}\mathrm{CH}_{2}\mathrm{P}\left(\mathrm{CH}_{3}\right)_{2}\)) shows two P (phosphorus) atoms capable of donating electron pairs, similar to ethylenediammine where two N (nitrogen) atoms can donate electron pairs. Both ligands can form chelate complexes. (b) The oxidation state of Mo in \(\mathrm{Na}_{2}\left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}(\mathrm{dmpe})\right]\) is 0. (c) Two possible isomers of the \(\left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}(\mathrm{dmpe})\right]^{2-}\) ion are cis-[Mo(CN)2(CO)2(dmpe)]^(2-) and trans-[Mo(CN)2(CO)2(dmpe)]^(2-), both displaying octahedral geometries with Mo at the center and ligands arranged around it.

Step-by-step solution

a) Drawing the Lewis Structure for dmpe and Comparing it with Ethylenediammine

1. Write the molecular formula for dmpe: \(\left(\mathrm{CH}_{3}\right)_{2}\mathrm{PCH}_{2}\mathrm{CH}_{2}\mathrm{P}\left(\mathrm{CH}_{3}\right)_{2}\) 2. Count the total number of valence electrons in the dmpe molecule: - There are 2 P (phosphorus) atoms with 5 valence electrons each (total 10). - There are 10 C (carbon) atoms with 4 valence electrons each (total 40). - There are 20 H (hydrogen) atoms with 1 valence electron each (total 20). - The total number of valence electrons is 10+40+20=70. 3. Draw the Lewis structure of dmpe by arranging the atoms and distributing the valence electrons: - Connect each P (phosphorus) atom to one CH2 group each, these groups are connected to each other, forming the backbone of the molecule. - Connect each CH2 group to two CH3 groups, one on each end. - Distribute the remaining valence electrons to form multiple bonds and satisfy the octet rule for each atom. 4. Compare dmpe with ethylenediammine as coordinating ligands: - Ethylenediammine (\(\mathrm{NH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{NH}_{2}\)) is a bidentate ligand capable of donating lone pairs of electrons from its two nitrogen atoms. - dmpe is also a bidentate ligand but is capable of donating lone pairs of electrons from its two phosphorus atoms. - Both ligands are able to form chelate complexes due to their ability to form multiple bonds with a metal ion, but they do this through different atoms (phosphorus in dmpe and nitrogen in ethylenediammine).

b) Determining the Oxidation State of Mo in the Given Complex

1. Write the formula for the complex: \(\mathrm{Na}_{2}\left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}(\mathrm{dmpe})\right]\) 2. Determine the charges contributed by the ligands in the complex: - Each CN (cyanide) ligand has a charge of -1, contributing -2 to the complex. - CO (carbonyl) ligands and dmpe are neutral with no charge contribution. 3. Calculate the charge of the metal ion (Mo) in the complex: - Since this is a complex, let's assign the charge of Mo as 'x'. - The overall charge of the complex is -2. - The complex is neutralized by 2 sodium (Na) ions, each with a charge of +1. - Solve for x: x - 2 (CN) + 0 (CO & dmpe) = (-2 overall charge) - x -2 = -2 - x = 0 4. The oxidation state of Mo in the complex is 0.

c) Sketching the Structure of the Given Ion, Including All Possible Isomers

1. Write the formula for the ion: \(\left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}(\mathrm{dmpe})\right]^{2-}\) 2. Identify the coordination number of Mo in the complex: - Coordination number represents the number of ligand bonds with the central metal ion. - Mo is coordinated to 2 CN ligands, 2 CO ligands, and 1 bidentate dmpe ligand, which contributes 2 lone pairs to the coordination sphere. - The coordination number of Mo in this complex is 2 + 2 + 2 = 6. 3. Draw the structure of the ion, considering possible isomers, by placing Mo at the center and arranging the ligands around it: - The structure of the \(\left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}(\mathrm{dmpe})\right]^{2-}\) ion is an octahedral complex, with the desired coordination number of Mo. - The possible isomers of this complex are determined by the placement of CN, CO, and dmpe ligands around the Mo center. - There are two structural isomers: one with cis-arranged CN ligands (CN ligands are adjacent) and one with trans-arranged CN ligands (CN ligands are opposite to each other). - Draw the cis-[Mo(CN)2(CO)2(dmpe)]^(2-) ion with CN ligands adjacent, CO ligands adjacent, and the dmpe ligand forming two bonds with Mo. - Draw the trans-[Mo(CN)2(CO)2(dmpe)]^(2-) ion with CN ligands opposite each other, CO ligands opposite each other, and the dmpe ligand forming two bonds with Mo. 4. In conclusion, two possible isomers of the \(\left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}(\mathrm{dmpe})\right]^{2-}\) ion are cis-[Mo(CN)2(CO)2(dmpe)]^(2-) and trans-[Mo(CN)2(CO)2(dmpe)]^(2-).

Key concepts

Ligands

In coordination chemistry, ligands play a crucial role as they are ions or molecules that bind to a central metal atom to form a coordination complex. The strength and type of bonding between a metal and its ligands significantly impact the properties of the complex.
When it comes to dmpe, or dimethylphosphinoethane, this ligand is a type of bidentate ligand. This means it has two "teeth" that can bind through its phosphorus atoms, creating strong bonds with the metal center.
Ethylenediamine, another common ligand often compared to dmpe, uses its nitrogen atoms to bind in a similar bidentate fashion. Both can form chelate complexes, where the metal is "hugged" by the ligand’s multiple bonding sites, stabilizing the complex and influencing its reactions and structure. Whether through nitrogen in ethylenediamine or phosphorus in dmpe, these ligands enhance the metal's ability to catalyze reactions by modulating electron density and sterics around the metal.

Oxidation States

Understanding the oxidation state of a metal in a complex is pivotal in predicting its reactivity, stability, and overall chemical behavior. By examining the complex \(\mathrm{Na}_{2}\left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}(\mathrm{dmpe})\right]\), we can determine the oxidation state of the molybdenum (Mo) central metal ion.
Generally, ligands contribute to the overall charge of the complex. For example, each cyanide (CN) ligand is negatively charged (\

Coordination Compounds

Coordination compounds are fascinating structures where metal atoms are surrounded by non-metal ligands. In the complex \( \left[\mathrm{Mo}(\mathrm{CN})_{2}(\mathrm{CO})_{2}(\mathrm{dmpe})\right]^{2-} \), coordination occurs through specific bond formations that stabilize the entire structure.
These compounds have a central metal atom, such as Mo, surrounded by different ligands that dictate the coordination number. This number, which is 6 in our example, tells us how many points of attachment around the metal exist. This includes:

• Two CN ligands
• Two CO ligands
• The bidentate dmpe ligand, offering two connections via the phosphorus atoms

Ultimately, these connections form an octahedral structure around Mo, allowing for distinct isomers. Octahedral complexes like this can show different physical and chemical properties depending on the arrangement of ligands, such as cis-trans isomerism, which impacts their color and reactivity.

Lewis Structures

Lewis structures are visual representations of molecules or ions, providing clarity on how atoms share electrons. Often, they help us predict the shape and bonding of molecules, guiding us in understanding their reactivity.
For the ligand dimethylphosphinoethane (dmpe), drawing its Lewis structure enhances our comprehension of its complexation properties. This involves arranging its phosphorus and carbon atoms, focusing on electron pairs and multiple bonding sites.
Creating the Lewis structure involves:

• Counting and distributing valence electrons to fulfill the octet rule.
• Arranging the molecular backbone around phosphorus atoms that are key to coordination.

By comparing dmpe's Lewis structure with a similar ligand, ethylenediamine, we see differences in the atoms used for electron pair donation and the overall electron distribution. Understanding how each ligand contributes its electrons clarifies how they bind metals in coordination complexes, impacting the overall structure and function of the complex.