Chemistry Exam Questions

Ligands

For a list of common ligand abbreviations, see the Crabtree textbook and lecture notes.

1, Z-type ligands are not so commonly encountered as the X and L types. (a) Explain what type of bonding scenario results from interaction of a transition metal fragment with a Z-type ligand; (b) which metal and ligand orbitals would be involved in a bonding interaction in an octahedral d6 metal complex? Assume M-Z bonding occurs on the z axis. 6 points

2. Consider two types of simple “bent” carbenes: methylene (H2C:) and aminocarbene (H(NH2)C:). (a) How would you classify these carbenes (L or X2)? (b) How do the relative pσ and pπ orbital energies change at carbon between methylene and aminocarbene? Using arrow notation, clearly label how the electrons occupy the pσ and pπ orbitals at carbon in each case. 6 points

3. (i) Sketch out the 3D structure of the molecule; (ii) determine the [MLlXxZz]n classification; (iii) provide the total valence electron count and formal oxidation state. Assume that common multidentate ligands bind with maximum denticity/hapticity, i.e. Cp = C5H5 = η5-C5H5. (some complexes do not have 18 valence electrons!). 36 points (3 per molecule)

4.  Consider the complex [Os(en)2(η2-C2H4)(η2-C2H2)]2+, where to two η2 ligands are trans to one another. (a) Identify the two limiting structures in terms of L-type and X-type donation for the two η2 ligands in the structure; (b) In the above complex, X-ray crystallography reveals that the C-C-H bond angle in coordinated “ethyne” is 127 degrees and 122 degrees in coordinated “ethene”. Sketch the 3D structure that best describes the bonding based on the X-ray crystallographic data. (d) What is the resultant [MLlXxZz]n classification, valence electron count, and formal oxidation state? 8 points

5. Suggest a mechanism for the below reaction, indicating names of the elementary reaction steps involved. 6 points

6. Consider the following ligand substitution reaction:

The above reaction could occur through a two-step dissociative or three-step associative mechanism, with the first step being reversible in both cases. It is very unlikely that any intermediate exceeds an 18e- valence electron count.

a. Draw the intermediate(s) for both these reaction mechanisms, clearly showing the structure and bonding for each intermediate. Except for the phosphines, no other ligand fully dissociates from the metal center. 4 points

b. Suppose a dissociative mechanism is occurring that adheres to the following rate law:

What would happen to kobs if excess PPh3 were added and k-1 >> k2 ? 1 point

What would happen to kobs if excess PPh2Me were added and k-1 >> k2 ? 1 point

c. Kinetic studies actually indicate that ΔS‡ = -10 cal⋅mol-1⋅K-1 and addition of excess PPh3 or PPh2Me do not inhibit the reaction rate. Based on this information, which mechanism is operative and which step is likely rate-limiting? 2 points

7. Consider the following alkylidene complexes:

a. For each complex, determine the [MLlXxZz]n classification, total valence electron count, and metal’s formal oxidation state. 6 points

b. Using crystal field theory, compare the singlet and triplet carbene configurations for the nickel complex and rationalize why the triplet configuration would be more energe ticallyfavorable. Consider the Ni-C and Ni-P bonds to be sitting on the xy plane while the z axis is sticking out of the page. 4 points

Total points: 80