Covalent Bond Classification Method

Some of these questions are explained in the assigned readings, so please make sure you have done the reading before working out your answers.

1. Give an example of an L-function ligand that is common to organometallic chemistry, and explain why it fits this classification.
2. Z-function ligands are not so commonly encountered as the X and L types. Explain what type of bonding scenario pertains to the interaction of a transition metal fragment with a Z-function ligand.
3. Explain why it is that an η⁶-benzene ligand should be classified as an L₃ ligand, whilst η⁵-cyclopentadienyl should be classified differently, as an L₂X ligand.
4. A carbene ligand such as methylene can be considered either as an LZ ligand or as an X₂ ligand. Explain why LZ is equal to X₂ with reference to the frontier molecular orbitals of methylene.
5. In the case of η⁷-cycloheptatrienyl, consideration of the frontier molecular orbitals could lead to a classification of L₃XZ. Draw an MO energy level diagram for the frontier molecular orbitals (π system) of η⁷-cycloheptatrienyl assuming D_7h symmetry, populate it with the correct number of electrons, and clearly circle the MO in the diagram that corresponds to the Z function.
6. Redraw the MO energy level diagram of the previous question, now populating it with electrons in such a way as to correspond to an L₂X₃ classification.
7. Once all the ligands in a molecule have been classified, it is then possible to classify the molecule itself according to [ML_lX_xZ_z]^Q. If the molecule is charged, then transformations are employed to convert the classification to its "equivalent neutral class". Transformations are of the type L⁺ becomes X, whilst X⁺ becomes Z, and X^- becomes L. In addition, L^- goes to LX. If after transformation to its equivalent neutral class the system then contains both an L and a Z function, the transformation LZ → X₂ is applied. Draw the molecular ion [Cp₂WH₃]⁺, assign it to its proper point group, find its [ML_lX_xZ_z]^Q classification, and convert it to the equivalent neutral class.
8. Sketch out the structure of the molecule [W(PMe₃)₂(C^tBu)(CH^tBu)(CH₂^tBu)] and find its [ML_lX_xZ_z]^Q classification.
9. Overwhelmingly, the majority of iron compounds are of the type ML₄X₂. Give a real example of a molecule of this type, and also give an example of a known iron organometallic compound that is not of the ML₄X₂ variety.
10. Calculate the Valence (VN), the electron count (EN), and ligand bond number (LBN) for the sandwich complex CpW(η⁷-C₇H₇). Explain how LBN is superior to "coordination number" in such a case.
11. In the case of a C_3v-symmetric molecule such as (ON)Cr(NⁱPr₂)₃ having a linear nitrosyl (bond angle O-N-Cr equal to 180°) it is correct to consider the nitrosyl ligand as an X₃ type. Find the Valence (VN), the electron count (EN), and ligand bond number (LBN) for this molecule. Now, explain why it is that the nitrosyl ligand in (ON)Cr(NⁱPr₂)₃ is correctly considered to be an X₃ type.