In this question, we need to identify the most unstable species among Ti(CH2H5)4, Ti(CH2Ph)4, Pb(CH3)4, and Pb(C2H5)4.



To identify the most unstable species, we need to consider the following factors:

1. Steric Hindrance: The bulkier the ligands, the more steric hindrance is present, and the more unstable the species becomes.

2. Polarizability: The more polarizable the ligands, the more unstable the species becomes.

Using these factors, we can rank the stability of the given species as follows:

1. Ti(CH2H5)4: This species has four ethyl ligands, which are relatively small and not very polarizable. Therefore, it is the most stable species among the given options.

2. Ti(CH2Ph)4: This species has four phenyl ligands, which are bulkier than ethyl ligands. However, they are still not very polarizable. Therefore, it is less stable than Ti(CH2H5)4.

3. Pb(CH3)4: This species has four methyl ligands, which are smaller than phenyl and ethyl ligands. However, they are more polarizable than ethyl ligands. Therefore, it is less stable than Ti(CH2H5)4 and Ti(CH2Ph)4.

4. Pb(C2H5)4: This species has four ethyl ligands, which are as bulky as phenyl ligands but less polarizable than methyl ligands. Therefore, it is the most unstable species among the given options.



Therefore, the most unstable species among Ti(CH2H5)4, Ti(CH2Ph)4, Pb(CH3)4, and Pb(C2H5)4 is Pb(C2H5)4.