Longest C-O bond length in [Fe(CO)4]2-

1. Explanation of the complex [Fe(CO)4]2-:

- The given complex is [Fe(CO)4]2-, which means it consists of an iron (Fe) atom bonded to four carbon monoxide (CO) ligands.
- Carbon monoxide (CO) is a strong π-acceptor ligand, meaning it can donate its π-electrons to the metal center.
- The Fe atom in [Fe(CO)4]2- has a d6 electron configuration, meaning it has six d-electrons available for bonding.

2. Factors affecting bond length:

- The bond length between the metal and the ligand depends on various factors, including the nature of the metal atom, the nature of the ligand, and the electronic configuration of the metal.
- In general, the bond length increases with an increase in the size of the metal atom and decreases with an increase in the strength of the metal-ligand bond.
- In the case of CO ligands, the bond length increases with an increase in the π-accepting ability of the ligand.

3. Analysis of the given complexes:

a) [Mn(CO)6]:

- The Mn atom in [Mn(CO)6] has a d5 electron configuration, and CO is a strong π-acceptor ligand.
- However, the Mn atom is larger than the Fe atom, which can result in longer bond lengths.
- Therefore, [Mn(CO)6] may have longer C-O bond lengths than free CO, but it may not have the longest C-O bond length among the given options.

b) Ni(CO)4:

- The Ni atom in Ni(CO)4 has a d8 electron configuration, and CO is a strong π-acceptor ligand.
- However, Ni is smaller than Mn and Fe, which can result in shorter bond lengths.
- Therefore, Ni(CO)4 is less likely to have the longest C-O bond length among the given options.

c) [Co(CO)4]-:

- The Co atom in [Co(CO)4]- has a d7 electron configuration, and CO is a strong π-acceptor ligand.
- Co is larger than Ni, but smaller than Mn and Fe.
- Therefore, [Co(CO)4]- may have longer C-O bond lengths than Ni(CO)4, but it is still less likely to have the longest C-O bond length among the given options.

d) [Fe(CO)4]2-:

- The Fe atom in [Fe(CO)4]2- has a d6 electron configuration, and CO is a strong π-acceptor ligand.
- Fe is smaller than both Mn and Co.
- Therefore, [Fe(CO)4]2- is likely to have the longest C-O bond length among the given options.

4. Conclusion:

- Based on the analysis, [Fe(CO)4]2- is expected to have the longest C-O bond length among the given options.
- The smaller size of the Fe atom compared to Mn and Co allows for longer bond lengths, despite CO being a strong π-acceptor ligand.