Question

Which of the following is correct about the structure of metal carbonyl?

• A. Only II, III and IV
• B. Only I, II and III
• C. Only II and III
• D. I, II, III and IV

Answer 1

Answer: (B)

Only I, II and III

Answer 2

• (I) $[Fe(CO)_5]$: Iron pentacarbonyl has a trigonal bipyramidal structure, consistent with VSEPR theory and the 18-electron rule for $d^6$ metal complexes. This statement is correct.
• (II) $[Cr(CO)_6]$: Chromium hexacarbonyl is an octahedral complex. The Cr(0) atom has a $3d^5 4s^1$ configuration. In the octahedral field, the 6 valence electrons fill the $t_{2g}$ orbitals as $(t_{2g})^6$, resulting in all paired electrons, making it diamagnetic. This statement is correct.
• (III) $[Ni(CO)_4]$: Nickel tetracarbonyl is a tetrahedral complex. The Ni(0) atom has a $3d^8 4s^2$ configuration. In the tetrahedral field, the 10 valence electrons fill the $e$ and $t_2$ orbitals as $(e)^4(t_2)^6$, resulting in all paired electrons, making it diamagnetic. This statement is correct.
• (IV) $[Co_2(CO)_8]$: Dicobalt octacarbonyl exists in two isomeric forms:
  1. A bridging isomer which has a Co-Co single bond, six terminal carbonyl ligands, and two bridging carbonyl ligands.
  2. A non-bridging isomer which has a Co-Co single bond and all eight carbonyl ligands are terminal. Since the statement claims $[Co_2(CO)_8]$ has two bridging and six terminal carbonyls, it only describes one of the isomers and is not universally true for all forms of $[Co_2(CO)_8]$. Therefore, this statement is incorrect.

Based on the analysis, statements (I), (II), and (III) are correct.