Q.11 The geometry of Fe(CO)5 is
(Given: Atomic number of Fe = 26)
(A) pentagonal planar
(B) square pyramidal
(C) trigonal bipyramidal
(D) trigonal pyramidal
Option Analysis
Pentagonal Planar (A)
This rare geometry involves five ligands in a flat pentagon (72° bond angles), typically seen in ions like [XeF₅]⁻ with D₅ₕ symmetry, not common for coordination number 5 transition metal complexes like Fe(CO)₅.
Square Pyramidal (B)
Features a square base with one apical ligand (one Fe-ligand bond ~90° shorter), common for CN=5 with sp³d hybridization in d⁸ systems like [Ni(CN)₅]³⁻, but Fe(CO)₅ shows equal Fe-CO bonds confirming no such distortion.
Trigonal Bipyramidal (C)
Correct for Fe(CO)₅; Fe (atomic number 26, [Ar]3d⁶4s²) in zero oxidation state pairs electrons for dsp³ hybridization (one 3d, 4s, three 4p orbitals), yielding three equatorial (120°) and two axial (90°) CO positions with D₃ₕ symmetry.
Trigonal Pyramidal (D)
Applies to tetrahedral electron geometry with one lone pair (e.g., NH₃, ~107° angles), irrelevant here as Fe(CO)₅ has five bonding pairs and no lone pairs on Fe.
Why Trigonal Bipyramidal Structure?
Iron pentacarbonyl, Fe(CO)₅, showcases Fe(CO)5 geometry trigonal bipyramidal structure, a cornerstone in coordination chemistry for CSIR NET aspirants studying metal carbonyls. With Fe atomic number 26 and zero oxidation state, this molecule achieves 18-electron stability through five CO ligands.
Hybridization and Electron Count
Fe([Ar]3d⁶4s²) pairs d-electrons under strong-field CO influence, freeing dsp³ orbitals (one d, s, three p) for five sigma bonds plus π-backbonding. No lone pairs yield pure trigonal bipyramidal (D₃ₕ), unlike fluxional behavior in solution.
Why Trigonal Bipyramidal?
Coordination number 5 favors trigonal bipyramidal (90°/120° angles) over square pyramidal due to equal Fe-CO bonds (~1.83 Å) and minimal repulsion. Examples: PCl₅(g), Fe(CO)₅.
Ruling Out Alternatives
- Pentagonal planar: Rare, planar 72° angles for [XeF₅]⁻, unstable for d⁶ Fe(0).
- Square pyramidal: Apical distortion absent; seen in [VO(acac)₂].
- Trigonal pyramidal: Lone-pair effect, not applicable.
Geometry Comparison Table
| Geometry | CN | Hybridization | Fe(CO)₅ Fit? | Example |
|---|---|---|---|---|
| Pentagonal Planar | 5 | d²sp³? | No | [XeF₅]⁻ |
| Square Pyramidal | 5 | sp³d | No | [Ni(CN)₅]³⁻ |
| Trigonal Bipyramidal | 5 | dsp³ | Yes | Fe(CO)₅ |
| Trigonal Pyramidal | 4 (AX₃E) | sp³ | No | NH₃ |
CSIR NET Exam Insight
This Fe(CO)5 geometry trigonal bipyramidal insight aids exam questions on VSEPR/hybridization in organometallics. Understanding coordination number 5 geometries is crucial for competitive exams.
