# Can Fe(CO)5 adopt both square pyramidal and trigonal bipyramidal geometries?

I know the hybridization of $$\ce{Fe(CO)5}$$ is $$\mathrm{dsp^3}$$. According to my book, coordination compounds with coordination number 5 can interchange between square pyramidal and trigonal bipyramidal geometries. Is it true for this compound? Doesn't seem so according to the picture:

Was my understanding wrong somewhere?

• The orbitals used in $\ce{[Fe(CO)5]}$ are 3d, 4s and 4p. If that makes it an $\mathrm{dsp^3}$ complex in your book then so be it. But then consider that orbitals such as 4p and 4d are energetically too far apart to take part in bonding together in a noteworthy manner. – Jan Aug 30 '16 at 9:48
• – Martin - マーチン Oct 18 '18 at 16:54

Your book was correct that a five coordinate metal complex is able to adopt both square pyramidal and trigonal bipyramidal geometries, and in both cases the sp3d hybridisation scheme applies (if you believe in hybridisation...).

Which geometry is adopted depends upon a combination of steric and electronic factors, and isn't necessarily trivial to predict, though MO theory and crystal field theory can help understand which might be more favourable.

The structure of iron pentacarbonyl is widely agreed to be trigonal bipyramidal, with three CO ligands occupying equatorial positions, and two CO ligands occupying axial positions.

Trigonal bipyramidal structure of iron pentacarbonyl

Whilst both geometries are possible (and indeed observed, IF5 is square pyramidal), I suspect that your confusion arises from the fact that iron pentacarbonyl is able to undergo a phenomenon known as Berry psuedorotation.

Mechanism of Berry psuedorotation

Berry psuedoroation is the process in which two of the equatorial ligands are switched out for the two axial ligands via a square planar intermediate. This process is sufficiently rapid that if observed by NMR, only one environment for the carbon of the CO ligand is observed (i.e. the psuedorotation is faster than the NMR timescale).

• No.Usually dsp3 refers to inner orbital complex while the latter refers to outer orbital complex..Maybe you follow a different convention? – user33702 Aug 29 '16 at 13:19
• I've never known there to be a difference other than the way people choose to write it, but I'm just an organic chemist, not an inorganiker – NotEvans. Aug 29 '16 at 13:28
• @MathJack A hybridisation scheme involving s-, p-, and d-type orbitals is garbage. They are far too much apart to considerably interact with each other. – Martin - マーチン Sep 27 '16 at 10:12
• @NotEvans. As far as i understand, IF5 has sp3d2 hyb. Hence the comparison between IF5 and Fe(CO)5, which I think is dsp3, does not really make sense (as in your answer). The square pyramidal structure of IF5 is very easily explained using VSEPR Theory. Moreover, I don't think that IF5 can show trigonal bipyramidal geometry. – Piyush Maheshwari May 21 '18 at 16:03
• This distinction between "sp3d" and "dsp3" is a relic of an incorrect and flawed theory. – orthocresol Dec 28 '18 at 15:26

The most recent single crystal structure investigation (high precision, $$R_1 = 1.9\%$$, $$wR_2 = 5.7\%$$) of molecular iron pentacarbonyl [1] suggests trigonal bipyramidal coordination environment (also mentioning anti-Berry pseudorotation), as shown by the calculated geometry index $$\tau_5$$:

\begin{align} \begin{cases} \tau_5 &= 0 \qquad &\text{square pyramidal geometry} \\ \tau_5 &= 1 \qquad &\text{trigonal bipyramidal geometry} \end{cases} \end{align}

$$\tau_5 = \frac{\beta - \alpha}{60^\circ} = \frac{178.93^\circ - 121.13^\circ}{60^\circ} = 0.96$$

where $$\alpha$$ and $$\beta$$ - two greatest valence angles of the iron coordination center calculated from the CIF-file (ICSD #74197).

$$\color{#909090}{\Large\bullet}~\ce{C}$$; $$\color{#FF0D0D}{\Large\bullet}~\ce{O}$$; $$\color{#E06633}{\Large\bullet}~\ce{Fe}$$.

The magnitude of the $$\ce{Fe-C}$$ axial vs equatorial band length distortion ($$0.44\%$$) is notably less than in an alogous p-block trigonal-bipyramidal species (e.g. in $$\ce{PF5}$$ $$\ce{P-F_{ax}} = \pu{1.577 Å}$$ and $$\ce{P-F_{eq}} = \pu{1.534 Å}$$; i.e. distortion $$2.8\%$$, by gas-phase electron diffraction).

There is a small angular distortion of the trigonal bipyramid in crystalline $$\ce{Fe(CO)6}$$ with the axial ligands closing on the unique carbonyl ($$\ce{C(1)-Fe-C(2)} = 89.47(5)^\circ$$, $$\ce{C(2)-Fe-C(2a)} = 178.94(10)^\circ$$), two equatorial angles opened from their ideal values ($$\ce{C(3)-Fe-C(1)} = 121.11(5)^\circ$$), and the third slightly closed ($$\ce{C(1)-Fe-C(1a)} = 117.8(1)^\circ$$). This distortion is of an anti-Berry pseudorotation type, i.e. in the direction away from a square-based pyramidal geometry in which $$\ce{C(1)-Fe-C(2)} = \ce{C(1)-Fe-C(3)} = ca. 105^\circ$$.

### Bibliography

1. Braga, D.; Grepioni, F.; Orpen, A. G. Organometallics 1993, 12 (4), 1481–1483. DOI 10.1021/om00028a082.

Here the hybridisation is dsp3 in $\ce{Fe(CO)5}$. It is basically an inner orbital complex. So, it is a spin-free orbital. The hybridisation plays a major role in determining the shape in these cases. So, when the hybridisation is dsp3 in most of the cases you get the shape as trigonal bipyramidal, whereas in sp3d we get square pyramidal.