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Most online sources suggest that the bond angle of $\ce{OBr2}$ is greater than the one of $\ce{OCl2}.$ It is explained by the electronegativity difference and electron repulsion within the VSEPR theory.

However, I think that the extent of back bonding would be more for $\ce{OCl2}$ and hence its bond angle should be greater. I have not been able to find any experimental data from reliable sources to verify my assumption.

Can someone please explain why I may be wrong and provide experimental data on bond angles along with it?

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  • $\begingroup$ It is indeed larger, but the difference is only 1.1 degree. This is negligible and a lot of different effects could cause it. $\endgroup$
    – permeakra
    Nov 22, 2016 at 21:38
  • $\begingroup$ Related: chemistry.stackexchange.com/q/18697/7475 $\endgroup$
    – Jan
    Nov 22, 2016 at 23:37
  • $\begingroup$ I found some old data (J. Chem. Soc. A, p658,1968) on $\ce{Cl2O}$ which gives an experimental bond angle of $111^{\circ}$ and bond length of $171$ pm. There are several sites that quote an angle of $105^{\circ}$ for $\ce{Br2O}$ but don't say whether this is measured. $\endgroup$
    – porphyrin
    Nov 23, 2016 at 9:20

2 Answers 2

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$\ce{F2O}$

Phase Method ∠F−O−F Reference
solid (−223.8 °C) powder x-ray diffraction 98.0(1.5)° [1], ICSD-430183
gas microwave spectroscopy 103.07(5)° [2]

$\ce{Cl2O}$

Phase Method ∠Cl−O−Cl Reference
solid (−183(2) °C) single-crystal x-ray diffraction 111.27(4)° [3], ICSD-407768
gas microwave spectroscopy 110.96(8)° [4]

$\ce{Br2O}$

Phase Method ∠Br−O−Br Reference
solid (−143 °C) single-crystal x-ray diffraction 114.2(2)° [5], ICSD-50198
gas microwave spectroscopy 112.24(20)° [6]

There are differences for the bonding angles for gaseous and solid phases due to packing effects and intermolecular interactions. The trend, however, remains: there is an increase of $\ce{X-O-X}$ angle from $\ce{F2O},$ $\ce{Cl2O}$ to $\ce{Br2O}$ as “a result of increasing repulsion forces between the heavier halogen atoms”. [5] Overlay of the three structures obtained via x-ray diffraction is shown below by merging the location of the common oxygen atom and fixing the direction of a single $\ce{X-O}$ bond as well as the plane of each molecule:

overlay of F2O, Cl2O, and Br2O

References

  1. Marx, R.; Seppelt, K. Structure Investigations on Oxygen Fluorides. Dalton Trans. 2015, 44 (45), 19659–19662. DOI: 10.1039/C5DT02247A.
  2. Pierce, L.; Jackson, R.; DiCianni, N. Microwave Spectrum, Structure, and Dipole Moment of $\ce{F2O}$. J. Chem. Phys. 1961, 35 (6), 2240–2241. DOI: 10.1063/1.1732240.
  3. Minkwitz, R.; Bröchler, R.; Borrmann, H. Tieftemperatur-Kristallstruktur von Dichlormonoxid, $\ce{Cl2O}.$ Zeitschrift für Kristallographie - Crystalline Materials 1998, 213 (4), 237–239. DOI: 10.1524/zkri.1998.213.4.237.
  4. Jackson, R. H.; Millen, D. J. Microwave Spectrum and Nuclear Quadrupole Coupling Coefficients for Chlorine Monoxide. Proc. Chem. Soc. 1959, 10.
  5. Hwang, I.-C.; Kuschel, R.; Seppelt, K. Structures of Bromine Oxygen Compounds. Z. Anorg. Allg. Chem. 1997, 623 (1–6), 379–383. DOI: 10.1002/zaac.19976230160.
  6. Brenschede, W.; Schumacher, H.-J. Über Die Darstellung Und Einige Eigenschaften Eines Bromoxyds von Der Formel Br 2 O. Z. Anorg. Allg. Chem. 1936, 226 (4), 370–384. DOI: 10.1002/zaac.19362260409.
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I think steric effects may play a role in that chlorine atoms are smaller than bromine atoms. The bromine atomic radius is about 0.15 Å larger than the chlorine atomic radius. The larger bond angle can be in part explained by the need to accommodate the larger atoms.

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