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While rummaging in my molecular model kit, I noticed that it contains a surprising number of $180^\circ$ oxygen atoms, i.e., oxygen that is meant to be covalently bonded to two partners in a linear geometry. Obviously this isn't a hard reference, but since the geometries contained in the kit usually represent common types of bonds, I started wondering when this actually occurs.

It's been a while since my organic chemistry course, but based on VSEPR, the first configuration I could imagine is a doubly positive oxygen with two double bonds on either side, that is, the isoelectronic configuration to carbon with a double bond on either side (as in $\ce{CO_2}$). But $\ce{O^{2+}}$ seems like quite an exotic configuration that I've never heard of. Alternatively, according to the Wikipedia article on linear configurations, the same might be possible in a linear (quasi-trigonal bipyramid) geometry given a central $\ce{O^{2-}}$ atom with two single bonds, isoelectronic to $\ce{Xe}$ in $\ce{XeF_2}$, which seems more in line with my intuition.

Are there other, more common linear oxygen configurations are there? And what are real-life examples of such bonds occurring?

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    $\begingroup$ Generally, the holes in that oxygen aren't for bonds. My experience has been that aside from the tetrahedral looking oxygen, they also have an oxygen that has 5 holes. Two antipodally placed, and 3 in a perpendicular plane. This is supposed to represent a sp2 hybridized oxygen atom. I've seen model sets with a similar carbon atom. They also have fancy blobs that represent orbitals that you can put into the holes that are out of plane. $\endgroup$
    – Zhe
    Apr 12, 2022 at 19:46
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    $\begingroup$ If you use the hole for a hydrogen bond, 180 degrees is possible (like in an alpha helix). flinnsci.com/globalassets/flinn-scientific/… $\endgroup$
    – Karsten
    Apr 13, 2022 at 23:18
  • $\begingroup$ @KarstenTheis That seems like the answer I was looking for! Not perfectly 180 degrees, but close enough and the most organic/biochemical suggestion so far. $\endgroup$ Apr 14, 2022 at 18:53

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Oxygen can be bonded with 180° bond angles in perovskite structures, such as silicate minerals formed deep in Earth's mantle (the perovskite form of calcium silicate has been identified at Earth's surface by being trapped under internal pressure in diamonds). Such minerals have octahedrally coordinated silicon atoms connected by linearly two-coordinate oxygens, with the empirical formula $\ce{MSiO3}$. The calcium silicate compound referred to above has the structure given below (red = oxygen, black = silicon, blue = calcium)[Source]:

Structure od davemaoite, calcium silicate perovskite.

Since the silicon in this silicate-perovskite structure is a $p$-block element and yet octahedrally coordinated, the covalent bonds to it are shared over multiple atoms and thus each silicon-oxygen linkage is partially ionic (cf. $\ce{SF6}$).

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    $\begingroup$ Very ingenious answer! Not quite the sort of thing one looks to build with a molecular model though... $\endgroup$ Apr 12, 2022 at 21:51
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    $\begingroup$ @orthocresol - maybe not for chemists, but materials science folk do odd things with ball and stick models… $\endgroup$
    – Jon Custer
    Apr 12, 2022 at 22:06
  • $\begingroup$ @JonCuster I would love to see some of that...! $\endgroup$ Apr 12, 2022 at 22:07
  • $\begingroup$ After posting the question, I also started thinking along these lines, good point. Since the kit is geared towards biochemistry, might there also be examples of such arrangements occurring in (organo-)metal coordination centers? $\endgroup$ Apr 13, 2022 at 17:55
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No, bond angles of virtually any $\ce{R-O-R}$ bond will differ from $180^\circ$, so there are no common linear oxygen configurations. Steric hindrance can cause a severe deviation from the ideal tetrahedral bond angle of $109.5^\circ$, but it will usually still be very far from $180^\circ$. For example, the paper below by Liedle et al. reports a bond angle for di-tert-butyl ether of $130.8^\circ$.


Reference:

J. Mol. Struct. 1989, 198, 1-15. https://doi.org/10.1016/0022-2860(89)80025-0

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