When would a R-O-R bond be linear?

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](https://en.wikipedia.org/wiki/Linear_molecular_geometry), 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?




Oxygen can be bonded with 180° bond angles in perovskite structures, such as [silicate minerals formed deep in Earth's mantle](https://en.wikipedia.org/wiki/Silicate_perovskite) (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](https://commons.wikimedia.org/wiki/File:Structure_of_Davemaoite.png#mw-jump-to-license)]:


[![Structure od davemaoite, calcium silicate perovskite.](https://i.stack.imgur.com/HUtMe.png)](https://i.stack.imgur.com/HUtMe.png)


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. $\ce{SF6}$).


 13

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>


 7