# Why can oxygen have a coordination number of 3 in a complex between silver and SiO2?

I am running an MD simulation of silver with $\ce{SiO2}$. Initially, my silver atom is neutral and oxygen has a coordination of 2 with silicon having a coordination of 4. After the deposition of silver atoms, I find that the coordination of silver atoms ranges up to 12 and oxygen has coordination of 3 where a bond is formed with silver. Since $\ce{Ag-Si}$ bonds are not present, silicon's coordination number does not change.

I want to understand why oxygen is able to have a coordination number of 3 as with 2 bonds with silicon, the valency is already satisfied. I know silver forms a fcc-lattice where the coordination number is 12 but I want to get a more in depth detail of this change of coordination number. The basic physics behind it.

• Welcome to Chemistry.SE. This is an interesting question. I have edited the question title to better get to the heart of what you want to know, which is why/how these coordination numbers are possible. If you have any questions about how our site works, please consider taking the tour and visiting our help center. – Ben Norris Jul 14 '15 at 10:53
• Just because it exist in an MD, it doesn't necessary mean it is real. From your description is not clear what calculation method you used, what kind of geometries you are talking about and what you consider coordination. You may want to share more details on these. The "coordination" of 12 neighbors in metallic silver is rather irrelevant to the question what is the coordination number of silver ions in complexes. – Greg Jul 14 '15 at 11:08
• Hi Greg, I have a fixed charged potential where the charges on the atoms remain fixed. As the silica lattice is amorphous and is to be kept neutral, I have qSi = 1.89 and qO = 0.945 and I simulate a deposition event of uncharged silver atoms on the lattice. The silver is having a coordination from 1-12 depending upon the site it is present on the lattice. I want to understand what leads to this variation and also the oxygen coordination of 3. – Curious Jul 14 '15 at 11:53
• Dunno why you have problem with this, you know coordination number but act as if know nothing about coordination complexes? – Mithoron Jul 15 '15 at 12:22

High coordination numbers are really not uncommon in solid-state compounds. For example, consider the structure of $\ce{Li2O}$, an anti-fluorite structure. In fluorite proper, calcium ions form a cubic closest packing structure and the fluoride ions occupy the tetrahedral voids. In $\ce{Li2O}$, it is reversed: the oxide ions form a cubic closest packing and lithium occupies the tetrahedral voids. That gives each lithium a coordination number of 4 and by maths and symmetry oxygen must have a coordination number of 8.
Tricoordinated oxide anions are also well-known in solid-state structures. The prime example would be the rutile structure of $\ce{TiO2}$ with hexacoordinated titanium and tricoordinated oxide ions.
In your specific case, the silver cations are acting as Lewis acids while the oxide anions (which have to lone pairs not directed towards neighbouring silicium atoms) can act as Lewis bases. A coordination number of three points to a somewhat covalent (and not so ionic) structure — which makes sense considering significant fragments of $\ce{SiO2}$ are still there.