Inner sphere complex

Inner sphere complex is a type of surface complex that refers to the surface chemistry changing a water-surface interface to one without water molecules bridging a ligand to the metal ion. Formation of inner sphere complexes occurs when ions bind directly to the surface with no intervening water molecules. These types of surface complexes are restricted to ions that have a high affinity for surface sites and include specifically adsorbed ions that can bind to the surface through covalent bonding.

Not to be confused with Inner sphere electron transfer.

Inner sphere complexes describe active surface sites that are involved in nucleation, crystal growth, redox processes, soil chemistry, alongside other reactions taking place between a cation and surface.[1] This affinity to surface sites can be attributed to covalent bonding.

When compared to outer sphere complexes that have water molecules separating ions from ligands, inner sphere complexes have surface hydroxyl groups that function as -donor ligands, increasing the coordinated metal ion's electron density.[2] This is an example of competitive complex formation, in which ligands will compete for space on an activation site of a metal ion.

Surface structures are able to reduce and oxidize ligands, whereas transport phenomena do not. Therefore, surface structure serves an important role in surface reactivity, with the coordination environment at the solid-water interface changing intensity or rate of a reaction.[1]

Applications in soil chemistry

Sorption reactions of inner sphere complexes are applicable in the transport and retention of trace elements in soil systems.[6] In particular, the sorbent materials found in nature are often metal-oxide inner sphere complexes.

In nature, this is particularly important for iron and manganese cycling, as both are effected by the redox potential of their environments for weathering to occur.[2] Oxoanions such as can hinder the dissolution and weathering of these metals. Reductive dissolution in these environments may take longer or be non-existent as a result. However, an understanding of this has led to greater usage of oxoanions in built environments where corrosion and weathering needs to be limited.[2]

Ion size of the central metal and of inorganic ligands also play a role in the weathering. Alkali earth metals have reduced sorption as their ion size increases due to decreased affinity to anionic charges, which increases their mobility through weathering as a result.[7]

For nonpolar ligands, van der Waals forces instead play a larger role in sorption interactions. Hydrogen bonding does also occur, but is not a part of the adsorption process itself.[8] Due to these factors, the soil quality influences the retention and depletion of nutrients, pollutants, and other ligands that perform sorption with the soil.[8]

Generally, the charged surface of a metallic ion can become charged via crystalline imperfections, chemical reactions at the surface, or sorption at the surface-active ion.[6] Clay minerals are an example of these interactions, and as such can explain chemical homeostasis in the ocean, biogeochemical cycling of metals, and even radioactive waste disposal.[9]

In engineering applications, the clay minerals can promote sodium ion adsorption in petroleum extraction, alongside the creation of environmental liners through the development of a stern layer.[9]

Additionally, water remediation can also be considered a by-product of inner sphere complexes found in clay and other mineral complexes.[10] This is theorized to occur due to metal-metal precipitation, such as in the case of iron-arsenic. However, pH can greatly affect the surface binding effectiveness in this case as well.

References
  1. Huntsberger JR (May 1, 1975). "Surface Chemistry and Adhesion- A Review of Some Fundamentals". Journal of Adhesion. 7 (4): 289–299. doi:10.1080/00218467608075060.
  2. Stumm W (May 5, 1995). "The Inner-Sphere Surface Complex". Aquatic Chemistry. Advances in Chemistry. 244. pp. 1–32. doi:10.1021/ba-1995-0244.ch001. ISBN 0-8412-2921-X.
  3. Shaw DJ (1992). Introduction to Colloid and Surface Chemistry. Great Britain: Butterworth Heinemann. pp. 151–159. ISBN 07506-11820.
  4. Pashley RM, Karaman ME (2004). Applied Colloid and Surface Chemistry. Great Britain: John Wiley & Sons, Ltd. pp. 8–9. ISBN 0-470-86882-1.
  5. "Index". Coordination Chemistry Reviews. 189 (1): 279. August 1999. doi:10.1016/s0010-8545(99)00205-2. ISSN 0010-8545.
  6. Smith KS (1999). "Metal Sorption on Mineral Surfaces: An Overview With Examples Relating to Mineral Deposits". Reviews in Economic Geology. 6A and 6B: 161–182. CiteSeerX 10.1.1.371.7008.
  7. "Introduction to the Sorption of Chemical Constituents in Soils | Learn Science at Scitable". www.nature.com. Retrieved 2019-11-16.
  8. Goldberg, Sabine (October 2014). "Application of surface complexation models to anion adsorption by natural materials: Surface complexation modeling of anion adsorption by soils". Environmental Toxicology and Chemistry. 33 (10): 2172–2180. doi:10.1002/etc.2566. PMID 24619924.
  9. Sposito G, Skipper NT, Sutton R, Park S, Soper AK, Greathouse JA (March 1999). "Surface geochemistry of the clay minerals". Proceedings of the National Academy of Sciences of the United States of America. 96 (7): 3358–64. Bibcode:1999PNAS...96.3358S. doi:10.1073/pnas.96.7.3358. PMC 34275. PMID 10097044.
  10. Aredes, Sonia; Klein, Bern; Pawlik, Marek (July 2012). "The removal of arsenic from water using natural iron oxide minerals". Journal of Cleaner Production. 29-30: 208–213. doi:10.1016/j.jclepro.2012.01.029.

Further reading
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