1. Complexes
Complex – Association of a cation and an anion or neutral molecule
All associated species are dissolved
None remain electrostatically effective
Ligand – the anion or neutral molecule that combines with a cation to form a complex
Can be various species
E.g., H2O, OH-, NH3, Cl-, F-, NH2CH2CH2NH2

2. Importance of complexes
Complexing can increase solubility of minerals if ions involved in reactions are complexed
Total concentration of species (e.g., complexed plus dissolved) will be higher in solution at equilibrium with mineral
E.g., Solution at equilibrium with calcite will have higher SCa2+ if there is also SO42- present because of CaSO4o complex

3. Some elements more common as complexes
Particularly true of metals
Cu2+, Hg2+, Pb2+, Fe3+, U4+ usually found as complexes rather than free ions
Their chemical behavior (i.e. mobility, toxicity, etc) are properties of complex, not the ion

4. Adsorption affected by complex
E.g., Hydroxide complexes of uranyl (UO22+) readily adsorbed by oxide and hydroxide minerals
OH- and PO4- complexes readily adsorbed
Carbonate, sulfate, fluoride complexes rarely adsorbed to mineral surfaces

5. Toxicity and bioavailability depends on complexes
Toxicity – e.g. Cu2+, Cd2+, Zn2+, Ni2+, Hg2+, Pb2+
Toxicity depends on activity and complexes not total concentrations
E.g., CH3Hg+ and Cu2+ are toxic to fish
other complexes, e.g., CuCO3o are not

6. Bioavailability – some metals are essential nutrients: Fe, Mn, Zn, Cu
Their uptake depends on forming complexes

7. General observations
Complex stability increases with increasing charge and/or decreasing radius of cation
Space issue – length of interactions
Strong complexes form minerals with low solubilities
Corollary – Minerals with low solubilities form strong complexes

8. High salinity increases complexing
More ligands in water to complex
High salinity water increases solubility because of complexing

9. Complexes – two types Outer Sphere complexes Inner Sphere complexes
AKA – “ion Pair”
Inner Sphere complexes
AKA – “coordination compounds”

10. Outer Sphere Complexes
Associated hydrated cation and anion
Held by long range electrostatic forces
No longer electrostatically effective
Metal ion and ligand still separated by water
Association is transient
Not strong enough to displace water surrounding ion
Typically smaller ions – Na, K, Ca, Mg, Sr
Larger ions have low charge density
Relatively unhydrated
Tend to form “contact complexes”

11. Outer Sphere complexes
Metal ion and ligand still separated by water
Association is transient
Not strong enough to displace water surrounding ion
Typically smaller ions – Na, K, Ca, Mg, Sr
Larger ions have low charge density
Relatively unhydrated
Tend to form “contact complexes”

12. Larger ions have low charge density
Relatively unhydrated
Tend to form “contact” ion pairs – with little water in between

13. Inner Sphere Complexes
More stable than ion pairs
Metal and ligands immediately adjacent
Metal cations generally smaller than ligands
Largely covalent bonds between metal ion and electron-donating ligand
Charge of metal cations exceeds coordinating ligands
May be one or more coordinating ligands

14. An Aquocomplex – H2O is ligand
Outer sphere – partly oriented water
Coordinating cation
Inner sphere – completely oriented water, typically 4 or 6 fold coordination

15. For ligand, L to form inner-sphere complex
Must displace one or more coordinating waters
Bond usually covalent nature
E.g.:
M(H2O)n + L = ML(H2O)n-1 + H2O

16. Size and charge important to number of coordinating ligands:
Commonly metal cations smaller than ligands
Commonly metal cation charge exceed charge on ligands
These differences mean cations typically surrounded by several large coordinating ligands
E.g., aquocomplex

17. Radius Coordinating Cation RR = Radius Ligand
Maximum number of ligands depends on coordination number (CN)
Most common CN are 4 and 6, although 2, 3, 5, 6, 8 and 12 are possible
CN depends on radius ratio (RR):
Radius Coordinating Cation
RR =
Radius Ligand

18. Maximum number of coordinating ligands
Depends on radius ratio
Generates coordination polyhedron

19. All coordination sites rarely filled
Only in aquo-cation complexes (hydration complexes)
Highest number of coordination sites is typically 3 to 4
The open complexation sites results from dilute concentration of ligands

20. Concentrations of solution
Water concentrations – 55.6 moles/kg
Ligand concentrations to mol/kg
5 to 6 orders of magnitude lower

21. Ligands can bond with metals at one or several sites
Unidentate ligand – contains only one site
E.g., NH3, Cl- F- H2O, OH-
Bidentate
Two sites to bind: oxalate, ethylenediamine

22. Various types of ligands

23. Multidentate – several sites for complexing
Hexedentate – ethylenediaminetetraacetic acid (EDTA)

24. Additional multidentate ligands

25. Thermodynamics of complexes
Strength of the complex represented by stability constant
Kstab also called Kassociation
An equilibrium constant for formation of complex

26. Al3+ + 4F- = AlF4- aAlF4- Kstab = (aAl3+)(aF-)4
Typical metals can form multiple complexes in water with constant composition
Al3+, AlF2+, AlF2+, AlF3
SAl = Al3+ + AlF2+ + AlF2+ + AlF3
Example:
Al3+ + 4F- = AlF4-
aAlF4-
Kstab =
(aAl3+)(aF-)4

27. Complexation changes “effective concentrations” of solution
Another example:
Ca2+ + SO42- = CaSO4o

28. Here the o indicates no charge – a complex
This is not solid anhydrite – only a single molecule
Still dissolved

29. Kstab = (aCa2+)(aSO42-) aCaSO4o is included in the Kstab calculations
It is a dissolved form

30. Examples of Kstab calculations and effects of complexing on concentrations