complexes complex – association of a cation and an anion or neutral molecule complex –...
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Complexes Complex – Association of a cation
and an anion or neutral molecule All associated species are dissolved None remain electrostatically effective
Importance of complexes
Complexing can increase solubility of minerals if ions involved in reactions are complexed Total concentration (SE) = complexed
plus dissolved Total concentration is higher in solution
than equilibrium with mineral E.g., Solution at equilibrium with calcite
will have higher SCa2+ if there is also SO4
2- present because of CaSO4o
complex
Importance of Complexes
Some elements more common as complexes Particularly true of metals Cu2+, Hg2+, Pb2+, Fe3+, U4+ usually found
as complexes rather than free ions The chemical behavior depends on
complex, not ion, e.g.: Mobility Bioreactivity: Toxicity & bioavialability
Mobility
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
Bioreactivity
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., CuCO3
o are not
Bioavailability
Some metals are essential nutrients: Fe, Mn, Zn, Cu Their uptake depends on forming
complexes
General observations
Complex stability increases with increasing charge and/or decreasing radius of cation Space issue – length of interactions High charge = stronger bond
Strong complexes form minerals with low solubilities Corollary – Minerals with high solubilities
form weak complexes
High salinity increases complexing More ligands in water to complex
High salinity water increases solubility because of complexing
Complexes – two types
No consistent nomenclature Outer Sphere complexes (weaker
bonds) AKA – “ion Pair”
Inner Sphere complexes (stronger bonds) AKA – “coordination compounds” AKA – “complex” (S&M)
These are ideal end-members – most complexes intermediate in structure
Outer Sphere Complexes Associated hydrated (usually) cation
and anion Held by long range electrostatic forces Fairly weak complex, but ions still no
longer “electrostatically effective” Separated by water molecules
oriented around cation Water separates ions making up
complex
Outer Sphere complexes Association is transient Not strong enough to displace water
surrounding ion Typically smaller cations
Na, K - monovalent so weaker bonds Ca, Mg, Sr - divalent so stronger bonds
Outer Sphere complexes Also larger ions (Cs & Rb) have low
charge density Relatively unhydrated Tend to form “contact complexes” – e.g.,
no water separation Still considered ion pairs, but no
intervening water
Inner Sphere Complexes More stable than ion pairs Form with ligands 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
Inner Sphere Complexes 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
Inner sphere – completely oriented water, typically 4 or 6 fold coordination
Outer sphere – partly oriented water
Coordinating cation
An Aquocomplex – H2O is ligand
+
Note – cross section, actually 3-D sphere
For ligand other than water 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
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 A good example is the “aquocomplex” +
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):
RR = Radius Coordinating Cation
Radius Ligand
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
Concentrations of solution Water concentrations – 55.6 moles/kg Ligand concentrations 0.001 to 0.0001
mol/kg 5 to 6 orders of magnitude lower
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
Thermodynamics of complexes
Strength of the complex represented by stability constant Kstab also called Kassociation
An equilibrium constant for formation of complex
Typical metals can form multiple complexes in water with constant composition Al3+, AlF2+, AlF2
+, AlF3, AlF4-
SAl = Al3+ + AlF2+ + AlF2+ + AlF3 + AlF4
-
Example:Al3+ + 4F- = AlF4
-
Kstab =(aAl3+)(aF-)4
aAlF4-
Since CaSO4º not solid anhydrite –a single molecule Dissolved – must include the CaSO4º in
thermodynamic calculations aCaSO4º ≠ mCaSO4º
Kstab =(aCa2+)(aSO42-)
aCaSO4o