Complexes Complex – Association of a cation and an anion or neutral molecule Complex – Association of a cation and an anion or neutral molecule All associated species are dissolved All associated species are dissolved None remain electrostatically effective None remain electrostatically effective

Importance of complexes Complexing can increase solubility of minerals if ions involved in reactions are complexed Complexing can increase solubility of minerals if ions involved in reactions are complexed Total concentration (  E) = complexed plus dissolved Total concentration (  E) = complexed plus dissolved Total concentration is higher in solution than equilibrium with mineral Total concentration is higher in solution than equilibrium with mineral E.g., Solution at equilibrium with calcite will have higher  Ca 2+ if there is also SO 4 2- present because of CaSO 4 o complex E.g., Solution at equilibrium with calcite will have higher  Ca 2+ if there is also SO 4 2- present because of CaSO 4 o complex

Importance of Complexes Some elements more common as complexes Some elements more common as complexes Particularly true of metals Particularly true of metals Cu 2+, Hg 2+, Pb 2+, Fe 3+, U 4+ usually found as complexes rather than free ions Cu 2+, Hg 2+, Pb 2+, Fe 3+, U 4+ usually found as complexes rather than free ions The chemical behavior depends on complex, not ion, e.g.: The chemical behavior depends on complex, not ion, e.g.: Mobility Mobility Bioreactivity: Toxicity & bioavialability Bioreactivity: Toxicity & bioavialability

Mobility Adsorption affected by complex Adsorption affected by complex E.g., Hydroxide complexes of uranyl (UO 2 2+ ) readily adsorbed by oxide and hydroxide minerals E.g., Hydroxide complexes of uranyl (UO 2 2+ ) readily adsorbed by oxide and hydroxide minerals OH - and PO 4 - complexes readily adsorbed OH - and PO 4 - complexes readily adsorbed Carbonate, sulfate, fluoride complexes rarely adsorbed to mineral surfaces Carbonate, sulfate, fluoride complexes rarely adsorbed to mineral surfaces

Bioreactivity Toxicity and bioavailability depends on complexes Toxicity and bioavailability depends on complexes Toxicity – e.g. Cu 2+, Cd 2+, Zn 2+, Ni 2+, Hg 2+, Pb 2+ Toxicity – e.g. Cu 2+, Cd 2+, Zn 2+, Ni 2+, Hg 2+, Pb 2+ Toxicity depends on activity and complexes not total concentrations Toxicity depends on activity and complexes not total concentrations E.g., CH 3 Hg + and Cu 2+ are toxic to fish E.g., CH 3 Hg + and Cu 2+ are toxic to fish Other complexes, e.g., CuCO 3 o are not Other complexes, e.g., CuCO 3 o are not

Bioavailability Some metals are essential nutrients: Fe, Mn, Zn, Cu Some metals are essential nutrients: Fe, Mn, Zn, Cu Their uptake depends on forming complexes Their uptake depends on forming complexes

General observations Complex stability increases with increasing charge and/or decreasing radius of cation Complex stability increases with increasing charge and/or decreasing radius of cation Space issue – length of interactions Space issue – length of interactions High charge = stronger bond High charge = stronger bond Strong complexes form minerals with low solubilities Strong complexes form minerals with low solubilities Corollary – Minerals with high solubilities form weak complexes Corollary – Minerals with high solubilities form weak complexes

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

Complexes – two types No consistent nomenclature No consistent nomenclature Outer Sphere complexes (weaker bonds) Outer Sphere complexes (weaker bonds) AKA – “ion Pair” AKA – “ion Pair” Inner Sphere complexes (stronger bonds) Inner Sphere complexes (stronger bonds) AKA – “coordination compounds” AKA – “coordination compounds” AKA – “complex” (S&M) AKA – “complex” (S&M) These are ideal end-members – most complexes intermediate in structure These are ideal end-members – most complexes intermediate in structure

Outer Sphere Complexes Associated hydrated (usually) cation and anion Associated hydrated (usually) cation and anion Held by long range electrostatic forces Held by long range electrostatic forces Fairly weak complex, but ions still no longer “electrostatically effective” Fairly weak complex, but ions still no longer “electrostatically effective” Separated by water molecules oriented around cation Separated by water molecules oriented around cation Water separates ions making up complex Water separates ions making up complex

Outer Sphere complexes Association is transient Association is transient Not strong enough to displace water surrounding ion Not strong enough to displace water surrounding ion Typically smaller cations Typically smaller cations Na, K - monovalent so weaker bonds Na, K - monovalent so weaker bonds Ca, Mg, Sr - divalent so stronger bonds Ca, Mg, Sr - divalent so stronger bonds

Outer Sphere complexes Also larger ions (Cs & Rb) have low charge density Also larger ions (Cs & Rb) have low charge density Relatively unhydrated Relatively unhydrated Tend to form “contact complexes” – e.g., no water separation Tend to form “contact complexes” – e.g., no water separation Still considered ion pairs, but no intervening water Still considered ion pairs, but no intervening water

Inner Sphere Complexes More stable than ion pairs More stable than ion pairs Form with ligands Form with ligands Ligand – the anion or neutral molecule that combines with a cation to form a complex Ligand – the anion or neutral molecule that combines with a cation to form a complex Can be various species Can be various species E.g., H 2 O, OH -, NH 3, Cl -, F -, NH 2 CH 2 CH 2 NH 2 E.g., H 2 O, OH -, NH 3, Cl -, F -, NH 2 CH 2 CH 2 NH 2

Inner Sphere Complexes Metal and ligands immediately adjacent Metal and ligands immediately adjacent Metal cations generally smaller than ligands Metal cations generally smaller than ligands Largely covalent bonds between metal ion and electron-donating ligand Largely covalent bonds between metal ion and electron-donating ligand Charge of metal cations exceeds coordinating ligands Charge of metal cations exceeds coordinating ligands May be one or more 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 – H 2 O is ligand + Note – cross section, actually 3-D sphere

For ligand other than water to form inner- sphere complex For ligand other than water to form inner- sphere complex Must displace one or more coordinating waters Must displace one or more coordinating waters Bond usually covalent nature Bond usually covalent nature E.g.: E.g.: M(H 2 O) n + L = ML(H 2 O) n-1 + H 2 O

Size and charge important to number of coordinating ligands: Size and charge important to number of coordinating ligands: Commonly metal cations smaller than ligands Commonly metal cations smaller than ligands Commonly metal cation charge exceed charge on ligands Commonly metal cation charge exceed charge on ligands These differences mean cations typically surrounded by several large coordinating ligands These differences mean cations typically surrounded by several large coordinating ligands A good example is the “aquocomplex” A good example is the “aquocomplex” +

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

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

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

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

Ligands can bond with metals at one or several sites Ligands can bond with metals at one or several sites Unidentate ligand – contains only one site Unidentate ligand – contains only one site E.g., NH 3, Cl - F - H 2 O, OH - E.g., NH 3, Cl - F - H 2 O, OH - Bidentate Bidentate Two sites to bind: oxalate, ethylenediamine Two sites to bind: oxalate, ethylenediamine

Various types of ligands

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

Additional multidentate ligands

Thermodynamics of complexes Strength of the complex represented by stability constant Strength of the complex represented by stability constant K stab also called K association K stab also called K association An equilibrium constant for formation of complex An equilibrium constant for formation of complex

Typical metals can form multiple complexes in water with constant composition Typical metals can form multiple complexes in water with constant composition Al 3+, AlF 2+, AlF 2 +, AlF 3, AlF 4 - Al 3+, AlF 2+, AlF 2 +, AlF 3, AlF 4 -  Al = Al 3+ + AlF 2+ + AlF AlF 3 + AlF 4 -  Al = Al 3+ + AlF 2+ + AlF AlF 3 + AlF 4 - Example: Example: Al F - = AlF 4 - K stab = (a Al3+ )(a F- ) 4 a AlF4 -

Another example: Another example: The o indicates no charge – a complex The o indicates no charge – a complex Ca 2+ + SO 4 2- = CaSO 4 o

Since CaSO 4 º not solid anhydrite –a single molecule Since CaSO 4 º not solid anhydrite –a single molecule Dissolved – must include the CaSO 4 º in thermodynamic calculations Dissolved – must include the CaSO 4 º in thermodynamic calculations a CaSO4º ≠ m CaSO4º a CaSO4º ≠ m CaSO4º K stab = (a Ca2+ )(a SO42- ) a CaSO4 o

Examples of K stab calculations and effects of complexing on concentrations Examples of K stab calculations and effects of complexing on concentrations