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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.

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Presentation on theme: "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."— Presentation transcript:

1 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 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

2 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 of species (e.g., complexed plus dissolved) will be higher in solution at equilibrium with mineral 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 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

3 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 Their chemical behavior (i.e. mobility, toxicity, etc) are properties of complex, not the ion Their chemical behavior (i.e. mobility, toxicity, etc) are properties of complex, not the ion

4 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

5 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

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

7 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 Strong complexes form minerals with low solubilities Strong complexes form minerals with low solubilities Corollary – Minerals with low solubilities form strong complexes Corollary – Minerals with low solubilities form strong complexes

8 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

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

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

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

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

13 Inner Sphere Complexes More stable than ion pairs More stable than ion pairs 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

14 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

15 For ligand, L to form inner-sphere complex For ligand, L 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

16 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 E.g., aquocomplex E.g., aquocomplex

17 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

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

19 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

20 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

21 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

22 Various types of ligands

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

24 Additional multidentate ligands

25 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

26 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 Al 3+, AlF 2+, AlF 2 +, AlF 3 Al = Al 3+ + AlF 2+ + AlF AlF 3 Al = Al 3+ + AlF 2+ + AlF AlF 3 Example: Example: Al F - = AlF 4 - K stab = (a Al3+ )(a F- ) 4 a AlF4-

27 Complexation changes effective concentrations of solution Complexation changes effective concentrations of solution Another example: Another example: Ca 2+ + SO 4 2- = CaSO 4 o

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

29 a CaSO4 o is included in the K stab calculations a CaSO4 o is included in the K stab calculations It is a dissolved form It is a dissolved form K stab = (a Ca2+ )(a SO42- ) a CaSO4 o

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


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