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Inorganic Chemistry BasicsLewis acid = electron pair acceptor (e.g. H+, Al3+, BF3, CO2, SiF4)

Lewis base = electron pair donor (e.g. NH3, PF3, Hal-, HS-, H2O, OH-)

Lewis acids and bases form Lewis acid/base adducts (e.g. BF3←NH3)

All metal ions (Mn+) are Lewis acids

Ligands are Lewis bases

Pearson’s concept of hard and soft acids and bases (HSAB):Hard: less easily polarizable (usually ions of high charge and/or small radius)

Soft: more easily polarizable (usually ions of low charge and/or large radius)

Hard acids prefer hard bases, soft acids prefer soft basesi.e., more stable acid-base complexes are formed with hard/hard or soft/soft combinations

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The HSAB concept is useful for predicting stability and reactivity:

CuF + HI CuI + HFs h h s s s h h

Periodic trends for acids (simplified):

Increasing

softness

Increasing

hardness

e.g. Mg2+, Al3+, Si4+

e.g. Mg2+, Ca2+, Sr2+, Ba2+

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H+, Li+, Na+, K+

Be2+, Mg2+, Ca2+, Sr2+, Mn2+

Sc3+, La3+, Co3+, Cr3+, Fe3+, Al3+, Ga3+, As3+

Ti4+, Zr4+, Th4+, U4+, Pu4+

BF3, BCl3, AlCl3, SO3

Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Sn2+, Pb2+

Rh3+, Ir3+, Bi3+

SO2

Pd2+, Pt2+, Pt4+

Cu+, Ag+, Au+, Cd2+, Hg2+, CH3Hg+, Tl+

Br2, Br+, I2, I+

Acids

Hard

Borderline

Soft

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Bases

Hard

Borderline

Soft

NH3, RNH2, N2H4

H2O, OH-, O2-, ROH, RO-, R2O

CH3COO-, CO32-, NO3

-, PO43-, SO4

2-, ClO4-

F-, (Cl-)

C6H5NH2, N3-, NO2

-, SO32-, Br-

H-, R-, C2H4, C6H6, CO

SCN-, R3P, R2S, RSH, RS-, S2O32-, I-

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Absolute hardness, η

(I – A)2

I = Ionization energy

A = Electron affinity

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Plot of charge/radius ratio against the ionization energy (M to M2+) for some divalent metal ions

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Biologically relevant TMs

Biologically relevant AE elements

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Biologically relevant ligands (bases)

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pKa Values of Coordinating Ligands

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Thermodynamics of Metal Ion Complexation

Formation (stability) constants

Overall stability constants

(βn = K1 x K2 x … Kn)

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Chelate effectChelation refers to coordination of two or more donor atoms from a single ligand to a central metal ion

The resulting complex is characterized by an unusual thermodynamic stability

The gain in stability upon chelation has sometimes be ascribed to a significant gain in entropy (however: this is not always the reason as enthalpy can be the determining factor)

Example: [Co(H2O)6]3+ + EDTA [Co(edta)]- + 6 H2O

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Examples of biologically relevant chelating ligands/chelates

The most advanced chelates are proteins!

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Monensin

Cyclic polyether

Antibiotic with specificity for Na+

Structure of valinomycin (K+-specific ionophore)

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Kinetic ConsiderationsTerminology:

Thermodynamic terms: STABLE and UNSTABLE

Kinetic terms: INERT and LABILE

Definition of labile (Taube):

Reaction half-life (i.e. the time of disappearance of half of the initial compound) of 1 minute or less

All of the following cyano-complexes are extremely stable! (Dissociation constant for the hydrolysis of [Ni(CN)4]2- to [Ni(H2O)6]2+ is 10-22 M-2)

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Water Exchange Rates

Diffusion controlled rates of water exchange for alkali metal ions

Calcium has the highest rate constant for abundant non-alkali metal ions (Role as second messenger)

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