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1CHM3400 - Lecture 31 – Mar 30
Coordination compounds (Chapter 12 469-483)
• Coordination complexes• Hard and soft ligands• Crystal field splitting• Octahedral, tetrahedral, square planar• Protein examples
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Coordination complexesCoordination complex = metal ion with ligands forming a complex
Metal (=acid) in center surrounded by ligands
Coordination number = number of ligands
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Crystal field theorySplitting of degenerate d orbitals on metal due to repulsion of valence electrons on metal and lone pair on ligands
Energy of orbital is raised due to head-on ligand interaction
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Octahedral complexesCrystal field splitting Δ
Lowering of d(xy), d(yz), d(xz)
Raising of d(x2-y2) and d(z2)
Crystal field stabilization energy (CFSE) = n(eg)(0.6Δ) - n(t2g)(0.4Δ)
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Crystal field splitting magnitude
I- < Br- < SCN- < Cl- < F- < OH- < H2O < NH3 < en < NO2- < CN- < CO
Energy of photon corresponds to energy level splitting
Predict energy from λ
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High spin – low spinDepending on the crystal field splitting, high or low-spin complex is observed
Strong field ligand
Weak field ligand
What rule predicts high spin?
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Other symmetries
Can you rationalize the different energy splittings?
Splitting for tetrahedral complexes smaller⇒………. spin complexes
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Molecular orbital theoryCrystal field theory is easy to understand, but it completely neglects the covalent character of metal-ligand bonds.
Non-bonding orbitals
Anti-bonding orbitals
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High- and low-spin Fe2+
Fe2+: d6
O2 bindingNo O2 binding
Remember cooperative binding for hemoglobin
Low-spin ⇒ more compact complex⇒ fits into heme group
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CO binding to hemoglobinWhy is CO binding to hemoglobin much stronger than O2 binding (200x stronger)?