Chapter_23_Lecture 4

Chapter_23_Lecture 4 - Crystal Field Theory Valence Bond...

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Crystal Field Theory Valence Bond Theory pictures and rationalizes bonding and shape of molecules B theory gives little insight into the olors f coordination VB theory gives little insight into the colors of coordination compounds and can be ambiguous with regard to magnetic properites Crystal Field Theory explains color and magnetism Highlights the “effects” on the d-orbital energies of the metal n as the ligands approach ion as the ligands approach 23-66
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Crystal Field Theory What is Color? White light is electromagnetic radiation consisting of “all” avelengths the “visible” range wavelengths ( ) in the visible range Opaque objects reflect light lear bjects ransmit li ht Cea objectsta s t gt If the object absorbs al l visible wavelengths, it appears “black” If the object reflects all visible wavelengths, it appears “white” Objects appear “colored” in white light because they absorb certain wavelengths and reflect or transmit others 23-67
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What is Color? Each color has a “complimentary” color An object has a particular color for two reasons It reflects (or transmits) light of that color or It absorbs light of the “complimentary” color Ex. If an object absorbs only red (compliment of green), it is interpreted as “green” Colors with approximate wavelength ranges Complimentary colors, such as red and green, lie opposite each other 23-68
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Crystal Field Theory Assumption : A complex ion forms as a result of electrostatic attractions between the positively charged metal cation and the negative charge of the ligands The negative charge of the ligand is either partial as in a polar neutral ligand like NH , or full, as in an anionic ligand like Cl - g 3 , , g In CF Theory, the properties of complexes result from the splitting of d-orbital energies Split d-orbital energies arise from “electrostatic” interactions between the metal ion and ligands 23-69
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Crystal Field Theory The ligands approach the metal ion along the mutually perpendicular x , y, and z axes (octahedral orientation), minimizing the overall energy of the system B & C Lobes of the d 2 2 and d 2 orbitals lie directly in line with the x2-y2 z2 y approaching ligands and have stronger repulsions D, E, F lobes of the d xy , d xz , and d yz orbitals lie “between ” the approaching ands so the repulsion are eaker ligands, so the repulsion are weaker For a quick review on d orbital shape, see Silberberg page 294 23-70
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Crystal Field Theory An energy diagram of the orbitals shows all five d orbitals are higher in energy in the forming complex than in the free metal ion, because of the repulsions from the approaching ligands Crystal Field Splitting Energy Forming Complex Crystal Field Splitting Energy - The d orbital energies are “ plit” with the two d nd d rbitals rbital set) higher in energy split with the two d x2-y2 and d z2 orbitals (
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Chapter_23_Lecture 4 - Crystal Field Theory Valence Bond...

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