Chapter_19_Notes

Chapter_19_Notes - Electronic structure and spectra Crystal...

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1 Electronic structure and spectra Shriver, Chapter 19 Crystal field theory An ionic model, considers metal and its ligands as point charges All 5 d-orbitals are isoenergetic in a spherical environment/crystal field (i.e., free atom) Different arrangements of charges/ligands cause different splittings of the d-orbitals into different energy levels, although the overall energy center (barycenter) does not change Octahedral symmetry d z2 and d x2-y2 orbitals (which point along the M-L bonds) are destabilized relative to d xy , d xz , d yz (which point in between the M-L bonds) the energy difference between them is Δ d z2 and d x2-y2 orbitals are doubly degenerate in energy (symbol e g ) d xy , d xz , d yz are triply degenerate in energy (symbol t 2g ) ! d spherical symmetry octahedral symmetry 0.6 ! 0.4 ! e g (z2, x2-y2) t 2g (xy, xz, yz) electronic transition (UV-visible spectrum) for a d 1 ion is t 2g e g and its energy is Δ (also called 10 Dq in some books) the magnitude of Δ depends on the nature of M larger for higher oxidation states of the same metal ion and across a row, due to the decreasing cation radius, resulting in more interaction with the ligands increases down a triad as the extension of the d orbitals increases the magnitude of Δ depends on the nature of L (empirically: the spectrochemical series) halides < OH - < H 2 O < NH 3 < PPh 3 < CN - < CO weak xtal field strong xtal field Crystal field stabilization energy (CFSE)/ligand field stabilization energy (LFSE) CFSE = (0.4 x – 0.6 y ) Δ
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2 where x is the number of t 2g electrons and y is the number of e g electrons should we fill all the t 2g orbitals first? yes, when the CFSE is greater than the electron pairing energy P when does this occur? when Δ is large i.e., in the second and third row i.e., when L is a strong-field ligand we put three unpaired e- into the t 2g orbitals, but the fourth, fifth and sixth e- will be spin- paired these will be low-spin complexes however, if P > CFSE (i.e., Δ is small), then the fourth and fifth e- will go unpaired into the e g orbitals these will be high-spin complexes similar considerations apply for seven e- e.g., low-spin d 4 : t 2g 4 e g 0 , 2 unpaired e- high-spin d 4 : t 2g 3 e g 1 , 4 unpaired e- e.g., low-spin d 7 : t 2g 6 e g 1 , 1 unpaired e- high-spin d 7 : t 2g 5 e g 2 , 3 unpaired e- note: there is no classification of high- and low-spin for octahedral complexes with d 0 , d 1 , d 2 , d 3 , d 8 , d 9 , d 10 electron configurations CFSE increases in high-spin complexes from d 0 to d 3 , decreases to d 5 , then increases to d 8 - complexes with d 3 and d 8 electron configurations have a strong preference to be octahedral - since there is no CFSE in d 0 , (weak field) d 5 and d 10 complexes, these show no strong structural preference Hydration enthalpies (water is a weak-field ligand) vary in the same way as the CFSE, superimposed on the general increasing hydration enthalpy from right to left across the
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This note was uploaded on 08/06/2008 for the course CHEM 173B taught by Professor Scott during the Winter '08 term at UCSB.

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Chapter_19_Notes - Electronic structure and spectra Crystal...

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