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Chap 24 Part 2

Chap 24 Part 2. Color and Magnetism. Color. Color of a complex depends on; (i) the metal, (ii) its oxidation state & (iii) ligands (i.e., everything). For example, pale blue [Cu(H 2 O) 6 ] 2+ versus dark blue [Cu(NH 3 ) 6 ] 2+ .

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Chap 24 Part 2

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  1. Chap 24 Part 2

  2. Color and Magnetism Color Color of a complex depends on; (i) the metal, (ii) its oxidation state & (iii) ligands (i.e., everything) For example, pale blue [Cu(H2O)6]2+ versus dark blue [Cu(NH3)6]2+. Partially filled d orbitals usually give rise to colored complexes because they can absorb light from the visible region of the spectrum. • The color of the complex is the sum of the light not absorbed (reflected) by the complex.

  3. Color and Magnetism Color

  4. Color and Magnetism Color • A plot of absorption intensity of light versus wavelength is called an absorption spectrum for the complex or compound. Since the spectrum for [Ti(H2O)6]3+ has a maximum absorption at 510 nm (green & yellow), & transmits all other wavelengths, the complex is purple.

  5. Color and Magnetism Magnetism Transition metal complexes that are paramagnetic have unpaired e-’s & those that are diamagnetic have no unpaired e-’s. • Consider the d6 Co metal ion: [Co(NH3)6]3+ has no unpaired electrons, but [CoF6]3- has four unpaired electrons per ion. (note, s e-’s are lost first before d e-’s in a metal cation) • We need to develop a bonding theory to account for both color and magnetism in transition metal complexes.

  6. Crystal-Field Theory Complex has lower E Crystal field theory (CFT) describes bonding & can account for many of the color and magnetic properties in transition metal complexes. • Lewis A/B model assumes bonding results from ligand e-’s donated into hybridized d metal orbital. • CFT assumes that the interaction between ligand & metal is electrostatic (pos. nuclei & neg. e-’s).

  7. Crystal-Field Theory An octehedral array of negative ligands shown as small (blue) dots approaching the five different d orbitals of a metal ion.

  8. Crystal-Field Theory Although there is an overall reduction in E, the negative ligands repel d e-’s giving rise to a slight increase in E. The E gap is called D or the CF splitting E. Two of the five d orbitals are higher in E.

  9. Crystal-Field Theory [Ti(H2O)6]3+

  10. Crystal-Field Theory • A Spectrochemical series is a listing of ligands in order of their ability to increase : Cl- < F- < H2O < NH3 < en < NO2- (N-bonded) < CN- • Weak field ligands (Cl- & F-) lie on the low end of the spectrochemical series. • Strong field ligands (CN-) lie on the high end of the spectrochemical series.

  11. Crystal-Field Theory 2 As Cr3+ goes from complexes with weak field ligands to strong field ligands,  increases and the color of the complex changes from green to yellow.

  12. Crystal-Field Theory Electron Configurations in Octahedral Complexes Recall that the s e-’s are lost first for the metal ion. So, Ti3+ is d1, V3+ is ad?? and Cr3+ is a d?? ion. • We apply Hund’s rule to the 2 sets of 5 d-orbitals. • The first three e-’s go into different d orbitals with their spins parallel. • We have a choice for the placement of the fourth electron: • if it goes into a higher energy orbital, then there is an energy cost associated with promotion (); • if it goes into a lower energy orbital, then there is an energy cost associated with e- spin pairing.

  13. Crystal-Field Theory Weak-field ligands (which have a small D) tend to favor adding electrons to the higher-energy orbitals (high-spin complexes) because D is less than the spin-pairing energy. Strong-field ligands (which have a large D) tend to favor adding electrons to lower-energy orbitals (low-spin complexes) because D is greater than the spin-pairing energy.

  14. Crystal-Field Theory Tetrahedral & Square-Planar In a tetrahedral field the dxy, dyz, & dxz orbitals are of higher E than the dx2-y2 and the dz2 orbitals. • Because there are only 4 ligands,  for a tetrahedral field is smaller than  for an octahedral field. • This causes all tetrahedral complexes to be high spin (unless told otherwise).

  15. Crystal-Field Theory Tetrahedral & Square-Planar Square planar complexes can be thought of as octahedral complexes with the two ligands along the z-axis removed. • As a consequence the four planar ligands are drawn in closer towards the metal. • Relative to the octahedral field, the dz2 orbital is greatly lowered in energy, the dyz, and dxz orbitals lowered in energy, the dxy, and dx2-y2 orbitals are raised in energy.

  16. Crystal-Field Theory Most d8 metal ions form square planar complexes. The majority of complexes are low spin (i.e. diamagnetic). Examples: Pd2+, Pt2+, Ir+, and Au3+.

  17. End of Chapter 24 • Chemistry of Coordination Compounds • Homework: • 7, 13, 14, 17, 19-21, 23-26, 28, 31, 38, 30, 42-44, 47

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