Chap 24 Part 2

Color and Magnetism  The color of the complex is the sum of the light not absorbed (reflected) by the complex.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+. Partially filled d orbitals usually give rise to colored complexes because they can absorb light from the visible region of the spectrum.

Color and Magnetism Color

 A plot of absorption intensity of light versus wavelength is called an absorption spectrum for the complex or compound. Color and Magnetism Color Since the spectrum for [Ti(H 2 O) 6 ] 3+ has a maximum absorption at 510 nm (green & yellow), & transmits all other wavelengths, the complex is purple.

 Consider the d 6 Co metal ion: [Co(NH 3 ) 6 ] 3+ has no unpaired electrons, but [CoF 6 ] 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. Color and Magnetism Magnetism Transition metal complexes that are paramagnetic have unpaired e - ’s & those that are diamagnetic have no unpaired e - ’s.

Crystal-Field Theory  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). Crystal field theory (CFT) describes bonding & can account for many of the color and magnetic properties in transition metal complexes. Complex has lower E

An octehedral array of negative ligands shown as small (blue) dots approaching the five different d orbitals of a metal ion. 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. Two of the five d orbitals are higher in E. The E gap is called  or the CF splitting E.

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

 A Spectrochemical series is a listing of ligands in order of their ability to increase  : Cl - < F - < H 2 O < NH 3 < en < NO 2 - (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. Crystal-Field Theory

As Cr 3+ goes from complexes with weak field ligands to strong field ligands,  increases and the color of the complex changes from green to yellow. 2

 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. Crystal-Field Theory Electron Configurations in Octahedral Complexes Recall that the s e - ’s are lost first for the metal ion. So, Ti 3+ is d 1, V 3+ is a d ?? and Cr 3+ is a d ?? ion.

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

 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). Crystal-Field Theory Tetrahedral & Square-Planar In a tetrahedral field the d xy, d yz, & d xz orbitals are of higher E than the dx 2 -y 2 and the dz 2 orbitals.

 As a consequence the four planar ligands are drawn in closer towards the metal.  Relative to the octahedral field, the dz 2 orbital is greatly lowered in energy, the d yz, and d xz orbitals lowered in energy, the d xy, and dx 2 -y 2 orbitals are raised in energy. 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.

Crystal-Field Theory Most d 8 metal ions form square planar complexes. The majority of complexes are low spin (i.e. diamagnetic). Examples: Pd 2+, Pt 2+, Ir +, and Au 3+.

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