Chem. 1B 11/17 Lecture Announcements I Lab Last Quiz Monday/Tues on Exp 10, 14 and Chapter 24 No Lab next Wednesday Experiment 10 report due Exam 3 Two weeks (and three lectures) from today On electrochemistry and Chapter 24 Last years exam did not cover last parts
Announcements II Mastering Ch. 24 assignment due 11/26 Todays Lecture Transition Elements (Ch. 24) Bonding in Coordination Complexes - Theory Chapter 24 Transition Metals Optical Isomer Demonstration Show with models of MX2YZ and MWXYZ Chapter 24 Transition Metals Coordination Complex Bonding Theory cont.
To understand how electrons in the d shells influence bonding, we must understand the shapes of d orbitals Two different classes of d orbitals occurs d dxy lies in xy plane Off axes orbitals z z
y x xz z y x dyz y x
Chapter 24 Transition Metals Coordination Complex Bonding Theory cont. Two different classes of d orbitals occurs On axes orbitals dz^2 dx^2 y^2 z z
y x y x Chapter 24 Transition Metals Coordination Complex Bonding Theory cont. E In octahedral binding, because the ligands
bring the electrons, lower energy results when the binding axes orbitals (dz2 and dx2-y2) are UNFILLED Or alternatively, the ligands cause a split in Metal in octahedral complex Free atom energy levels of d shell orbitals D On axis Off axis Chapter 24 Transition Metals Coordination Complex Bonding Theory cont.
How does d orbital splitting affect coordination complexes? Electrons go to low energy states first Example: [Cr(CN)6]3- has 4 1 = 3 d shell electrons they should occupy the three off-axes orbitals On axis Off axis Chapter 24 Transition Metals Coordination Complex Bonding Theory cont. When we add more than 3 electrons (e.g. 4 electrons), there are two possibilities:
fill bottom orbitals first or go to top orbitals Filling depends on D gap (larger leads to low spin states first shown, while smaller leads to high spin states Chapter 24 Transition Metals Coordination Complex Bonding Theory Role of Ligands Particular metals, such as Fe, can form complexes with different properties (e.g. colors or magnetic properties) depending on ligands
Ligands affect size of D gap Strong ligands result in large D gap, while weak ligand results in smaller D gap (with the idea that more tightly held electrons will overlap more with d Chapter 24 Transition Metals Coordination Complex Bonding Theory Role of Ligands and Metal Ligand Strength (see text for full range) CNstrongest NH3 H2O
Cl- Iweakest Weak Field Ligands tend to give high spin states Chapter 24 Transition Metals Coordination Complex Magnetic and Light Absorbing Properties Magnetic Properties:
Compounds or atoms with unpaired electrons are magnetic (since half filled shells will have electrons with the same spin) Example: Fe [Kr]4s23d6 will have 4 E unpaired electrons and is magnetic 3d 4s Other metals, e.g. Zn (d10), are not Chapter 24 Transition Metals Coordination Complex Magnetic Properties cont. Octahedral Complexes will have d
electrons split into to energy states by ligand field Large D gap complexes give rise to low spin states that are less magnetic vs. high spin states Examples: [Fe(CN)6]4- vs. [Fe(Br)6]4large D small D Chapter 24 Transition Metals Coordination Complex Light Absorbing Properties Gap between on- and off-axes d orbitals can also lead to transitions between two states Example: [Cr(CN)6]3
Absorption of light causes electronic transition from low energy to high energy state: Chapter 24 Transition Metals Coordination Complex Light Absorbing Properties cont. Many coordination complexes absorb visible light (lgreen light ~ 525 nm or E = hc/l = 3.8 x 10-19 J) The larger the D gap, the greater the E, and the smaller the l value energy Visible colors go ROYGBIV (red, orange, yellow, green, blue, indigo, violet from longer to shorter
wavelength) Chapter 24 Transition Metals Coordination Complex Light Absorbing Properties cont. Example: [Co(H2O)6]2+ (used for the Drierite color demonstration) Color is pink/purple (but pink is red + white
= seen color because complex absorbs other colors) Using color wheel (text) expected absorbance is in green (measured in Chem 31 as 510 nm) Color wheel used because we see reflected light Chapter 24 Transition Metals Coordination Complex Other Geometries Besides octahedral geometries, tetrahedral and square planar geometries have different overlaps with d orbitals resulting in different d orbital splitting In tetrahedral complexes, the complex can be
positioned (see Fig. 24.17) where ligand bonds interact with off-axis d orbitals (dxy, dxz, and dyz) making these orbitals higher in energy and on-axis d orbitals lower in energy (however Metal in tetrahedral complex Off axis D and high spin states) with small D values On axis Chapter 24 Transition Metals Coordination Complex Other Geometries
In square planar geometry, overlap is most with dx^2 y^2 (but is more complex as shown below) Square planar geometry is common for d8 ions in which dx2 y2 orbitals are unoccupied (low Metal in square planar spin) complex dx2 y2 dxy dZ2 dxz
dyz on axis and off axis in xy plane Chapter 24 Transition Metals Questions 1. Which two d orbitals do octahedral complexes overlap with the most? 2. Which d orbital is there the greatest overlap in square planar complexes? 3. Give the number of unpaired electrons for the following metals in octahedral complexes for low spin states/high spin states a) Fe3+ - octahedral b) Co2+ octahedral
c) Cu2+ - tetrahedral d) Mn3+ - octahedral Chapter 24 Transition Metals Questions cont. 4. Ti3+ is purple while Ti4+ is uncolored. Explain. 5. For which of the following metals in octahedral complexes does the ligand NOT play a role in the number of unpaired electrons? a) Mn2+ b) Fe3+ c) Co2+ d) Ni2+ 6. [Fe(en)3]3+ undergoes a ligand replacement reaction and forms [FeX6]3-. The new complex absorbs at shorter wavelengths. What do we know about the strength of X as a ligand?
