Transition Metals Bonding Theory Overview

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 year s exam did not cover last parts of Ch. 24

Announcements II Mastering Ch. 24 assignment due 11/26 Today s 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 Off axes orbitals dxz dyz z z z y y y dxy lies in xy plane x x 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 y x x

Chapter 24 Transition Metals Coordination Complex Bonding Theory cont. 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 energy levels of d shell orbitals Free atom Metal in octahedral complex E 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 gap (larger leads to low spin states first shown, while smaller leads to high spin states second shown)

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 gap Strong ligands result in large gap, while weak ligand results in smaller gap (with the idea that more tightly held electrons will overlap more with d shell electrons)

Chapter 24 Transition Metals Coordination Complex Bonding Theory Role of Ligands and Metal Ligand Strength (see text for full range) I- CN- NH3 H2O Cl- weakest strongest Weak Field Ligands tend to give high spin states Metal Ion Strength (greater charge, Fe3+ vs. Fe2+, increases )

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 unpaired electrons and is magnetic E 3d 4s Other metals, e.g. Zn (d10), are not magnetic

Chapter 24 Transition Metals Coordination Complex Magnetic Properties cont. Octahedral Complexes will have d electrons split into to energy states by ligand field Large 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]4- large small

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 ( green light ~ 525 nm or E = hc/ = 3.8 x 10-19 J) The larger the gap, the greater the E, and the smaller the 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 E = ? If we switched to NH3 as a ligand (stronger), what shift would be expected?

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 with small values and high spin states) Metal in tetrahedral complex Off axis 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 spin) Metal in square planar complex on axis and off axis in xy plane dx2 y2 dxy dZ2 dxz dyz

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+ 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? d) Ni2+