Chem. 1B – 12/1 Lecture

Announcements I Exam 3 Results –Average was 66.3% –Distribution (narrower than other exams) –Problems where students did poorly (electrolysis problem, corrosion problem, bonus problem, most of Chapter 24 questions) –On the rest of the Electrochemistry part, students did reasonably well –Key has been posted ScoreN s4 80s19 70s39 60s30 50s23 <5016

Announcements II Class Average and Grading –Post Exam 3 Ave = 71% (excluding students not taking Exam 3, but including true score – see last bullet) –If we were finishing the semester, I would probably reduce cut-offs by 1 or 2% (but I need to wait on homework, rest of lab, and final exam scores before doing so) –Blackboard score is mostly correct except: doesn’t include in-class bonus points yet (scores could go up by up to ~0.8%) some have non-entered scores (instead of zeros) – then blackboard’s score is too high lecture only students (can me to find true score)

Announcements III Mastering Questions –Chapter 24 included several problems we hadn’t covered yet. I expect to treat these and problems with errors as bonuses, but will need to see scores from Mastering so grading is fair Lab –Done with Quizzes –Experiment 12 was due –Finishing Scheduled Lab work –Lab Final on Wed./Thurs. next week (only on experiments covered in class)

Announcements IV Make-Up Quiz –In lecture on 12/8 –Five Question – Multiple Choice – on Chapter 24 (any topics possible) –Optional; will replace your lowest quiz score (if higher) Today’s Lecture –Coordination Compounds (Chapter 24) Crystal Field Theory (we are skipping applications subsection) –Organic Chemistry (Chapter 20) Carbon – carbon bonds Alkanes

Chapter 24 Transition Metals Coordination Complex – Bonding Theory – Review from last time –In octahedral binding, because the ligands bring the electrons, lower energy results when the binding axes orbitals (d z2 and d x2-y2 ) are UNFILLED –Or alternatively, the ligands cause a split in energy levels of d shell orbitals E Free atom Metal in octahedral complex On axis Off axis 

Chapter 24 Transition Metals Coordination Complex – Bonding Theory – Review from last time –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 – Review from last time –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) –Metal Ion Strength (greater charge, Fe 3+ vs. Fe 2+, increases  ) strongest CN - weakest NH 3 Cl - I-I- H2OH2O 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]4s 2 3d 6 will have 4 unpaired electrons and is magnetic Other metals, e.g. Zn (d 10 ), are not magnetic E 4s 3d

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 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(H 2 O) 6 ] 2+ (used for the terrible 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 NH 3 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 ) where ligand bonds interact with “off-axis” d orbitals (d xy, d xz, and d yz ) 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 On axis Off axis 

Chapter 24 Transition Metals Coordination Complex – Other Geometries –In square planar geometry, overlap is most with d x^2 – y^2 (but is more complex as shown below) –Square planar geometry is common for d 8 ions in which d x2 – y2 orbitals are unoccupied (low spin) Metal in square planar complex d x2 – y2 d xy on axis and off axis in xy plane d Z2 d xz d yz

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) Fe 3+ - octahedralb) Co 2+ – octahedral c) Cu 2+ - tetrahedrald) Mn 3+ - octahedral

Chapter 24 Transition Metals Questions – cont. 4.Ti 3+ is purple while Ti 4+ 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) Mn 2+ b) Fe 3+ c) Co 2+ d) Ni [Fe(en) 3 ] 3+ undergoes a ligand replacement reaction and forms [FeX 6 ] 3-. The new complex absorbs at shorter wavelengths. What do we know about the strength of X as a ligand?

Chapter 20 Organic Chemistry Introduction –Organic Chemistry is a major area of study (we offer 7 organic chemistry classes at the undergraduate level) –In ~1 week, we only have time to introduce basic principles of organic chemistry

Chapter 20 Organic Chemistry Overview –Nature of Carbon – Carbon Bonds –Hydrocarbons (structure, naming and isomers) –Reactions –Aromatic Hydrocarbons –Functional Groups

Chapter 20 Organic Chemistry Nature of Carbon – Carbon Bonds –Carbon is one of the few elements that form fairly stable bonds with itself –Most alkanes, while combustible in air (more stable as CO 2 + H 2 O), have negative  G f º –Carbon “likes to” form 4 bonds ([He]2s 2 2p 2, but mostly forms sp to sp 3 hybrid bonds) –Simplest hydrocarbon is CH 4, methane, in which sp 3 hybridization occurs (tetrahedral geometry)

Chapter 20 Organic Chemistry Nature of Carbon – Carbon Bonds –As carbon – carbon bonds are common, in alkanes, they also occur with sp 3 hybridization (tetrahedral for each C atom) –Example alkane is ethane: CH 3 CH 3

Chapter 20 Organic Chemistry Nature of Carbon – Carbon Bonds –Hydrocarbons containing double bonds are known as alkenes –Hybridization is sp 2 (see ethene structure below – drawn in 3D) C H H C H H remaining p orbital forms  bond

Chapter 20 Organic Chemistry Nature of Carbon – Carbon Bonds –Simplest alkene is ethene (also called ethylene and structure is CH 2 =CH 2 ) –alkenes are hydrocarbons with one or more double bonds