Absorbs λ that is than above; so! is ; but the

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1 5.111 Lecture Summary #9 Friay, November 1, 014 Reaing For oay: in 4 th an 5 th eitions Reaing for Lecture #30: in 5 th e an in 4 th e. opic: I. Spectroscopic heory: Both Ligan ype an Geometry Make a Difference II. Crystal Fiel heory: etraheral Case III. Crystal Fiel heory: Square Planar Case IV. ther Geometries an Applications I. Spectroscopic heory: Both Ligan ype an Geometry Make a Difference Nickel Demonstration Recall the color of light transmitte is complementary to the color of light that is absorbe. Re is complementary to green; orange is complementary to blue; yellow is complementary to violet. [NiCl 6 ] 4 (greenish) Absorbs λ that is ; so! o is ;Cl is a H fiel ligan [Ni(H ) 6 ] + (bluegreen) Absorbs λ that is than above; so! o is than above; is a fiel ligan than above H EDA NiEDA (blue) Absorbs λ that is than above; so EDA is a! o is fiel ligan than above than above; Also [Ni(H ) 6 ] + (bluegreen) imethylglyoxine (mgh) Ni(mgH) (re) Absorbs λ that is than above; so! is ; but the complex is square planar an not octaheral. Geometry matters. Square planar geometry must allow for a large splitting of the energy of the orbitals. (We will look at this in a few minutes.) violet blue green yellow orange re λ 400nm 430nm 490nm 560nm 580nm 60nm 800nm 1

2 II. Crystal Fiel heory: etraheral Case z axis M y axis L xaxis out of the page y x z tetraheral z axis M y axis xaxis out of the page yz xy xz tetraheral here is repulsion between the ligan negative point charges an the orbitals that are 45 off axis ( yz, z, y ) than there is between the ligan negative point charges an the orbitals that are on axis (z an x y ). As a result of the above, there is greater orbital estabilization for yz, z, y than for an x y z ( of octaheral). z an x y have the energy with respect to each other (egenerate). yz, z, y have the same energy with respect to each other (egenerate). he tetraheral crystal fiel splitting energy ( ) is than for octaheral complexes because the point charges are not irecte at any orbital set.

3 E y z (e ) g + 3 (t ) + xy 5 xz yz 5 3 average energy of 5 x y z (e) 5 orbitals with ligans y z yz (t g ) (Spherical crystal fiel) (ctaheral crystal fiel) (etraheral crystal fiel) o an are the octaheral an tetraheral crystal fiel splitting energy, respectively Again, the positioning of the orbitals is opposite for tetraheral an octaheral y, xz, an yz orbitals are now calle t an x y an z are e is than o because the point charges are not irecte at any orbital set in a tetraheral crystal fiel. Because is small, many tetraheral complexes are. ou can assume that they are all high spin! Because the overall energy in the tetraheral crystal fiel is maintaine, t orbitals go up in energy by /5, an the e orbitals go own in energy by 3/5. etraheral Example for Cr 3+ (a) figure out electron count (b) raw tetraheral crystal fiel splitting iagram, label orbitals, an fill in electrons E (t ) average energy of orbitals with ligans (Spherical crystal fiel) (e) (etraheral crystal fiel) (c) Write n electron configuration: () How many unpaire electrons? (e) If this compoun is [CrCl 6 ] 3 an the wavelength of most intensely absorbe light is 740 nm, preict the color of the complex. 3

4 III. Crystal Fiel heory: Square Planar Case axis axis M n+ y z axis Square planar repulsion much less repulsion than ligan point charges in octaheral crystal fiel. irecte at orbitals Less repulsion than Destablize compare for x y an for xy to all other orbitals axis M n+ axis axis yz xz y Square planar stabilize compare stabilize compare repulsion than for z, to y an x y to y an x y z, an z. Less repulsion than for x y since orbitals are 45 off axis in y. E average energy of orbitals with ligans (Spherical crystal fiel) x y z +3/5 /5 (tg) xy xz yz (ctaheral crystal fiel) M n+ (eg) (square planar crystal fiel) M n+ yz xy z x y xz he overall energy of the square planar crystal fiel is also maintaine, but the relative energies of each of the orbitals are more complicate an you are not expecte to know them. 4

5 IV. ther Geometries axis axis What about square pyramial? L M n+ axis M n+ axis axis axis square planar square pyramial z is for square pyramial compare to square planar z an yz are for square pyramial compare to square planar y an z are egenerate for square pyramial An Applications to metalloenzymes (Nickel enzyme example) Nickel epenent enzymes are responsible for removing ~100 million tons of C from the atmosphere each year an proucing ~1 trillion kg of acetate from greenhouse gases an other carbon sources. We want to know what these nickelbase catalysts look like, so that we can mimic this chemistry. o probe the geometry of the nickel cofactor, spectroscopy was use an it was foun that the Ni + ( 8 ) center was iamagnetic. Preict whether it has square planar, tetraheral, or octaheral geometry? y E y z y y z yz z xy z yz y z (ctaheral z yz (etraheral crystal fiel) crystal fiel) (Square planar crystal fiel) Answer: he Ni center in the enzyme must be. 5

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