Chemistry 64 Winter 1994 NAME: FINAL EXAMINATION THIS EXAMINATION IS WORTH 100 POINTS AND CONTAINS 5 QUESTIONS. THEY ARE NOT EQUALLY WEIGHTED.

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1 1 Chemistry 64 Winter 1994 NAME: FINAL EXAMINATION THIS EXAMINATION IS WORTH 100 POINTS AND CONTAINS 5 QUESTIONS. THEY ARE NOT EQUALLY WEIGHTED. YOU SHOULD ATTEMPT ALL QUESTIONS AND ALLOCATE YOUR TIME ACCORDINGLY ALL BOOKS AND PAPERS OTHER THAN THE TABLES WHICH HAVE BEEN HANDED OUT SHOULD BE PLACED ON THE FLOOR DURING THIS EXAMINATION Tanabe Sugano Diagrams and the Periodic Table are provided as detachable Supplementary Material at the back of this exam. Magnetic Moment information, the Spectrochemical Series, and Abbreviations used are provided below. Spin-only formula: µ s = 2{S(S+1)} 1/2 BM = {n(n+2)} 1/2 BM Spectrochemical Series Increasing o (fixed metal): I < Br < Cl ~ SCN ~ N 3 < F < OH < CH 3 CO 2 < oxalate 2 < H 2 O < NCS < EDTA 4 < NH 3 ~ py < en ~ tren < o phen < NO 2 < H ~ CH 3 < CO ~ CN Abbreviations Used: en=ethylenediamine; Me=methyl; bipy=bipyridine

2 2 FOR GRADING USE ONLY Question 1 (18)... Question 2 (23)... Question 3 (28)... Question 4 (19)... Question 5 (12) TOTAL (100)...===========

3 3 Question 1. (18 pts) NOTE-THIS IS THE ONE ABOUT THE LAB! (a) (6 pts) Why was the recrystallization of [Cr(NH 3 ) 6 ](NO 3 ) 3 carried out in acidic solution? What product might have resulted if the ph had been >7? (b) (6 pts) Transition metal hexaquo complexes [M(H 2 O) 6 ] n+ are often colored. Give 2 transition metal ions whose hexaquo compounds are colorless, and explain their lack of color.

4 4 (c) (6 pts) Rhodium acetate [Rh(O 2 CMe) 2 ] 2 has the same structure as the Cr(II) acetate which you prepared in lab, and the same MO scheme as that used for chromium acetate can be used to describe metalmetal bonding in this Rh compound. What is the Rh-Rh bond order in rhodium acetate? Predict the spin-only magnetic moment for rhodium acetate. Question 2 (23 pts) (a) (3 pts) Give the expected sign (+ or ) for S, H, and V for the following reaction. Explain your answers. [PtCl 4 ] 2 + NH 3 > [PtCl 3 (NH 3 )] + Cl

5 5 (b) (6 pts) There are 3 possible square-planar isomers of the formula Pt(CO)(NH 3 )(Cl)(Br). Given the following order of the trans-effect: CO > Br > Cl > NH 3 and the following order of leaving group ability: Cl > Br > NH 3 > CO Suggest syntheses of all three possible isomers starting from PtCl 4 2. Make sure that you assign the correct charge to any intermediates which you draw!!

6 6 (c) (6 pts) Circle the less intense electronic spectral band among each of the following pairs, and explain the reason for your choice: [Cr(NH 3 ) 6 ] 3+ : 4 A 2g 4 T 2g or [Cr(en) 3 ] 3+ : 4 A 2g 4 T 2g cis -[CoCl 2 (NH 3 ) 4 ] + : 1 A 1g 1 T 1g or trans -[CoCl 2 (NH 3 ) 4 ] + : 1 A 1g 1 T 1g [Co(H 2 O) 6 ] 2+ ; 4 A 2g ---> 4 T 1g or [CoCl 4 ] 2 ; 4 A 2 ---> 4 T 1

7 7 (d) (4 pts) An idealized portion of the Tanabe-Sugano diagram for the d 2 electron configuration is shown below. Match the 2 excited state electron configurations illustrated (1 or 2) with the appropriate spectral term line on the diagram (X or Y) and explain your answer. X E/B e g e g Y t 2g t 2g 1 2 /B (e) (4 pts) Match the energy of the electronic spectral band with the appropriate complex, and explain your answer. 20,100 cm 1 [Ti(H 2 O) 6 ] 3+ 22,300 cm 1 [Ti(CN) 6 ] 3 Question 3. (28 pts) Explain the following experimental observations:

8 8 (a) (4 pts) The rates for substitution of water ligands by labelled water in [Cr(H 2 O) 6 ] 2+ and [Cu(H 2 O) 6 ] 2+ are much larger than those for the other first row [M(H 2 O) 6 ] 2+ ions. (b) (4 pts) The rate of acid hydrolysis of [Ru(NH 3 ) 5 X] 2+ depends on the halide X in the order Cl > Br > I.

9 9 (c) (4 pts) The rate of anation of [Ti(H 2 O) 6 ] 3+ by Y depends on the nature of Y in the order CH 3 CO 2 H < H 2 O < NCS < CH 3 CO 2. (d) (4 pts) Under acidic conditions hydrolysis of a chloride ligand of cis- [Co(Me 2 NCH 2 CH 2 NMe 2 ) 2 Cl 2 ] + is faster than analogous hydrolysis of cis-[co(h 2 NCH 2 CH 2 NH 2 ) 2 Cl 2 ] +.

10 10 (e) (4 pts) Under basic conditions hydrolysis of a chloride ligand of cis- [Co(Me 2 NCH 2 CH 2 NMe 2 ) 2 Cl 2 ] + is slower than analogous hydrolysis of cis-[co(h 2 NCH 2 CH 2 NH 2 ) 2 Cl 2 ] +. (f) (4 pts) The rate of electron transfer between [Co(NH 3 ) 5 (H 2 O)] 3+ and [Cr(H 2 O) 6 ] 2+ is approximately 10 7 times faster at ph 10 than at ph 6. In contrast, the rate of electron transfer between [Co(NH 3 ) 5 (H 2 O)] 3+ and [Ru(H 2 O) 6 ] 2+ is essentially unchanged over this ph range.

11 11 (g) (4 pts) The relative rates of the following electron transfer reactions are: [Os(bipy) 3 ] 2+ + [Os(bipy) 3 ] 3+ [Os(bipy) 3 ] 3+ + [Os(bipy) 3 ] 2+ 5x10 4 [Co(H 2 O) 6 ] 2+ + [Co(H 2 O) 6 ] 3+ [Co(H 2 O) 6 ] 3+ + [Co(H 2 O) 6 ] 2+ 5 RATE (M 1 s 1 )

12 12 Question 4 (19 pts) (a) (9 pts) The electronic spectra of [Co(NH 3 ) 5 (X)] 2+ (where X is a halide) are shown below. [Note that 1 kk = 1000 cm 1.] Assume that the Tanabe-Sugano diagrams for rigorously octahedral complexes can be used for these compounds of lower symmetry. Assign the spectral bands to specific electronic transitions. If this is not possible for a given band, assign it to a type of transition. Explain (1) the relative intensities of the bands (2) their relative energies (3) the change in relative energy as X is changed

13 13 (b) (6 pts) For the complex [Co(CO) 4 ], sketch the expected electronic spectrum due to d-d transitions. Be sure to label and include scales for the axes in your drawing.

14 14 (c) (4 pts) Predict the effect of lowering the temperature on the intensity and the bandwidth of a typical band in the electronic spectrum of an octahedral transition metal complex. Question 5 (12 pts) (a) (2 pts) Sketch the HOMO and the LUMO of CO.

15 15 (b) (4 pts) Sketch transition metal orbitals of the appropriate symmetry to interact with the HOMO and LUMO above, and define 'backbonding'. (c) (6 pts) The C-O stretch in the IR spectrum of the complex salt [Ag(CO)] + [B(OTeF 5 ) 4 ] appears at 2204 cm -1. Compare this value to those for other transition metal carbonyls and for CO itself and explain it by using the bonding picture developed above.

16 16 PERIODIC TABLE 1 H Li Na K Rb Cs Fr 87 (223) Be Mg Ca Sr Ba Ra 21 Sc Y La Ac Ti Zr Hf Unq (261) V Nb Ta (262) Unp Cr Mn Mo Tc (98) W Unh Re Uns (263) (262) Fe Ru Os Uno (265) 27 Co Rh Ir Une (266) Ni Pd Pt Cu Ag Au Zn Cd Hg B Al Ga In Tl C 6 N Si P Ge As Sn Sb Pb Bi O Se Te Po 84 (209) S F 9 2 He Ne Cl Ar Br I At (210) 85 Kr Xe Rn (222) Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho (145) Th Pa U Np Pu Am Cm Bk Cf Es Er Tm Yb Lu Fm Md No Lr (237) (244) (243) (247) (247) (251) (252) (257) (258) (259) (260)

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25 Tanabe-Sugano Diagrams 25

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B I N G O B I N G O. Hf Cd Na Nb Lr. I Fl Fr Mo Si. Ho Bi Ce Eu Ac. Md Co P Pa Tc. Uut Rh K N. Sb At Md H. Bh Cm H Bi Es. Mo Uus Lu P F.

B I N G O B I N G O. Hf Cd Na Nb Lr. I Fl Fr Mo Si. Ho Bi Ce Eu Ac. Md Co P Pa Tc. Uut Rh K N. Sb At Md H. Bh Cm H Bi Es. Mo Uus Lu P F. Hf Cd Na Nb Lr Ho Bi Ce u Ac I Fl Fr Mo i Md Co P Pa Tc Uut Rh K N Dy Cl N Am b At Md H Y Bh Cm H Bi s Mo Uus Lu P F Cu Ar Ag Mg K Thomas Jefferson National Accelerator Facility - Office of cience ducation