Name ___________________________

Inorganic Chemistry
Exam 4
May 1, 2003

All work must be shown for any credit to be given.

(4 pts.) 1. Describe an example of hydrogen bonding and how the hydrogen bonding effects the physical or chemical properties.

Water
In water the hydrogen bonding cause the molecules to cluster together creating larger units of material. This causes the boiling point to be unusually high for the molecular weight of the material. In the solid state, it causes the ordering of the molecules that results in the very open grid structurethat reduces the density, causes the solid to expand, and causes the solid to float in its own liquid.

DNA
In DNA it is the hydrogen bonding that holds the 2 strands together. Because adenine, A, can form two hydrogen bonds to thymine, T, and guanine, G, can form three hydrogen bonds to cytosine, C, there are always equal amounts of A & T and equal amounts of C & G.

The following questions concern the contribution made by the X-ray photo of beta-DNA made by Rosalind Franklin. Reference the website http://www.mrsec.wisc.edu/edetc/DNA/index.html

(3 pts.) 2. How did the X-ray photo answer the question, "What is the shape of the molecule? Is it a straight chain? Are there two or three strands in the helix?"

The so-called "cross" pattern in the photo suggests a helical structure. Upon seeing the photo, Watson is reported to have said "The instant I saw the picture my mouth fell open and my pulse began to race.... the black cross of reflections which dominated the picture could arise only from a helical structure... mere inspection of the X-ray picture gave several of the vital helical parameters."Franklin concluded that the missing fourth layer line agreed with the double helix "if one molecule is displaced from the other by about three-eighths of the fibre-axis period"

(3 pts.) 3. How did the X-ray photo answer the question, "Are the strains of the helix in phase? If not, how are they out of phase?"

Watson's and Crick's model would suggest that the double helix is "a pair of templates, each of which is complementary to the other", which, prior to duplication, separate and unwind into two chains. "Each chain then acts as a template for the formation on to itself of a new companion chain, so that eventually we shall have two pairs of chains where we only had one before. Franklin's X-ray data also suggested "that there are only two molecules in the primitive unit cell" and that the two chains were offset, which was consistent with the Watson and Crick model.

(3 pts.) 4. How did the X-ray photo answer the question, "Are the phosphates on the outside and the bases on the inside of the helix or vice versa?"

Franklin also stated that the features of her X-ray photo suggested that "a crystallographically important part of the molecule lies on a helix of this diameter. This can only be the phosphate groups or phosphorus atoms."

(3 pts.) 5. How did the X-ray photo answer the question, "How are the bases involved? How many are there per coil (period) of polymer chain? How are they bonded?

In the Watson and Crick model, DNA is composed of two chains joined by complementary bases that are hydrogen bonded to each other. There are 10 residues, nucleotides, in the 3.4 nm period of the chain. The periodicity of 0.34 nm, deduced from the diffraction data, is due to the nucleotide repeat distance. The 3.4 nm layer lines are attributed by Wilkins and Franklin to the repeat distance or period of the chain. This suggested 10 nucleotides per period of chain.

(4 pts.) 6. In terms of ligand exchange, define the terms:

• labile - exchange t1/2 less than 1 min. at RT
• inert - exchange t1/2 more than 1 min. at RT

(4 pts.) 7. Give possible geometries for octahedral coordination species for:

• Dissociative mechanism - octahedral coordination has six ligands about the central atom, in a dissociative mechanism, one ligand is lost, resulting in five coordination
• Associative mechanism - octahedral coordination has six ligands about the central atom, in a associative mechanism, one ligand is added, resulting in seven coordination

(6 pts.) 8. Given the following reduction potential data, calculate the reduction potential for the process:

Cr₂O₇^-2 à Cr⁺²

in Cr₂O₇^-2, Cr is in the +6 oxidation state.
in Cr⁺³, Cr is in the +3 oxidation state
thus the first step is a 3 electron change per atom

in Cr⁺³, Cr is in the +3 oxidation state
in Cr⁺², Cr is in the +2 oxidation state
thus a 1 electron change per atom

E^o = ((2*+1.33v) + (1*-0.41V)/4 = +0.90v

(4 pts.) 9. Describe the quadrouple bond with regard to what orbitals overlap as well as the number and type of molecular orbitals formed.

A quadrouple bond is composed of four bonds. As usual the first bond between two atoms is the sigma bond and is the result of d_z² orbitals overlapping. The second and third bonds between two atoms are the usual pi bonds and are the result of d_xz-d_xz and d_yz-d_yzorbitals on the two metals overlapping. The fourth bond results from overlapping d_x²-y²-d_x²-y² orbitals from the two metal atoms.

(5 pts.) 10. Give the rate law expression for the following reaction:

rate = - d[ML₆]/dt = k[ML₆][L']

(6 pts.) 11. Give the valence electron counts for the following species. Which one(s) obey the 18-e rule (EAN Rule)?

Cr(CO)₆

EAN = 24 + 6(2) = 36

Fe₃(CO)₁₂

EAN = 26 + 2(1) + 4(2) = 36

both obey the 18 electron rule

(2 pts.) 12. The metal in the center of the porphine ring of chlorophylls is magnesium.

(2 pts.) 13. The metal in the center of the porphine ring of hemeglobin is iron.

(2 pts.) 14. The metal in the center of the porphine ring of Vitamin B₁₂ is cobalt.

(2 pts.) 15. The metal atoms in questions 12., 13., & 14. are all coordinated to nitrogen atoms of the porphine ring.

(2 pts.) 16. What is necessary for a compound to be considered organometallic?

Organometallic compounds are expected to contain metal to carbon bonds.

(2 pts.) 17. Give an example of a molecule or ion which can act as a p-Donor in an organometallic compound.

Carbon monoxide is the most common material that is thought to form p-Donor .

(4 pts.) 18. Draw the molecular orbitals that result for alkene to metal donation and metal to alkene back donation between the platinum atom and the ethylene molecule.

(2 pts.) 19. What is meant by CVD?

CVD- chemical vapor deposition

(4 pts.) 20. In the respiration cycle, oxygen and carbon dioxide are exchanged in the cells and in the lungs. These exchanges are both illustrations of LeChatier’s Principle which states that "systems at equilibrium which are disturbed, shift the equilibrium in such a way as to lessen the effect of the stress". Explain what is the "stress" at the cell end of the cycle? What is the "stress" at the lung end of the cycle?

at the cell, the high concentration of the carbon dioxide is the stress

at the lungs, the stress is in the form of high concentration of oxygen

(4 pts.) 21. What is the relationship between the structure of perovskite and the high-temperature superconductors.

Three unit cells of perovskite and 1 unit cell of the high-temperature superconductors are about the same size. The unit cell of the superconductor contains less oxygen that the three unit cells of perovskite.

(6 pts.) 22. Show the sequence of steps for producing both cis- and trans- platin, [Pt(NH₃)₂Cl₂].

(3 pts.) 23. Explain why the Fe⁺² ion in deoxyhemoglobin is above the plane of the porphyrin.

It is too large to fit into the hole in the porphyrin until it coordinates with the di-oxygen molecule. It then shrinks in size and drops down into the hole.

(3 pts.) 24. What is the role of Fe⁺² to Fe⁺³ in the respiratory cycle.

It allows the electron tranfer in the cytochrome molecules.

(6 pts.) 25. For Taube’s "inner sphere" mechanism, give the reaction intermediate and explain how Cl-35 was used to prove the mechanism for the following reaction:

(3 pts.) 26. What are the typical approximate regions in the IR spectrum for terminal, 2-centered, and 3-centered bridging carbonyls.

terminal - 1900-2100 cm^-1
2-centered - 1700-1900 cm^-1
3-centered - 1500-1750 cm^-1

(8 pts.) 27. Give a use for 8 of the 10 first row transition metals.

Any reasonable use is accepted.

I neither received not gave any assistance on this examination nor did I see anyone else give or receive assistance on this examination.

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