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    Peaks and Fragments with Mass Spectrum

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    Synthesis and Characterization of Mo2(O2CCH3)4 and K4Mo2Cl8: Compounds that contain Metal-Metal Quadruple bonds.

     Procedure
    Place 2.0g of molybdenum hexacarbonyl and .10g of trimethylamine-N-oxide and a few boiling chips in the three-neck flask. Lightly grease the center neck of the flask and attach the reflux condenser to it.
    Insert the needle, connected to the nitrogen line, through the septum in the side neck. Slowly turn on the nitrogen gas.
    Purge the system with nitrogen for 10 min before continuing. Why?
    Add 100mL of ortho-dichlorobenzene, 2.0 mL hexanes, and 4.0mL of a 10:1 mixture of acetic acid/acetic anhydride into the reaction flask via a syringe.
    Place the Thermowell on the supplied plate and position it on the ring stand. Place the flask in the Thermowell and secure the whole assembly to the ring stand. Adjust the heat as needed so the vapors condense about half way up the reflux condenser. Continue heating the mixture for 1 hr and 45 min. occasionally, gently agitate the setup to ensure mixing and to wash the sublimed Mo(CO)6 off the walls of the flask.
    After the reflux is complete, let the flask cool. Once cooled to room temp, filter the solution. Wash the yellow molybdenum acetate solid twice using 30mL of absolute ethanol, then twice using 30mL of diethyl ether. Record an actual yield, and calculate a percent yield based upon Mo(CO)6.
    - Actually I did not get any product. So there was no actual yield. But I want to know the percent yield.
    To a 125 mL Erlenmeyer flask, add .4g of Mo2(O2CCH3)4 and .75g of finely ground potassium chloride. Slowly add 25mL of concentrated hydrochloric acid. Stir the mixture occasionally and gently for one hr. filter the mixture to isolate the insoluble magenta colored K4Mo2Cl8.
    Wash the solid twice using a small volume of absolute ethanol and allow the product to dry. Record the actual yield and calculate a percent yield based on Mo2(O2CCH3)4.
    - The actual yield was 1.982g. I want to know the percent yield.

    1. Include complete balanced reactions of both preparations, from starting materials to products.
    2. Trimethylamine-N-oxide is used as an initiator for the Mo2(O2CCH3)4 preparation. Suggest a reaction by which Trimethylamine-N-oxide activates molybdenum hexacarbonyl.
    3. Mo2(O2CCH3)4 has a bond order of four and is diamagnetic. If this compound was oxidized or reduced by one electron, as follows, what would be the resulting bond orders?
    Mo2(O2CCH3)4 + e-  [Mo2(O2CCH3)4]-
    Mo2(O2CCH3)4 - e-  [Mo2(O2CCH3)4]+
    4. Would the charged products be diamagnetic or paramagnetic? Explain
    5. The peaks due to heavy fragments in the mass spectrum of Mo2(O2CCH3)4 show that this compound is a dimer. However, there is not a single peak corresponding to the molar mass of Mo2(O2CCH3)4, instead there is a family peaks. Find and examine the entries for carbon, oxygen, and molybdenum in a table of the isotopes and suggest an explanation.
    6. Has the oxidation state of the molybdenum been altered in the conversion of Mo(CO)6 to Mo2(O2CCH3)4? Show this by calculating the oxidation states from the empirical formulas.
    7. Has the oxidation state of the dimolybdenum core been altered in the conversion of Mo2(O2CCH3)4 to K4Mo2Cl8? Show this by calculating the oxidation states from the empirical formula.
    8. In the far infrared spectrum (600-150 cm-1) of K4Mo2Cl8, two peaks are found. Explain their origin. What kind of peaks (if any) would you expect to find in the regular infrared region (4000-600 cm-1)?

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    Solution Preview

    In order to calculate yields, you work out the number of moles of your product by dividing the mass you obtain by the molar mass. For your second reaction this is 1.982/631.9 = 0.00314 moles. You then need to work out the number of moles of complex you started with. This is 0.4/428.058 = 0.0009344 moles. Theoretically then you can get a maximum of 0.0009344 moles of product, equating to (0.0009344 x 631.9) = 0.59 grams.... you got 1.982, which i'm afraid is impossible... the sample you weighed to get your yield must have been wet with solvent (this is often the case as some complexes can take hours to dry even if they look it). If you explain this in your report, im sure you will be marked fairly, as you ...

    Solution Summary

    Peaks and fragments with mass spectrum are examined. The infrared spectrum is examined.

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