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3.3: Exam 3

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  • 1. (29 pts) The simplest stable phosphorous sulfide, tetraphosphourous trisulfide, P4S3 is shown below. The bands observed in the IR and Raman spectra of P4S3 in gas phase, melt and solution are listed.



    Infrared Data:

    ν / cm-1


    Gas (550 °C)

    Raman Data: Δ / cm-1

    Melt (250 °C)                                           CS2 (25 °C)



    218 221 218 223
    286 292 287 291
    339 347 339 343
    414 420 420 423
    438 446 440 444

    a. (20 pts) Determine the normal modes of vibration for P4S3 and how they transform.

    b. (9 pts) Which are Raman and IR active?


    2. (41 pts) A molecular orbital analysis of transition metal dihydrogen complexes provides critical insight into the bonding interactions between metals and hydrogen and established an elegant framework in which the reactivity between H2 and transition metal complexes can be interpreted.




    a. (10 pts) Construct the molecular orbital diagram of a side-on bonded Cr(CO)5(H2) from group fragment orbitals.

    b. (6 pts) Pictorially illustrate the σ and π interactions that stabilize the formation of the dihydrogen complex.

    c. (15 pts) These interactions can effectively be used to rationalize several aspects of TM dihydrogen chemistry. In this regard, explain the following observations:

    i. (5 pts) d6 metals appear to form the most stable TM dihydrogen complexes

    ii. (5 pts) many TM dihydrogen complexes synthesized to date have ancillary π-accepting ligands

    iii. (5 pts) first row transition metals stabilize dihydrogen compounds while third row metals tend to promote dihydride compounds

    d. (10 pts) Construct the MO diagram for an end-on bonded H2 complex; and explain why (using the end- and side-on MO diagrams) end-on complexes are not favored energetically relative to side-on complexes.


    3. (30 pts) The nitrogen chemistry of early transition metals was established with the preparation of the Ti complexes from the Bercaw group at Caltech during the mid-1970s. One of the compounds is shown below. Construct the qualitative molecular orbital diagram for the dinuclear titanium complex from the frontier orbitals of the bent Cp2Ti fragment (in C2v symmetry) and the appropriate frontier molecular orbitals of nitrogen. Label the MO with appropriate symmetry labels, identify the nature of the bond (i.e., σ, σ*, π, π*) and fill up the MO with the appropriate number of electrons.





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