Chemistry 115 Quiz 7

May 31, 2001 Name ____________________________________

  1. In which complex do we expect to find the larger crystal field D?

a. [Cr(H2O)6] 2+ or [Cr(H2O)6] 3+ ? Explain briefly.

Since the metal and ligands are the same, it is the charge on the metal that will cause the difference; the larger charge on Cr(H2O)63+ will cause greater interaction and hence a larger crystal field.
  1. [Cr(en)3] 3+ or [CrCl6] 3-? Explain briefly.

Since both complexes contain the same metal ion Cr3+, the ligand will be responsible for the difference; the en is higher in the spectrochemical series, which means that it will cause a larger crystal field.

2. Which complex should absorb light of the highest frequency? [Cr(H2O)6] 3+, [Cr(en)3] 3+ or [CrCl6] 3-? Explain briefly.

All three complexes contain Cr3+ with different ligands; according to placement of the ligands inthe spectrochemical series, the CrCl63- will have the smallest crystal field while Cr(en)33+ will have the largest crystal field.

Absorption of the highest frequency light corresponds to the most energetic electron transition, which in turn would occur with the largest crystal field split.
  1. Consider the complex [CoF6]4-.
  1. Would it be more likely to be high spin or low spin? Explain briefly.

    2. Co2+ is a first transition series metal with a small charge - thus it will be the ligand that determines whether the complex is high or low spin. F- is one of the weakest ligands in the spectrochemical series.
  2. Draw the valence bond diagram expected for it. Co2+ = [Ar] 3d7
    3.

    [Ar]

    _||_ _||_ _|_ _|_ _|_

    xx

    xx xx xx

    xx xx ___ ___ ___
     

    3d

    4s

    4p

    4d

    sp3d2 = octahedral hybrid
  3. Draw the crystal field diagram expected for it.
    4.
     

    __|__

    __|__

    		  

    Eg = dx2 - y2, dz2
         

    |

    		  
         

    D

    Is small
         

    |

    		  

    __||__

    __||__

    __|__

    		  

    T2g = dxy, dxz, dyz
  4. How many unpaired electrons do you expect for it? 3
  1. Consider [CoCl4] 2- and [PtCl4] 2-. One is tetrahedral while the other is square planar.
  1. Which is which structure? Explain briefly.

Same ligand but Co2+ is 1st Transition Series while Pt2+ is 3rd Transition Series. 2nd and 3rd transition series metal complexes are always low spin. Thus square planar which is the low spin structure would be the PtCl42- while CoCl42- would be tetrahedral.

b. Draw valence bond diagrams for both.

[CoCl4] 2- - Co2+ = [Ar]3d7

[Ar]

_||_ _||_ _|_ _|_ _|_

xx

xx xx xx

___ ___ ___ ___ ___
 

3d

4s

4p

4d

sp3 hybrid = tetrahedral

[PtCl4] 2- - Pt2+ = [Xe] 4f14 5d8

[Xe]

_||_ _||_ _||_ _||_ xx

xx

xx xx ___

___ ___ ___ ___ ___

4f14

5d

6s

6p

6d

dsp2 hybrid = square planar