Quadruple Bonded Cr

Synthesis of a Quadruple Bonded Cr-Cr Complex.

Introduction

Metal-metal bonds is a type of bond that is common with many metals that has lower oxidation state. This bond is formed between two metals which involves two atoms with eight electrons. The ligands that supports the quadruple bonds are pi donor not pi acceptor. In the experiment described below, this type of bonding will be observed between two atoms of chromium in a dichromium complex. Although this type of bonding, especially the quadruple bond observed in this experiment is not too common, it is observed with metals such as chromium and molybdenum. To form a quadruple bond, delta orbitals are also occupied by electrons, something not seen in most MO diagrams.1

With this experiment also comes the discussion of metallic character. A complex that has no unpaired electrons is said to be diamagnetic and is weakly repelled by a magnetic field. The magnetic susceptibility of such a species is equal to or less than 0. Paramagnetic species do contain unpaired electrons and are slightly attracted to a magnetic field. These species have a small, but positive magnetic susceptibility.

Experimental Section

All experimental procedures were adapted and modified by the pre-existing literature cited.2

3Dichromium Complex:

Apparatus was assembled so that the reaction could take place in the absence of air. A diagram taken from the literature shows the appropriate setup.

A solution of chromium(III) chloride hexahydrate (3.0 g) in water (10 mL) was prepared. Zinc (2.4 g) was then added to the chromium chloride and the reaction vessel was sealed with a rubber stopper and separatory funnel so that air could not go inside the Erlenmeyer ( 1). A beaker of water was also connected to the side arm of the flask with a tube so that hydrogren gas could bubble the water. Concentrated hydrochloric acid (9.0 mL) was added to the separatory funnel and allowed to enter the reaction mixture dropwise. Upon the addition of acid along with stirring of the reaction mixture, effervescence along with a color change from green to blue was observed. Gas also bubbled into the beaker of water. A slurry of sodium acetate trihydrate (18 g) in cold water (20 mL) was made. A tube connecting the metal reaction mixture was inserted into the slurry mixture and once the metal mixture had become clear blue, a clamp was placed on the hose going into the beaker of water so that hydrogen gas buildup would force out the blue metal mixture into the sodium acetate slurry. Upon the addition of the metal mixture into the slurry, a deep red was observed.

Once all the metal mixture was added, the red solution was capped and cooled under running water. Upon cooling, a red precipitate was observed. This solid was collected by filtration and washed with water, ethanol, ether, and dried. UV-Vis, FT-IR, and metal susceptibility was taken for the sample and recorded along with the mass of the sample. The sample was then dried in the air and a green oxidized product was observed. UV-VIS, FT-IR, and metal susceptibility were also taken for the green product and the spectra were compared.

Results

See attached spectra for UV and FT-IR data.

Magnetic susceptibility for red product = 0.

Magnetic susceptibility for green product = 0.01 x 10-4

Yield of Red compound = 0.204 g

Theoretical Yield = g

% Yield = yield

Discussion

In this lab, we studied a quadruple bond between two chromium metals. The MO diagram below shows this bond for the Cr(II) state. As we can tell from the diagram, the bond order is 4 suggesting the quadruple bond and we see the delta orbital's. We also can see that the complex is diamagnetic, which would correlate to a magnetic susceptibility of 0 that we obtained for the red product.

Upon exposure to air, the oxygen causes the chromium to get oxidized to the Cr(III) oxidation state. I would assume that oxidation does not always occur with both chromiums and our green sample is not full oxidized. If both chromiums were oxidezed then we would result in dimagnetic sample again with 6 electrons paired. However, because we got a value for the magnetic susceptibility for the green sample then we can conclude that the sample is paramagnetic and one of the chromiums were oxidized. The MO diagram below also shows this change.

We can also conclude from the magnetic susciptiblity that paramagnetic sample (Cr+2) would weight more than dimagnetic sample (Cr+3). This is because paramagnetic samples get attracted to the magnetic field where as dimagnetic samples get repleed by the magnetic field.

During the reaction zinc was used as a reducing agent to change the Cr(III) to Cr(II) in the first step. Along with this, HCl was added as a counterion for the newly oxidized zinc and a source of H2 gas. Heat was used to help catalyze oxidation of our Cr(II) complex to the Cr(III) complex.

Analysis by IR and UV data is not as clear as the magnetic susceptibility. The two IR spectra for the different colored compounds look nearly identical to me. I would expect a small change to occur due to the oxidation change, but not much because the atoms are still the same. Unfortunately, I am unable to locate a change between the two spectra. We are able to recognize some of the other portions of the spectra though. We see CH peaks around 2900 and C-O peaks around 1500. The UV spectra also do not show a huge change between peaks. It does look as if the peak around 560 for the red spectra shifts to 550 upon oxidation. The peak around 430 stays the same. I can conclude that the 560 peak is the one that is involved with the quadruple bond.

The low yield can be attributed to the fact that not all of the robin egg blue solution was able to transfer to the sodium acetate. In fact, during my washings with water, I used the aspirator and possibly lost a large portion of my sample.

References

(1) Miessler, Gary L.; Donald Arthur Tarr (1999). "9". Inorganic Chemistry. pp. 454,455. ISBN 0138418918, 9780138418915

(2) Handout from CHM 5020 at Wayne State University.

(3) Tsai, Y. Chen, H. Chieh, C. Chang, Yu, J. Lee, G. Wang, Y. Kuo, T. Journey from Mo-Mo Quadruple Bonds to Quintuple Bonds. J.A.C.S communication [Online] 2009, 131, pp 12534-12535.

(4) http://www.git.or.th/eng/testing_center_en/lab_notes_en/glab_en/2008/ruby_from_tanzania_en.html (Accessed October 19, 2009).

(5) Quadruple bond. http://en.wikipedia.org/wiki/Quadruple_bond (Accessed October 18, 2009).

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