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such Zn à Zn2+ + 2e- E?

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such as +4 and
+5 also exist1. For this experiment only the
+2 and +3 oxidation states are considered. Chromium(III) is the most stable
form of chromium as chromium(III) complexes are, largely, kinetically inert. This
is a result of the d3 octahedral configuration having a large ligand
field stabilisation energy1. Chromium(II) is highly unstable
and is easily oxidised to chromium(III) in air, as such in this experiment
Schlenk techniques are used to create an inert atmosphere to prevent this.

 

The complex
synthesised, chromium(II) acetate, Cr2(CH3COO)4.2H2O,
is a dimer. The structure of this complex has been studied in great detail. Both
chromium atoms are surrounded by a distorted octahedron2. The chromium atoms are held
together by 4 acetate ligands which are in the equatorial positions, for these Cr-O
bonds the s, px, py, pz and dx2-y2 orbitals
of chromium are used1. There is a quadruple bond
between the two chromium ions resulting in one of the chromium ions in the
axial position, this bonding can be seen in figure 32,3. As result of the  s2p4d2 configuration, giving a quadruple bond,
there are two non-bonding pzdz2 hybrid orbitals that
point out either end of bond. This allows the bonding of the 2 H2O
ligands to the complex as in each water molecule the oxygen atom donates its lone
pair into the orbital and as such the final axial position is occupied by a
water molecule1.

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Figure 1:
Structure of Chromium(II) Acetate

In the first
reaction, the chromium(III) chloride is reduced to chromium(II) by zinc in an
acidic solution3:

Cr3+
+ e- à
Cr2+          E? = -0.41 V

Zn
à
Zn2+ + 2e-          E?
= 0.76 V

2 Cr3+
+ Zn à
2 Cr2+ + Zn2+

 

The second
reaction that occurs upon the addition of the pale blue solution to sodium
acetate is4:

2 CrCl2.4H2O
+ 4 Na(CH3COO) à Cr2(CH3COO)4.2H2O
+ 4 NaCl + 2 H2O

 

Due to its
instability chromium(II) acetate is often used as a precursor to other chromium
compounds5. Chromium(II) acetate is also
very useful in the polymer industry. Chromium(II) acetet and organic peroxide
systems can be used to initiate the polymerisation of vinyl monomers. In this
process, the Cr2+ ions are used to reduce benzoyl peroxide. This
reduced peroxide goes onto initiate polymerisation6. The chromium(II) acetate complex
can also be used in the graft copolymerisation of styrene on rubber containing
halogens7.  

 

Experimental

This procedure
was carried out using Schlenk techniques to ensure an inert atmosphere of
nitrogen was maintained.

Chromium(III)
chloride hexahydrate (9.8743 g; 0.037 mol) in a solution of conc. HCl (25 mL)
and water (15 mL) was added to granulated zinc (10.0524 g; 0.154 mol). The
mixture was left for 30 minutes until the solution had turned from deep green
to pale blue. A solution of sodium acetate (15.4333 g; 0.188 mol) in 20 mL was
prepared and degassed. Three quarters of the blue chromium(II) solution was
added, via syringe, to the sodium acetate and a colour change from blue to deep
red was observed. The product, a red solid, was collected by filtration and
washed with degassed water (10 mL), degassed ethanol (10 mL) and dry ether (20
mL) and was left to dry under vacuum for 2.5 hours.  The product was transferred to a nitrogen
filled dry-bag where samples for characterisation were made up and the mass
determined (4.2 g; 0.011 mol; 30%). IR
(L, cm-1) 3367, 3269, 1561, 1449, 1415, 1354, 1049, 1030, 677.

 

Results

Mass obtained =
4.2 g

Moles obtained =

Chromium(III)
chloride hexahydrate is the limiting reagent. However, as 2 moles of CrCl3
are needed to synthesise one mole of chromium(II) acetate the theoretical yield
is 0.019 mol.

Percentage
yield =

 

Magnetic Moment Calculation:

C
= 1.05

l
= 2.3 cm

m
= 0.0978 g

R0
= -69

R
= -37

MW
= 376.2 gmol-1

T
= 289 K

 

 

cdia
can be found by calculating the diamagnetic and constitutive corrections for the
atoms in the molecule, which can be found using Pascal’s constants8:

 

As , where n is the
number of unpaired electrons:

 

 

Figure 3: IR spectrum of chromium(II) acetate

 

Discussion

The yield of
chromium(II) acetate was 60% which is depressed in comparison to the yields of
70-80% published by Reeve3 or 95% published by Hatfield
et al.4. This yield was reduced
particularly by loss in the dry-bag due to reduced dexterity. As only three
quarters of the chromium(II) solution was transferred into the funnel, the
yield calculation will have been affected as the chromium was the limiting
reagent.

 

The relevant peaks in the IR
spectrum, figure 2, can be seen in table 1 below:

 

Experimental Frequency (cm-1)

Literature Frequency
 (cm-1)

Bond Vibration

3367
(w)
3269
(w)

3550-3200

H-O-H
Stretching

1561
(m)

1560

C=O
sym. Stretching

1449
(s)

1350-1450

C-H
Bending

1415
(s)

1415

C=O
asym. Stretching

677
(s)

~650

Cr-O
Stretching

Table 1:
Characteristic absorption frequencies in the IR spectrum of chromium(II)
acetate, figure 2

The absorption
frequencies in the IR spectrum obtained from the product synthesised agree with
the frequencies found in the literature9. The low frequency of the C=O
stretching frequencies show that they are bridging carbonyl ligands. As a
result, it can be concluded that it is likely the correct product was obtained.

 

The magnetic
moment calculation gives that there are either -2.4 or 0.4 unpaired electrons
in the complex. As -2.4 unpaired electrons is a result that does not make
sense, it can be concluded that the complex has 0.4 unpaired electrons which
rounds down to 0 unpaired electrons. This result suggests that the complex is
diamagnetic which supports the hypothesis made on the structure of chromium(II)
acetate shown in figure 12. As the result is slightly
higher than 0 it suggests that there is an impurity present in the complex that
is paramagnetic. As this measurement was taken out of the dry box it is is
likely that this paramagnetic species is chromium(III) acetate which forms when
chromium(II) acetate is exposed to air.  

 

In1970, Cotton
et al.10 showed that the Cr-Cr bond
length in hydrated chromium (II) actetate was 2.362 Å using x-ray crystallography.

The short distance between these two Cr ions and
the complex’s diamagnetic nature provide evidence of a quadruple bond. This quadruple
bond is made up of a sigma bond, 2 pi bonds and a delta bond. The sigma bond is
a result of the overlap of the pzdz2 hybrid orbitals of
both chromium ions, the overlap of the dxz orbitals in each chromium
and the two dyz orbitals gives a pi bond. Finally, the delta bond is
formed as a result of an overlap of dxz orbitals11.

Figure 3:
Molecular Orbital Diagram for the formation of the quadruple Cr-Cr bond

If the complex
synthesised in this experiment is heated in
vacuo, the dehydrated chromium(II) acetate complex is formed. The Cr-Cr
bond length in this complex was measured by Cotton et al.10 using x-ray crystallography and
was found to be 2.288 Å10. This is shorter than the Cr-Cr bond in the hydrated complex. This
decrease in bond length is a result of the increased Cr-O axial bond length,
from 2.272 Å in the hydrated complex to 2.327 Å, and the oxygen atom’s displacement from the Cr-Cr axis. This
increases the s contribution to the Cr-Cr bond, making
it stronger and thus decreasing its length10.

 

Conclusions

The hydrated
chromium(II) acetate complex was synthesised successfully with a 60% yield. The
structure of the complex was confirmed using magnetic moment measurements and
IR spectroscopy.

 

References

1         C. Housecroft
and A. G. Sharpe, Inorganic Chemistry, Pearson, 4th edn., 2012.

2         J. N. van
Niekerk, F. R. L. Schoening and J. F. de Wet, Acta Crystallogr., 1953, 6,
501–504.

3         J. C. Reeve, J.

Chem. Educ., 1985, 62, 444.

4         M. R.

Hatfield, H. Matheson and J. Kleinberg, in Inorganic Syntheses, ed. L.

F. Audrieth, John Wiley & Sons, Inc., 3rd edn., 1950, pp. 148–150.

5         L. R. Ocone,
B. P. Block, J. P. Collman and D. A. Buckingham, in Inorganic Syntheses,
ed. H. F. J. Holtzlaw, John Wiley & Sons, Inc., 8th edn., 1966, pp.

125–132.

6         M. Lee and Y.

Minoura, J. Chem. Soc. Faraday Trans. 1 Phys. Chem. Condens. Phases,
1978, 74, 1726.

7         M. Lee, H.

Nakamura and Y. Minoura, J. Polym. Sci. Polym. Chem. Ed., 1976, 14,
961–971.

8         G. A. Bain and
J. F. Berry, J. Chem. Educ., 2008, 85, 532.

9         G. Socrates, Infrared
and raman characteristic group frequencies?: tables and charts., John Wiley
& Sons, 2007.

10       F. A. Cotton,
C. E. Rice and G. W. Rice, J. Am. Chem. Soc., 1977, 99,
4704–4707.

11       S. F. Rice, R.

B. Wilson and E. I. Solomon, Inorg. Chem., 1980, 19, 3425–3431.

 

 

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