Introduction
The current interest in metal-metal bonding stems from the rhenium cluster compounds
studied in the past which have very strong and short metal to metal bonds, leading to speculation
of quadruple bonds in between the metals. However, since these rhenium cluster compounds are
difficult to study, the first row transition metal ions, Cr(II) and Cu(II), can be studied instead.
Generally, first row transition metals do not extend sufficiently for good overlap but exceptions
to this are Cr(II) and Cu(II), which form acetate complexes which are structurally similar to the
Re and Mo carboxylates and exhibit interesting metal-metal bonding.
In this experiment, chromium(II) acetate hydrate and copper(II) acetate hydrate were
synthesized. There structures are shown in Figure 1.0 and 2, respectively. These complexes share
a very similar structure as each contain a metal-metal bond, and form a bidentate linkage to four
acetate ligands via the oxygen donor atoms. Another similarity is that these complexes are
hydrated around the metal
Figure 1.0: Chromium(II) acetate hydrate
Figure 2.0: Copper(II) acetate hydrate
The purpose of this experiment was to synthesize the acetate complexes and determine
their magnetic moments using the mass magnetic susceptibility calculated by the Evans balance.
Since the strength of the metal-metal bond cannot be accurately determined by thermodynamic
measurements, the magnetic moment along with the metal-metal bond force constant can be used
to assess the stability of the complex as they are both indicators of the bond strength.
Magnetic susceptibility is the degree of magnetization of a material in response to an
applied magnetic field. The Evans balance measures the change in current required to keep a set
of suspended permanent magnets in balance after their magnetic fields interact with the sample.
The molar susceptibility was determined through corrections from the mass susceptibility, which
was used to calculate the effective magnetic moment. From this effective magnetic moment, the
numbers of unpaired electrons were determined which can indicate the structure of the metal
complexes
The magnetic moment is influenced by the orbital and the spin motion. However, the spin
contribution is more significant as it influences the magnetic moment in symmetric orbitals.
Electrons that are paired and exhibit negative magnetic susceptibility are termed to be
diamagnetic and are repelled slightly by electric fields whereas paramagnetic metals have
unpaired electrons and have a positive magnetic susceptibility.
Experimental
A. Synthesis of Chromium (II) Acetate Hydrate
A 35.1176 g sample of Na(CH3CO2)3H2O was dissolved in 30.25 mL of deoxygenated water
under the fume hood. 4.4999g of zinc mesh and 5.5073 g of CrCl36H2O were dissolved in 20 mL
of deoxygenated water in a filter flask and covered with parafilm. A beaker was filled with
distilled water and covered with parafilm. A clamped tube was inserted into the filter flask from
the sodium acetate flask while another unclamped tube was inserted into the beaker of water. 10
mL of concentrated HCl was added to the funnel of the closed stopcock. The HCl was added
drop by drop to a dark green sodium acetate solution and stirred. Once all of the HCl was added,
a royal blue solution was formed. The clamped tube was loosened and nitrogen gas was added to
the sodium acetate flask. A red precipitate formed in the filter flask as a result of the transferred
sodium acetate solution. The filter flask containing the red, viscous chromous acetate precipitate
was covered with parafilm and cooled in ice. This solution was filtered by suction on a Buchner
funnel and washed with four portions of 20 mL of ice-cold deoxygenated water. Afterwards, the
product was washed with 12 mL of alcohol and two portions of 12 mL of ether. The yield of the
brick red precipitate was determined to be 0.8102 g
B. Synthesis of Copper (II) Acetate Hydrate
3.1287 g of copper(II) sulfate pentahydrate was dissolved in 60 mL of distilled water in a
water bath at 40oC. Approximately 7.7 mL of 50% ammonia was added to the light blue solution
which changed the solution into an intense blue colour. 1.0452 g of sodium hydroxide were
added to the solution and the solution was stirred for 35 minutes at 60oC which formed a light
blue precipitate. The solution was cooled; filtered by suction using a Buchner funnel and the
precipitate was washed with three 25 mL portions of warm distilled water. The precipitate was
then dissolved in 9.5 mL of 10% acetic acid which formed a dark blue solution. This solution
was filtered by suction and the product was allowed to dry on the funnel. The yield was
determined to be 2.4449 g
C. Measuring Mass Magnetic Susceptibility
After the complex was synthesized, it was crushed into powder form using a mortar and
pestle. The mass magnetic susceptibility and mass were determined for the empty glass capillary
tube. Fine powdered solid was inserted into a glass capillary tube and the mass magnetic
susceptibility was measured three times by the Evans balance. All parameters including mass of
the sample and tube were recorded or measured and the capillary tube was cleaned using
acetone. This was done for both complexes.
Results
Table 1.0: Quantitative Results for Chromium(II) Acetate Hydrate
Mass of empty tube: 1.7828 g
Mass of sample and tube: 1.8105 g
Mass of sample: 0.0277 g
Reading of empty tube : -057
Reading of sample tube 1: 125
Reading of sample tube 2: 129
Reading of sample tube 3: 129
The average of R: 127.67
Length of sample: 1.7 cm
Balance calibration constant: 1.002
Room temperature: 294.0 K
The chromium(II) acetate hydrate was red and turned into brick red upon washing with reagents
which matches literature characteristics
Table 2.0: Quantitative Results for Copper(II) Acetate Hydrate
Mass of empty tube: 1.7738g
Mass of sample and tube: 1.8496 g
Mass of sample: 0.0758
Reading of empty tube : -060
Reading of sample tube 1: 053
Reading of sample tube 2: 053
Reading of sample tube 3: 053
The average of R: 053
Length of sample: 2.5 cm
Balance calibration constant: 1.002
Room temperature: 294.0 K
The copper(II) acetate hydrate was a turquoise blue colour which matches literature
characteristics of this complex
Calculations
Determination of Magnetic Moments (chromium(II) acetate hydrate)
The following equations are needed to determine the magnetic moment
1. Mass magnetic susceptibility (ignoring volume susceptibility)
g = CL(R-Ro) / (1 x 109 (m))
L = sample length in centimetres
m = sample mass in grams
C = balance calibration constant
R = reading from the digital display with the sample
Ro = reading from the digital display when just empty tube
g = (1.002)(1.7cm)(127.67- (-57)) / (1 x 109 (0.0277 g))
g = 1.1356 x 10-5 erg G-2 g-1
2. Molar susceptibility:
m = g x (molecular weight in g/mol)
Where g is the mass magnetic susceptibility
m = 1.1356 x 10-5 erg G-2 g-1 x 376.2 g/mol
m = 4.2722 x 10-3 erg G-2 mol-1
3. Molar susceptibility corrections:
m corr = m + ∑correction
Where ∑correction is the sum of all diamagnetic corrections
Cr2(CH3CO2)4(H2O)2
∑correction = [2(13) + 4(29) + 2(13)]x 10-6 erg G-2 mol-1 = 168 x 10-6 erg G-2 mol-1
m corr = 4.2722 x 10-3erg G-2 mol-1 + 168 x 10-6 erg G-2 mol-1
m corr = 4.4402 x 10-3 erg G-2 mol-1
4. Effective Magnetic Moment
μeff = 2.84(m corr T)1/2
Where m corr = Corrected molar susceptibility
T = in Kelvins
μeff = 2.84(4.4402 x 10-3 erg G-2 mol-1(294.0))1/2
μeff can be approximated to be the μs if there is only spin contribution so,
μeff = 3.24484146 BM
μs = 4.8-5.0 BM for Cr(II) high spin = 4 unpaired electrons
μs = 3.0-3.3 BM for Cr(II) low spin = 2 unpaired electrons
μs = 3.7-3.9 BM for Cr(III) with 3 unpaired electrons
Determination of Magnetic Moments (copper(II) acetate hydrate)
The following equations are needed to determine the magnetic moment
1. Mass magnetic susceptibility (ignoring volume susceptibility)
g = CL(R-Ro) / (1 x 109 (m))
g = (1.002)(2.5cm)(53- (-60)) / (1 x 109 (0.0758 g))
g = 3.734 x10-5 erg G-2 g-1
2. Molar susceptibility:
m = g x (molecular weight in g/mol)
m = 3.734 x10-5 erg G-2 g-1 x 376.2 g/mol
m = 1.4048 x 10-3 erg G-2 mol-1
3. Molar susceptibility corrections:
m corr = m + ∑correction
Cu2(CH3CO2)4(H2O)2
∑correction = [2(13) + 4(29) + 2(13)] x 10-6 erg G-2 mol-1 = 168 x 10-6 erg G-2 mol-1
m corr = 1.4048 x 10-3 erg G-2 mol-1+ 168 x 10-6 erg G-2 mol-1
m corr = 1.5729 x 10-3 erg G-2 mol-1
4. Effective Magnetic Moment
μeff = 2.84(m corr T)1/2
μeff = 2.84(1.5729 x 10-3 erg G-2 mol-1 (294.0))1/2
μeff = 1.931248904 BM
Table 3.0: Results for chromium(II) acetate hydrate
Chromium(II) acetate hydrate
The value of g: g = 1.1356 x 10-5 erg G-2 g-1
The value of m: 4.2722 x 10-3 erg G-2 mol-1
The value of μeff: 3.24484146 BM
The number of the unpaired electrons: 2 eAccording to experimental results this complex approximately corresponds to values of magnetic
moment that have 2 unpaired electrons (Cr2+ in low spin) as indicated by the chart given in
tutorial by Prof. Greenberg
Table 4.0: Results for copper (II) acetate hydrate
Copper(II) acetate hydrate
The value of g: 3.734 x10-5 erg G-2 g-1
The value of m: 1.4048 x 10-3 erg G-2 mol-1
The value of μeff: 1.931248904 BM
The number of the unpaired electrons: 1 eAs seen off the table given in tutorial by Prof. Greenberg, the experimental magnetic moment
approximately corresponds to 1 unpaired electron in this complex
Discussion
The purpose of this experiment was to synthesize chromium(II) acetate hydrate and
copper(II) acetate hydrate. These two complexes exhibit some sort of interesting metal-metal
bond that resembled the Re quadruple metal to metal bond. The magnetic moment of these two
complexes were determined with the aid of the Evans balance that computes the magnetic
susceptibility of both complexes. These magnetic moments can be used as indicator of the bond
strength as well as how many electrons are in the structure of the complex. Determination of the
magnetic moment allowed the electronic configuration of the metal centres to be analyzed.
In the procedure, the nitrogen bubbling was omitted due to time constraints but it would
have been more beneficial as it would decrease the amount of oxygen around the product at all
times as discussed later. The deoxygenated water was used to minimize the exposure of the
solution to oxygen which can oxidize the chromium (II) metal to chromium (III) metal and zinc
was actually used to reduce the chromium(III) from CrCl36H2O to chromium(II) metal.
The mass magnetic susceptibility was converted to molar suspectibility and corrected by
adding the molar susceptibilities that are exhibited by other atoms in that complex to determine
the effective magnetic moment. In this experiment, it is assumed that the magnetic moment can
be approximated by the spin contribution only. The magnetic moment for chromium(II) acetate
hydrate and copper(II) acetate hydrate were determined to be approximately 3.245 BM and 1.931
BM, respectively.
The expected metal-metal bond from chromium(II) would have been 4.8-5.0 BM if it was
high spin d4 or 3.0-3.3 BM if it was low spin d4.Since the experimental magnetic moment fits
into the range given for the low spin d4 chromium(II) metal, it can be determined that the
complex was accurately synthesized and the metal centre did not yet oxidize. The complex
synthesized in this experiment was considered to have a chromium(II) metal centre with low spin
and two unpaired electrons, which would make this metal diamagnetic. The copper(II) acetate
hydrate was expected to contain two d9 copper(II) ions with a magnetic moment between 1.8-2.1.
The magnetic moment may have some orbital momentum contribution causing it to deviate from
the spin-only approximation due to its relatively larger radii. The experimentally determined
complex was accurately synthesized as the magnetic moment fit into this range, indicating one
unpaired electron and a paramagnetic metal centre. If itwere paramagnetic, then the magnetic
susceptibility should be much higher than the 053 reading, which leads to the metal-metal bond
molecular orbital as the coupling on these metals sequentially leads to low spin acetate
complexes.
By examining literature values, chromium(II) acetate hydrate was found to have Cr(II)
high spin with 4 unpaired electrons and copper(II) acetate hydrate was found to have 1 unpaired
electron. Therefore, both these metal centres would be paramagnetic. However, the dimer
complexes they form are both diamagnetic. When a molecular orbital diagram is created for the
chromium(II)-chromium(II) bond (Figure 3.0), when the metal-metal bond is formed, there is
one sigma bond, two pi bonds and one delta bond.
Figuree u 3.0. Molecular orbital diagram of a chromium(II)-chromium(II) bond
Since all the electrons filled in the molecular orbital diagram are all paired, it can be
concluded that the chromium(II) acetate hydrate complex is a diamagnetic complex. The bond
order can be calculated using ½ (bonding electrons – antibonding electrons). In this case, the
chromium(II)-chromium(II) bond has eight bonding electrons and zero antibonding electrons,
thus the bond order is four which corresponds to a quadruple bond between the two
chromium(II) atoms in the complex synthesized. Therefore, this complex would have a metal-
metal bond that very strong and short. This would account for the shorter bond length of 236 pm
in the complex compared to the 250 pm bond length in metallic chromium. The complex is
actually a dimer with a chromium-chromium quadruple bond formed from the overlap of four dorbitals (dxz, dyz, dx2-y2, and dz2) from each metal. Also deducible is that the structure at each
chromium metal is octahedral as it has a coordination number of six.
The copper(II) ion exhibits similar characteristics as the chromium(II) ion as it is a
paramagnetic metal, but upon forming a copper(II)-copper(II) bond, there are a total of eighteen
electrons. These are used to form one sigma bonding orbitals, two pi bonding and antibonding
orbitals, and two delta bonding and antibonding orbitals. All the bonding and antibonding
orbitals have no effect on the bond order because they cancel each other out. Instead, the bond
order is just the two paired sigma electrons which give a bonder order of one. Therefore, there is
one single bond between the two copper metals. It is a relatively weaker bond which accounts for
the longer bond length of 264 pm than its regular metallic bond length. The complex forms a
dimer of two octahedral complexes with copper(II) metal centres similar to the chromium(II)
acetate hydrate. Therefore, it can be concluded that the metal-metal bond strength is relatively
stronger in the chromium(II) acetate hydrate dimer complex than the copper(II) acetate hydrate
dimer complex, all by using the magnetic moment determined from the Evans balance.
In any experiment, there are great discrepancies from experimental error; however the
ones present in this experiment are critical to the magnetic moment value. A major error in this
experiment is the oxygen insensitivity as there were many steps at which chromium could have
been oxidized which would alter the complex formed and its electronic configuration. An
example of this is during filtration, as some of the precipitate had a colour change of red to darkand unsaturated red colour. In this step, the precipitate was very vulnerable to oxidation during
the whole procedure of cleaning it with deoxygenated water, alcohol and ether. Sometimes, the
apparatus would still be wet with water droplets from previous cleaning. Even the distilled water
can become a source of oxygen which can cause chromium(II) to oxidize by donating a valence
electron to oxygen. A way to minimize the presence of oxygen is streaming nitrogen gas
throughout the experiment as nitrogen readily reacts with oxygen in the air to form nitrogen
oxides so oxygen in the air around the product is depleted. Other possible ways for oxygen to
interact were from the deoxygenated water that was sitting out too long. Instead, it should have
been gotten from the source every time before usage. Also, oxygen was allowed to interact when
the parafilm was removed or reapplied. Other errors could have risen from the Evans balance as
any sort of vibration or temperature or pressure change around the machine could disturb the
system and distort the readings. In addition, tight perfect packing of the precipitate was not
perfectly achieved and could have led to discrepancies in the magnetic susceptibility readings.
The IR spectrum provided for each of the two complexes were very similar in terms of
the wavelengths at which peaks occur indicating asymmetrical and symmetrical stretches in the
bonds of the atoms. The transmittance values are slightly higher for the copper acetate complex
for every peak because of its relatively weaker bond strength compared to the strong quadruple
bond and thus the copper(II) acetate hydrate can have stretches in its bonds to be longer and give
a higher transmittance values. The wavelengths in the 1500 cm-1 range are due to vibronic
stretches from the acetone ligand.
References
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(accessed November 7, 2010)
2. “Copper(II) Acetate.” Wikipedia. http://en.wikipedia.org/wiki/Copper(II)_acetate
(accessed November 7, 2010).
3. Copper, F. A.; Walton, R. A. After 155 Years, A Crystalline Chromium-Chromium
Carboxylate with a supershort Cr-Cr Bond. J. Am. Chem. Soc. 2000, 122, 416-417.
4. “Determination of Magnetic Moment.” Blackboard Academic Suite.
http://portal.utoronto.ca (accessed November 5, 2010).
5. “Experiment 6 Determination of Magnetic Moments.” Blackboard Academic Suite.
http://portal.utoronto.ca (accessed November 5, 2010).
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