8 th April 2002/MM

Transitions Metals: Synthesis of an Inorganic Compound (trans-dinitrobis(ethylenediamine)cobalt(III) nitrate)

Objective

• To synthesize a transition metal complex containing cobalt, in the form of Co(III), and ethylenediamine.
• To characterize the resulting compound spectroscopically.

Grading

You will be assessed on

• the correctness and thoroughness of your observations (both weeks).
• the ability to calculate a percent yield (Week 1).
• understanding of the concept of a limiting agent (Week1).
• explaining solubility trends (Week 2).
• calculation of molar absoptivity (Week 2).
• calculation of magnetic moment and explanation (Week 2).

Link to the Report Form here.

Introduction

A large class of complex inorganic ions seem to behave as single species. An example is ferricyanide, [Fe(CN)₆]^-3. These complex ions (and neutral complex compounds as well) have distinctive properties that may be quite unlike those associated with their constituent molecules and ions, each of which is capable of independent existence. (For example, the Fe⁺³ and CN^- ions found in the ferricyanide complex ion exist as independent species in other compounds.) The transition metals are well known for forming a large number of complex ions. In this experiment we will synthesize a transition metal complex containing cobalt, in the form of Co(III), and ethylenediamine.

In general, a complex ion or compound consists of a central atom closely surrounded by a number of other atoms or molecules that can bind to the central atom by "donating" electrons to that atom. The surrounding species are called ligands. The nearest neighbor atoms to the central atom constitute the inner (or first) coordination sphere.

Stereochemistry

The most common coordination numbers (the number of individual ligands bound) are two, four, and six, with geometries illustrated in Fig 1:

Fig 1. Common geometries for complex ions. (A) linear, (B) square planar, (C) tetrahedral, and (D) octahedral

Complexes of Cu(I), Ag(I), Au(I) and some of Hg(II) form linear structures such as Cu(CN)₂^-, Ag(NH₃)₂⁺, etc. Four-fold coordination (C) is not too common with transition metals, and the square planar geometry occurs in complexes of Pd(II), Pt(II), Ni(II), Cu(II), and Au(III). Six-fold coordination is the most common and the geometry that we are interested in.

A ligand that is capable of occupying one position in the inner coordination sphere and forming one bond to the central atom is called a monodentate ligand. Examples are F^-, Cl^-, OH^-, H₂O, NH₃ and CN^-. If the ligand has two groups that are capable of bonding to the central atom, it is called a bidentate ("two teeth") ligand. An example of a bidentate ligand is ethylenediamine (CH₂NH₂CH₂NH₂), which is commonly abbreviated "en". Both nitrogen atoms in "en" can bond to the central atom in a complex.

Complex ion salts with the same chemical formulas often behave differently because the same number of atoms can be arranged into different forms called isomers. Hydrate isomerism is illustrated by the following example: There are three distinct compounds with the formula Cr(H₂O)₆Cl₃. One of these, violet in color, reacts immediately with AgNO₃ to precipitate all of the chlorines as AgCl. The second is light green but only 2/3 of the chlorine is precipitated as AgCl, The third is dark green and only 1/3 of the chlorine is precipitated as AgCl. The last compound has only one reactive chlorine, so apparently two chlorines in this compound are bonded tightly to the Cr and are not available for reaction. We might thus write this compound as [CrCl₂(H₂O)₄]Cl·(H2O)₂, where the species within the brackets are regarded as ligands bonded fairly strongly to the central chromium, and this species would behave as a single ion in solution. i.e., in aqueous solution,

The light green compound with 2 reactive chlorines is apparently [CrCl(H₂O)₅]Cl₂·H₂O, while the violet compound with 3 reactive chlorines is [Cr(H₂O)₆]Cl₃. Closely related to hydrate isomerism is ionization isomerism, where an ion takes the place of water. Consider two different compounds with the formula Co(NH₃)₅SO₄Br. One of these, [Co(NH₃)₅(SO₄)]Br, appears red, whereas the other, [Co(NH₃)₅Br]SO₄, appears violet.

In addition to these coordination sphere isomers there are geometrical isomers, which have coordination spheres of the same composition but different geometrical arrangement. Geometrical isomers are distinct compounds and can have different physical properties (although often not too different) such as color, crystal structure, melting point, and so on. For example, dichloro diammine platinum (II) occurs in the square planar geometry (B) so the chlorine ligands can be either next to one another (cis) or opposite from one another (trans). The compound you will synthesize has an octahedral geometry with two (bidentate) "en" ligands, and two nitro (NO₂) ligands. The geometrical isomer you will make is the trans form, in which the NO₂ ligands are not adjacent to one another. This difference between cis and trans octahedral isomers is shown in Fig 2.

Fig 2. The trans and cis geometrical isomers for octahedral complexes with two bidentate and monodentate ligands specifically dinitrobis(ethylenediamine)Co(III). The two black balls represent the NO2 monodentate ligands and the two pairs of linked white balls represent the two bidentate ethylenediamine ligands. Cis and trans describe the relationship between the two NO₂ ligands.

In the procedure that follows we start with a cobalt solution made from the salt hexaquacobalt(II) nitrate, [Co(H₂O)₆](NO₃)₂. When this salt dissolves it ionizes to form NO₃^- and Co(H₂O)₆²⁺.) We wish to prepare a Co(III) compound of ethylenediamine, so we must add ethylenediamine (en) and oxidize the Co(II). Because Co(II) is more reactive than Co(III), we allow it to react with (en) first, and then oxidize the resulting complex ion. In aqueous solution (en) reacts with water to produce OH^- ions which can also bind to Co(II), so the pH is adjusted close to 7 by adding HNO₃. (Other acids would introduce new ligands to compete for the Co.) NaNO₂ is added to provide the ligands that will be trans in the final compound. Lastly, Co(II) is oxidized to Co(III) by bubbling oxygen through the solution.

Procedure

• Use your 10 mL graduated cylinder to measure out 20 mL of the 20% by weight solution of ethylenediamine in dilute HNO₃.
• Pour it into a clean 125 mL Erlenmeyer flask, then rinse the cylinder with about 5mL of deionised water and add the rinse water to the flask. Set this aside for a moment and prepare the second set of reactants as described below.
• Weigh out 9.0 g of hexaquacobalt(II) nitrate and 6.0 g sodium nitrite using a rough balance. Add these reactants to approximately 15 mL of deionised water in an Erlenmeyer flask. After they have dissolved, add the neutralized ethylenediamine solution prepared above. Record your observations.
• For the next set of instructions, refer to the diagram below. Fit a piece of rubber tubing over an "IG" (inert gas) tap and open the valve slowly to obtain a gentle flow of oxygen. Then insert a Pasteur pipet into the other end of the rubber tubing. CAUTION: Too high a gas flow might blow the pipet out of the tubing and cause serious injury. Always adjust the valve carefully while pointing your pipet in a safe direction. Test the flow by immersing the pipet tip in a beaker of water--it should bubble vigorously, but not enough to cause much splashing. When the flow is set to your satisfaction, immerse the tip of the pipet in the Erlenmeyer flask containing the reaction mixture. Secure the flask to a stand with a clamp because the reaction mixture may need about 10 minutes of moderately vigorous bubbling to reach completion. Record your observations.

  Figure 3. The bubbling apparatus.

• After about 10 minutes of bubbling, turn off the gas flow and immerse the flask in ice water. This will cause further crystallization. After approximately 5 minutes in the ice bath, pour the flask's contents through the filter crucible while it has suction applied using the setup shown below. Record your observations.

  Figure 4. Schematic diagram showing sintered-glass filter crucible mounted on suction flask with rubber filter adapter. Clamp the filter flask to a support post to prevent breakage.

• The crystals will remain in the crucible while the solution passes through. Wash your crystals by slowly pouring approximately 5 mL of ethanol over them while suction is applied. Why do we wash with ethanol? Answer on lab report form.
• The next step is recrystallization to obtain a more purified product. Transfer the product crystals to a 250 mL beaker. Add about 80 mL of deionised water and stir to dissolve the crystals. Gently heat the beaker over a Bunsen burner, gradually bringing it to a boil. Allow the solution to boil gently until its volume has been reduced to about 50 mL. Then shut off the flame. When the solution has cooled to near room temperature, place the beaker into an ice bath. Crystal growth should be immediately apparent. After a few minutes in the ice bath, transfer these crystals into the filter crucible. To help with this transfer you may use a rubber policeman on the end of a stirring rod, but remember that your crystals are water-soluble. If you use water in the transfer you will lose the product. Apply suction and rinse the crystals three times with separate 5 mL portions of ethanol. Scrape the crystals onto a watch glass and place in your desk to dry. This compound will be used in the second week of the experiment.

In terms of the materials used, the overall reaction is:

4{[Co(H₂O)₆](NO₃)₂} + 8 NaNO₂ + 8 C₂H₄(NH₂)₂ + 4 HNO₃ + O₂(g) --->

4 trans-[Co(en)₂(NO₂)₂]NO₃ + 8 NaNO₃ + 26 H₂O

However, the actual reaction in solution involves ions and the en species exists partially in the form of

NH₂ CH₂CH₂NH₃⁺. From the reaction and quantities used, calculate the theoretical yield and your percentage yield.

Solubility

The solubility of a compound is an important consideration in its' usefulness and enables predictions to be made if the compound will completely dissociate, in say water, as to whether a reaction will go to completion. Obviously, compounds vary in their ablility to dissolve in say water, and thus produce solutions of varying concentrations. The terms "slightly soluble", "soluble", and "insoluble" are used qualitatively to describe such solutions. The approximate solubilites of substances are indicated by the descriptive terms in the table below. Factors that influence solubility are the temperature and nature of the solvent: both are needed in order to make a valid comparison.

_______________	Descriptive Solubility Terms	______________________
Term	Grams solvent/1 g of solute	Grams solute/100g of solvent
Very soluble	less than 1	more than 100
Freely soluble	1-10	10-100
Soluble	10-30	3-10
Sparingly soluble	30-100	1-3
Slightly soluble	100-1,000	0.1-1
Very slightly soluble	1,000-10,000	0.01-0.1
Insoluble	more than 10,000	less than 0.01

One useful property of transition metals is that, by definition, they have at least one oxidation state with an incomplete d or f subshell. Since electrons spin and generate a magnetic field, the magnetic properties of transition metals are of great interest in determining the oxidation state, electronic configuration, and so on. Most organic compounds and main group element compounds have all their electrons paired. Such molecules are diamagnetic and have very small magnetic moments. Many transition metal compounds, however, have one or more unpaired electrons, and are termed paramagnetic. The number of unpaired electrons on a given metal ion determines the magnetic moment, m, affecting it both by virtue of their spin and their orbital motion. The spin part is the more important, and a close estimate of the magnetic moment can be obtained using the equation

µ_s = g _`/ [S(S + 1)]

where g is the gyromagnetic ratio for an electron (~ 2) and S is the total spin of the unpaired electrons (at 1/2 each).

For one unpaired electron (as in Ti³⁺)

 µ_g = 2 _`/ [1/2(1/2+1)] = 1.732

The units of the magnetic moment are Bohr magnetons (BM). Actual magnetic moments are somewhat larger than the spin-only values obtained above, because of the orbital contribution. investigate which solvents your complex is soluble, sparingly soluble or insoluble in. For information on the operation of the magnetic susceptibility balance and calculation of the magnetic moment, please click here.

In addition to characterizing your complex by extent of solubility in various solvents and evaluating the magnetic moment, you will run a visible spectrum in order to calculate its' molar absorptivity.

Procedure

Solubilities

Here is a list of solvents that are available for you to use: acetone, acetonitrile, chloroform, hexane, methylene chloride, DMSO (dimethyl sulfoxide) ,THF (tetrahydrofuran), toluene, water. Test a minimum of 4 solvents and find at least 2 of which that exhibit some degree of solubility.

Measurement of visible spectrum

For this section, you will need to make up at least 3 standard solutions in order to plot your graph and find its' molar absorptivity. Looking at the color of your complex, should give you an idea of the dilutions that you need to make.

Measurement of Magnetic Susceptibility

There are a number of techniques that were used to determine the magnetic susceptibility of transition metal complexes. Your method will involve the Evans balance which 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 magnetics are on one end of a balance beam, and after interacting with the sample, change the position of the beam. This change is registered by a pair of photodiodes set on opposite sides of the balance beam's equilibrium position. The diodes send signals to an amplifier that in turn supplies current to a coil that will exactly cancel the interaction force. A digital voltmeter, connected across a precision resistor, in series with the coil measures the current directly and this is displayed on the digital readout.

The general expression for the mass magnetic susceptibility, X_g, for the Evans balance is

X_g =

where L = sample length in centimeters

m = sample mass in grams

C = balance calibration constant (different for each balance; printed on the

back of the instrument)

R = reading from the digital display when the sample (in the sample tube)

is in place in the balance

Ro = reading from the digital display when the sample (in the sample tube)

is in place in the balance

X'_v= volume susceptibility of air (0.029 X 10^-6 ergoG^-2 cm^-3 )

A = cross-sectional area of the sample

The calibration standards usually employed in magnetic susceptibility measurements are Hg[Co(SCN)₄] or [Ni(en)₃]S₂O₃, and have values of 1.644 X 10^-5 and 1.104 x 10^-5 ergoG^-2 cm^-3, respectively. The volume susceptibility of air is usually ignored with solid samples, so that the mass magnetic susceptibility equation can be rewritten as follows:

Xg = CL(R - R_o)/(1x 10⁹(m))

Where X_g is in centimeter-gram-second (cgs) units of ergoG^-2 g^-1.

For this part of your experiment, you will determine the magnetic moment of your starting and synthesized Co complex, comment and explain the results.