0.9 Transition metals  (Page 2/3)

$[{\text{CrCl}}_2(H_{2}O{)}_4]\text{Cl}\cdot (H_{2}O{)}_2\to [{\text{CrCl}}_2(H_{2}O{)}_4{]}^{+}+{\text{Cl}}^{-}+\text{water}$

The light green compound with two reactive chlorines is apparently $[\text{CrCl}(H_{2}O{)}_5]{\text{Cl}}_2\cdot H_{2}O$ , while the violet compound with three reactive chlorines is $\text{Cr}(H_{2}O{)}_6{\text{Cl}}_3$ .

Closely related to hydrate isomerism is ionization isomerism, where an ion takes the place of water. Consider two different compounds with the formula $\text{Co}({\text{NH}}_3{)}_5{\text{SO}}_4\text{Br}$ . One of these, $[\text{Co}({\text{NH}}_3{)}_5({\text{SO}}_4)]\text{Br}$ , appears red, whereas the other, $[\text{Co}({\text{NH}}_3{)}_5\text{Br}]{\text{SO}}_4$ , 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, dichlorodiamine 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 $({\text{NO}}_2)$ ligands. The geometrical isomer you will make is the trans form, in which the ${\text{NO}}_2$ 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 (“en”) and monodentate $({\text{NO}}_2)$ ligands specifically dinitrobis(ethylenediamine)Co(III). The two black balls represent the ${\text{NO}}_2$ ligands and the two pairs of linked white balls represent the two ethylenediamine ligands. Cis and trans describe the relationship (relative position) between the two ${\text{NO}}_2$ ligands. 

In the procedure that follows we start with a cobalt solution made from the salt hexaquacobalt(II) nitrate, $[\text{Co}(H_{2}O{)}_6]({\text{NO}}_3{)}_2$ . When this salt dissolves it ionizes to form two ions of ${\text{NO}}_3^{-}$ and one of $\text{Co}(H_{2}O{)}_6^{2+}$ . We wish to prepare a Co(III) compound of ethylenediamine, so we must add ethylenediamine (en) and oxidize the Co(II) to Co(III). 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 ${\text{OH}}^{-}$ ions which can also bind to Co(II), so the pH is adjusted close to 7 first by adding ${\text{HNO}}_3$ . (Other acids would introduce new ligands to compete for the Co.) ${\text{NaNO}}_2$ 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.

Experimental procedure

• Use your 10 mL graduated cylinder to measure out 20 mL of the 20% by weight solution of ethylenediamine in dilute ${\text{HNO}}_3$ .
• Pour it into a clean 125 mL Erlenmeyer flask. Rinse the graduated cylinder with about 5mL of deionised water (DI water from white handle faucet) 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 ( ${\text{NaNO}}_2$ ) using a rough balance (Record mass on report form). Add these reactants to approximately 15 mL of DI water in an Erlenmeyer flask. After they have dissolved, add the neutralized ethylenediamine solution prepared in steps 1-2. Record your observations.
• For the next set of instructions, refer to the diagram below. Fit a piece of rubber tubing over an inert gas "IG" tap (on benchtop) 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.