Objectives

Grading

Your will be determined according to the following:

Introduction

The transition metals are the largest “group” (classification) of elements from the periodic table. These can be found in nature as ores or in its elemental form, such as gold. All transition metals have more than one oxidation state. Most transition metals (TMs) can complex with other species (called ligands in “TM Complex” jargon) by giving their electrons to them, forming a complex. These ligands, which are the nearest neighbor atoms to the metal center, constitute the inner (or first) coordination sphere. Complexes may be either neutral or charged and have distinctive properties that may be quite unlike those associated with their constituent molecules and ions, each of which is capable of independent existence. An example of a charged complex is ferricyanide, [Fe(CN)6]−3[Fe(CN)6]−3 size 12{ \[ ital "Fe" \( ital "CN" \) rSub { size 8{6} } \] rSup { size 8{ - 3} } } {}. The Fe+3Fe+3 size 12{ ital "Fe" rSup { size 8{+3} } } {} and CN−CN− size 12{ ital "CN" rSup { size 8{ - {}} } } {} ions found in the ferricyanide complex ion exist as independent species and 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, Co(III), and ethylenediamine.

Stereochemistry

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

Figure 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 (A) such as Cu(CN)2−Cu(CN)2− size 12{ ital "Cu" \( ital "CN" \) rSub { size 8{2} } rSup { size 8{ - {}} } } {}, Ag(NH3)2+Ag(NH3)2+ size 12{ ital "Ag" \( ital "NH" rSub { size 8{3} } \) rSub { size 8{2} } rSup { size 8{+{}} } } {}, etc. Four-fold coordination (C) is not too common with transition metals, and the square planar geometry (B) occurs in complexes of Pd(II), Pt(II), Ni(II), Cu(II), and Au(III). Six-fold coordination (D) is the most common and in fact the one we will study in this laboratory exercise.

A ligand that is capable of occupying only one position in the inner coordination sphere by forming only one bond to the central atom is called a monodentate (“one tooth”) ligand. Examples are F−F− size 12{F rSup { size 8{ - {}} } } {}, Cl−Cl− size 12{ ital "Cl" rSup { size 8{ - {}} } } {}, OH−OH− size 12{ ital "OH" rSup { size 8{ - {}} } } {}, H2OH2O size 12{H rSub { size 8{2} } O} {}, NH3NH3 size 12{ ital "NH" rSub { size 8{3} } } {} and CN−CN− size 12{ ital "CN" rSup { size 8{ - {}} } } {}. If the ligand has two groups that are capable of bonding to the central atom, it is called a bidentate ("two teeth") ligand, and so forth. An example of a bidentate ligand is ethylenediamine (CH2NH2CH2NH2)(CH2NH2CH2NH2) size 12{ \( ital "CH" rSub { size 8{2} } ital "NH" rSub { size 8{2} } ital "CH" rSub { size 8{2} } ital "NH" rSub { size 8{2} } \) } {}, which is commonly abbreviated "en". Both nitrogen atoms in "en" can bond to the central atom in a complex at the same time.

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)6Cl3Cr(H2O)6Cl3 size 12{ ital "Cr" \( H rSub { size 8{2} } O \) rSub { size 8{6} } ital "Cl" rSub { size 8{3} } } {}. One of these, violet in color, reacts immediately with AgNO3AgNO3 size 12{ ital "AgNO" rSub { size 8{3} } } {} to precipitate all of the chlorines as AgCl. The second is light green but only ⅔ of the chlorine is precipitated as AgCl. The third compound is dark green and only ⅓ of the chlorine is precipitated as AgCl. The last compound has only one reactive Cl, 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]⋅(H2O)2[CrCl2(H2O)4]⋅(H2O)2 size 12{ \[ ital "CrCl" rSub { size 8{2} } \( H rSub { size 8{2} } O \) rSub { size 8{4} } \] cdot \( H rSub { size 8{2} } O \) rSub { size 8{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,

[ CrCl 2 ( H 2 O ) 4 ] Cl ⋅ ( H 2 O ) 2 → [ CrCl 2 ( H 2 O ) 4 ] + + Cl − + water [ CrCl 2 ( H 2 O ) 4 ] Cl ⋅ ( H 2 O ) 2 → [ CrCl 2 ( H 2 O ) 4 ] + + Cl − + water size 12{ \[ ital "CrCl" rSub { size 8{2} } \( H rSub { size 8{2} } O \) rSub { size 8{4} } \] ital "Cl" cdot \( H rSub { size 8{2} } O \) rSub { size 8{2} } rightarrow \[ ital "CrCl" rSub { size 8{2} } \( H rSub { size 8{2} } O \) rSub { size 8{4} } \] rSup { size 8{+{}} } + ital "Cl" rSup { size 8{ - {}} } + ital "water"} {}

The light green compound with two reactive chlorines is apparently [CrCl(H2O)5]Cl2⋅H2O[CrCl(H2O)5]Cl2⋅H2O size 12{ \[ ital "CrCl" \( H rSub { size 8{2} } O \) rSub { size 8{5} } \] ital "Cl" rSub { size 8{2} } cdot H rSub { size 8{2} } O} {}, while the violet compound with three reactive chlorines is Cr(H2O)6Cl3Cr(H2O)6Cl3 size 12{ ital "Cr" \( H rSub { size 8{2} } O \) rSub { size 8{6} } ital "Cl" rSub { size 8{3} } } {}.

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)5SO4BrCo(NH3)5SO4Br size 12{ ital "Co" \( ital "NH" rSub { size 8{3} } \) rSub { size 8{5} } ital "SO" rSub { size 8{4} } ital "Br"} {}. One of these, [Co(NH3)5(SO4)]Br[Co(NH3)5(SO4)]Br size 12{ \[ ital "Co" \( ital "NH" rSub { size 8{3} } \) rSub { size 8{5} } \( ital "SO" rSub { size 8{4} } \) \] ital "Br"} {}, appears red, whereas the other, [Co(NH3)5Br]SO4[Co(NH3)5Br]SO4 size 12{ \[ ital "Co" \( ital "NH" rSub { size 8{3} } \) rSub { size 8{5} } ital "Br" \] ital "SO" rSub { size 8{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 (NO2)(NO2) size 12{ \( ital "NO" rSub { size 8{2} } \) } {} ligands. The geometrical isomer you will make is the trans form, in which the NO2NO2 size 12{ ital "NO" rSub { size 8{2} } } {} ligands are not adjacent to one another. This difference between cis and trans octahedral isomers is shown in Fig 2.

Figure 2

Fig 2. The trans and cis geometrical isomers for octahedral complexes with two bidentate (“en”) and monodentate (NO2)(NO2) size 12{ \( ital "NO" rSub { size 8{2} } \) } {} ligands specifically dinitrobis(ethylenediamine)Co(III). The two black balls represent the NO2NO2 size 12{ ital "NO" rSub { size 8{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 NO2NO2 size 12{ ital "NO" rSub { size 8{2} } } {} ligands. 

In the procedure that follows we start with a cobalt solution made from the salt hexaquacobalt(II) nitrate, [Co(H2O)6](NO3)2[Co(H2O)6](NO3)2 size 12{ \[ ital "Co" \( H rSub { size 8{2} } O \) rSub { size 8{6} } \] \( ital "NO" rSub { size 8{3} } \) rSub { size 8{2} } } {}. When this salt dissolves it ionizes to form two ions of NO3−NO3− size 12{ ital "NO" rSub { size 8{3} } rSup { size 8{ - {}} } } {} and one of Co(H2O)62+Co(H2O)62+ size 12{ ital "Co" \( H rSub { size 8{2} } O \) rSub { size 8{6} } rSup { size 8{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 OH−OH− size 12{ ital "OH" rSup { size 8{ - {}} } } {} ions which can also bind to Co(II), so the pH is adjusted close to 7 first by adding HNO3HNO3 size 12{ ital "HNO" rSub { size 8{3} } } {}. (Other acids would introduce new ligands to compete for the Co.) NaNO2NaNO2 size 12{ ital "NaNO" rSub { size 8{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

Figure 3

Figure 3. The bubbling apparatus.

Figure 4

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.

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

4 { [ Co ( H 2 O ) 6 ] ( NO 3 ) 2 } + 8 NaNO 2 + 8C 2 H 4 ( NH 2 ) 2 + 4 HNO 3 + O 2 ( g ) → 4 trans − 4 { [ Co ( H 2 O ) 6 ] ( NO 3 ) 2 } + 8 NaNO 2 + 8C 2 H 4 ( NH 2 ) 2 + 4 HNO 3 + O 2 ( g ) → 4 trans − size 12{4 lbrace \[ ital "Co" \( H rSub { size 8{2} } O \) rSub { size 8{6} } \] \( ital "NO" rSub { size 8{3} } \) rSub { size 8{2} } rbrace +8 ital "NaNO" rSub { size 8{2} } +8C rSub { size 8{2} } H rSub { size 8{4} } \( ital "NH" rSub { size 8{2} } \) rSub { size 8{2} } +4 ital "HNO" rSub { size 8{3} } +O rSub { size 8{2} } \( g \) rightarrow 4 ital "trans" - {}} {}

[ Co ( en ) 2 ( NO 2 ) 2 ] NO 3 + 8 NaNO 3 + 26 H 2 O [ Co ( en ) 2 ( NO 2 ) 2 ] NO 3 + 8 NaNO 3 + 26 H 2 O size 12{ \[ ital "Co" \( ital "en" \) rSub { size 8{2} } \( ital "NO" rSub { size 8{2} } \) rSub { size 8{2} } \] ital "NO" rSub { size 8{3} } +8 ital "NaNO" rSub { size 8{3} } +"26"H rSub { size 8{2} } O} {}

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

NH2CH2CH2NH3+NH2CH2CH2NH3+ size 12{ ital "NH" rSub { size 8{2} } ital "CH" rSub { size 8{2} } ital "CH" rSub { size 8{2} } ital "NH" rSub { size 8{3} } rSup { size 8{+{}} } } {}. From the reaction and quantities used, calculate the theoretical yield and your percentage yield.

1. Synthesis

A. Volume of 20% ethylenediamine solution used ______ (r = 0.980 g/mL)

a. Observations

b. Questions