Synthesis of a Coordination Compound



Synthesis of a Coordination Compound

Purpose

• To produce a coordination compound.

• To calculate the percent yield of the synthesis.

Introduction

Chemical reactions involving the transfer of protons (H+ ions) are commonly classified as Bronsted-Lowry acid-base reactions. Many reactions that occur without a proton transfer can be classified as Lewis acid-base reactions. Substances with empty electron orbitals that can accept a pair of electrons are Lewis acids; those substances that can donate a pair of electrons in the formation of a covalent bond are called Lewis bases. The reaction of a Lewis acid and a Lewis base results in the formation of an adduct, which has a bond called a coordinate covalent bond. The Lewis base supplies both of the electrons required for the formation of this bond. The term coordination refers to the process in which one or more Lewis bases donate a pair of electrons to a Lewis acid to form adducts or complexes.

The transition metal ions, with their many empty valence orbital, often act as Lewis acids toward atoms having nonbonding electron pairs to form coordination complexes. The Lewis bases, the electron-pair donors in the complex, are called ligands. The number of ligand atoms, molecules, or units attached to the central ion is defined as the coordination number of the complex. The following are examples of such complexes with coordination numbers of 2, 4, and 6 respectively.

Many ligands are bifunctional or polyfunctional in nature; that is, they are able to act as Lewis bases at more than one site. When one ligand can coordinate through two sites, it is called bidentate. When the bidentate (literally “two teeth”) ligand forms a ring structure with the metal ion, it is termed a chelate ring. The formation of transition metal complexes and chelates greatly increases the solubility of the metal ions in aqueous solution. The ligand used in this experiment, 1,2-diaminoethane (H2NCH2CH2NH2), commonly called ethylenediamine, is a bidentate ligand. You will prepare a complex of nickel(II) that contains three ethylenediamine ligands.

NiCl2.6H2O(aq) + 3 H2NCH2CH2NH2(aq) ( [Ni(H2NCH2CH2NH2)3]Cl2(s) + 6H2O(l)

Collected evidence indicates that iron and copper are essential to almost every form of life. In fact, iron and copper seem to be the only metals capable of involvement in the important respiratory metalloproteins responsible for transport of oxygen. Respiratory metalloproteins attracted early recognition because of their vivid colors, red for those containing iron and blue for those containing copper. Most of these complexes are of the hemoglobin type, in which a protein is attached to a heme group. In heme, the four ligand groups form a square planar complex with the iron; the remaining fifth and sixth coordination positions of iron are perpendicular to the plane of the ring. In hemoglobin, a complex protein is attached to the fifth position and water or oxygen to the sixth position.

Many mollusks and aquatic arthropods (crabs and lobsters) have respiratory pigments called hemocyanins that are carried in solution in the blood. The hemocyanins are proteins much like hemoglobin, except that copper(II) replaces the iron of the hemes. When oxygenated, hemocyanins are blue rather than red. Other examples of metallocomplexes include myoglobin, chlorophyll, and cyanocobalamin (Vitamin B12), which contain the metals iron, magnesium, and cobalt respectively.

Procedure

1. Accurately weigh a 4.5-5.0 g sample of nickel(II) chloride hexahydrate into a clean dry 100 mL beaker. Note the appearance of the compound.

2. Completely dissolve this sample by stirring in a maximum of 10 mL of distilled water. Note the color of the solution.

3. Slowly add 18.0 - 20.0 mL (measured to nearest 0.1 mL) of 25% ethylenediamine solution to the beaker while constantly stirring.

4. Add 50 mL of acetone in 10 mL increments, stirring well after the addition of each portion.

5. Continue to stir until precipitation begins. This may require a few minutes of fairly vigorous scratching of the wall of the beaker with the stirring rod.

6. After precipitation has begun, allow the beaker to stand undisturbed for a few minutes to cool to room temperature. Note the appearance of the mixture.

7. Cool the beaker in an ice bath to maximize precipitation of the product. The product is ionic and soluble in water, but is not soluble in the less polar acetone.

8. Set up a suction filtration apparatus. Turn on the aspirator; completely wet the filter paper with a small amount of distilled water.

9. Transfer the slurry (the mixture of the solid and liquid) to the funnel. If some of the precipitate gets through or around the filter paper at first, turn off the suction, pour the filtrate back into the beaker, and refilter as before. If any precipitate remains in the beaker, add a few mL of acetone to the beaker, scrape the particles loose using a rubber policeman on the end of a stirring rod, and transfer them into the funnel. Allow the vacuum to draw air through the precipitate for about two minutes.

10. Turn off the suction. Break up the solid with a spatula, being very careful not puncture the filter paper, and then add 10 mL of acetone to the solid as a wash. Let stand 2-5 minutes.

11. Pull this was liquid through the solid, then repeat the entire process (including breaking up the solid) with two additional 10 mL portions of acetone.

12. Pour the wash liquids from the flask, rinse them down the sink with a great amount of water, and pull air through the solid for about five minutes to dry it. By this time the product should be a free-flowing, microcrystalline powder. Record the appearance of the product.

13. Carefully transfer the product to a weighed vial labeled with your name. Leave the material in the vial with the cap off to dry overnight.

14. The next day, determine the mass of the product.

Calculations

1. Calculate the mass of pure ethylenediamine used. Because ethylenediamine is a thick, viscous, strongly alkaline liquid, it is commonly used in solution rather than as a pure liquid. The mass of the ethylenediamine is determined from the volume of the solution and its density. The solution used in this experiment is 25.0% by mass ethylenediamine. The solution has a density of 0.950 g/mL.

2. Calculate the number of moles of each reactant used.

3. Determine which reactant is the limiting reagent.

4. Calculate the theoretical yield of the complex. The molar mass of the complex, tris(ethylenediame)nickel(II) chloride, is 310.0 g/mole.

5. Calculate percent yield.

Data

|Mass of NiCl2.6H2O | |

|Appearance of NiCl2.6H2O | |

|Color of solution of NiCl2.6H2O | |

|Volume of ethylenediamine solution | |

|Concentration of ethylenediamine solution | |

|Appearance of ethylenediamine solution | |

|Appearance of reaction mixture | |

|Appearance after addition of acetone | |

|Color of product (complex) | |

|Mass of empty labeled vial | |

|Mass of empty labeled vial + product | |

|Mass of product (actual yield) | |

Results

|Mass of ethylenediamine | |

|Moles of NiCl2.6H2O | |

|Moles of ethylenediamine | |

|Limiting reagent | |

|Theoretical yield | |

|Percent yield | |

Questions

1. Explain why it is suggested that a minimum amount of water be used to dissolve the NiCl2.6H2O.

2. Assuming all other things are equal and constant, explain why you can’t increase the percent yield by increasing the amount of limiting reagent.

3. Why do you suppose the yield of this synthesis is less than 100%?

4. A student reports a 110% yield. Assuming the calculations are correct, how could you explain this erroneous result?

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