Group Work 1
Reactions
Here are the structures of dmg and en.
a. Which atoms in en attracted to the Ni(2+)? (What do they have that the Ni(2+) desires?)
The N atoms because they have
electron pairs, and (-) is attracted to (+)
b. How do you think the dmg binds to Ni(II)?
Through the N atoms as well, since they have lone pair electrons and they have the right geometry for both NÕs to bind
c. What is similar about the ligands dmg and en from the perspective of the nickel ion? (pretend you are the Ni! What do you seeÓ?)
As far as the Ni(II) is concerned, the dmg and en both are 2 N-atom donor ligands, and very similar in shape and size.
Procedure for dmg reaction.
a -Take 3 testtubes. Use the disposable pipettes to transfer 0.10 M Ni(2+) and 0.10 M dmg solutions into each tube according to the following table:
Tube 1 Tube 2 Tube 3
mL (Ni2+) 1 1 1
mL dmg 2 1 0.5
b- What happens? What do your eyes tell you is going on in the testtubes?
A rose-colored ppt forms.
c - The Ni(2+) and dmg solutions are the same concentration, 0.1 M.
What is the significance of adding these two chemicals as a series?
When added as a variable ratio, one can compare the amount of product. The reaction that gives the most product has the optimal molar ratio.
d Ð What conclusions about the product stoichiometry can you draw from a comparison of the results in the test tubes? What would you do to determine the stoichiometry of the product of the reaction? Check your answer with dr. b. or a PLI.
Not much; it's hard to tell by
looking which tube has the most product.
You would need to filter the ppt, dry and weigh it to compare the results of the experiment.
e. Write a balanced equation for the reaction(s) your observed. Note that the Ni(2+) complex formed is neutral (uncharged). What must then be true of the dmg ligand bound to Ni(2+)? Draw the structure of the complex.
[Ni(H2O)6]2+ + 2 dmg ˆ Ni(dmg)2 + 2 [H+] —one -OH group on each dmg loses a proton.
B. Procedure
a- Take 5 vials. Transfer (as above) 0.10 M Ni(2+) and 0.10 M en solutions according to the following sequence. Cap the vials and invert the tubes to mix reagents.
Tube # 1 2 3 4 5
mL (Ni2+) 3 4 3 2 1
mL en 0 1 2 3 4
b- Based on your visual information, how many different species (i.e. different Ni complexes) are present in this series of testtubes?
There are four distinct colors: green, turquoise, blue and violet.
c. If you have the information that Ni(II) prefers to be octahedral with 6 attached molecules (ligands), can you speculate about what the stoichiometries are for the different complexes? Write balanced equations for the formation of each complex. Determine the acids and bases in your equations. Try to draw structures of the different complexes. (Use next page as necessary)
the turquoise solution: [Ni(H2O)6]2+ + en --> [Ni(en)(H2O)4]2+ + 2 H2O
the blue solution: [Ni(en)(H2O)4]2+ + en --> [Ni(en) 2 (H2O)2]2+ + 2 H2O
the violet solution: [Ni(en) 2 (H2O)2]2+ + en--> [Ni(en) 3]2+ + 2 H2O
Ni(II) is lewis acid; H20 and en are lewis bases
d. Now analyze the reactions you have done today in terms of acids and bases.
Based on your observations, which moleculethe solvent water H2O or the added ligand, enis more strongly attracted by Ni? How would you describe this difference using Lewis Acid/Base terminology?
Since much less en or dmg is present than H2O and the reaction still proceed forward to
completion, the en and dmg are stronger Lewis bases.
e. In which reaction today is there an example of both Lewis Acid/base reaction and Bronstead acid/base reaction?
In the dmg reaction. The dmg ligand reacts as a Bronstead acid to lose a H+ before
binding to Ni(II).
The dmg binds to Ni(II) as a Lewis base.
click here for Beyond Groupwork Answers