Download 21 - Department of Chemistry | Oregon State University

Chemistry 226H
Worksheet 2b
1.
Oregon State University
Dr. Richard Nafshun
What is a Lewis base? What is a Lewis acid?
Let me start by stating that we are familiar with many bases and acids. Those we know
to be bases are Lewis bases and those we know to be acids are Lewis acids. Our previous
ideas of bases and acids came from Arrhenius, Bronsted, and Lowry. These ideas
involved protons—bases accept a proton and acids donate a proton. Gilbert Lewis
expanded the definitions of bases and acids to include species that didn't necessarily
transfer a proton.
A Lewis base donates a pair of electrons to make a new bond.
A Lewis acid accepts a pair of electrons to make a new bond.
We previously identified ammonia as a base because of its ability to accept a proton:
:NH3 (aq) + H2O (l)  H:NH3+ (aq) + OH- (aq)
Ammonia is a Lewis base because it donates a pair of electrons to make a new bond (this
pair of electrons is shown in the above reaction).
Consider the reaction of a silver ion and two ammonia molecules to produce the
transition metal complex [Ag(NH3)2]+.
Ag+ (aq) + 2 :NH3 (aq)  [H3N:Ag:NH3]+ (aq)
As before, ammonia is a Lewis base (each ammonia molecule is donating a pair of electrons to form a new bond). Because no proton is
being transferred we could not classify ammonia as a base in the Arrhenius, Bronsted, or Lowry sense. Lewis broadened the definitions of
bases and acids.
What is the Lewis acid is the reaction:
Ag+ (aq) + 2 :NH3 (aq)  [H3N:Ag:NH3]+ (aq)?
It is the silver ion—it is accepting a pair of electrons to form a new band (actually it is
accepting two pairs of electrons to form two new bonds).
HCl is a strong acid (it dissociates 100%). It is a Lewis acid because the proton accepts a
pair of electrons from water to form a new bond:
HCl (aq) + H2O (l)  Cl- (aq) + H3O+ (aq)
2.
Does water act as a Lewis acid or base? Draw the Lewis structure and explain.
Yes, water acts like a Lewis base—especially in transition metal chemistry. The lone
pairs of electrons on the oxygen atom are donated to a metal ion to form a new bond.
where M is the metal ion. A Lewis base donates a pair of electrons to form a new bond.
3.
Does Co3+ act as a Lewis acid or base? Draw the Lewis structure and explain.
Yes, Co3+ like the transition metal ions acts as Lewis acids. They accept a pair (or
several pairs) of electrons to form a new bond (several new bonds). On the left side of
the structure below I have written the chemical formula of ammonia with the nitrogen at
the end to indicate the nitrogen is the atom bonded to the metal ion.
4.
Sketch [Pt(H2O)4]2+ (square planar). How will the square planar configuration affect the
d-orbitals of Pt?
Energy
5.
Sketch [Ni(ox)2]2- (tetrahedral).
ox is the oxalate bidentate
6.
Sketch [Co(NH3)6]2+.
7.
The coordination number for Fe in K[Fe(en)] is:
CN = 2. There are two teeth biting into the metal ion. The ethylene diamine is a
bidentate.
8.
Does [Fe(CN)5Cl]3- have cis-/trans- isomers? Explain.
No cis- and trans- isomers. There are five CN- ligands and one chloride ion ligand. The
chloride ion will always be surrounded by five CN- ligands. There is only one structure
for [Fe(CN)5Cl]3- and it is polar.
9.
Sketch cis- and trans- [Co(H2O)4Cl2]+. Are either polar?
10.
Co3+ has 6 d-electrons. Explain.
Co is in Group 9 (it has nine valence electrons). Co3+ has six valence electrons (9-3=6).
The valence electrons in the transition metal ions are d electrons.
11.
How many d-electrons does Cu2+ have?
Cu is in Group 11 (it has eleven valence electrons). Cu2+ has nine valence electrons (112=9). The valence electrons in the transition metal ions are d electrons.
12.
The following was taken from a salad dressing bottle:
Ingredients: Water, soybean oil, vinegar (red wine, white wine), high fructose corn syrup, shallots, contains less than 2 % of:
blackberry puree, raspberry puree, lemon juice, minced garlic*, chives, parsley, spice, salt, xanthan gum, sodium benzoate, sorbic
acid, and calcium disodium edta (to preserve freshness).
Why is edta added? Why "calcium disodium edta" instead of just edta?
EDTA is added to prevent metal atoms from clumping and producing colored “chunks”
and making the dressing look rotten. EDTA has a charge of 4- when the four protons are
gone; Ca2+ and 2Na+ balance this.
13.
Does [FeCl3(CN)3]3- have mer- and fac- isomers? Explain. Draw them if they exist. Are
either polar?
Yes! The "3,3" combination yields mer- (meridian) and fac- (face) isomers. In the merisomer, the three similar ligands are oriented around a meridian. In the fac- isomer, the
three similar ligands are oriented in a triangular face. Both are polar.
Or, with some shading to see the orientation:
14.
How many d-electrons does V+ have? Sketch the energy level diagram for low-spin V+.
Sketch the energy level diagram for high-spin V+.
V is in Group 5 (it has five valence electrons). V+ has four valence electrons (5-1=4).
The valence electrons in the transition metal ions are d electrons.
High Spin:
15.
What is the spectrochemical series?
CN- > NO2- > en > NH3 > NCS- > H2O > F- > ClLarge d-splitting
Low spin
Strong field
Small d-splitting
High spin
Weak field
Lower Δo →