1. TM I-Intro to Complexes
CHM 122
Week 12, I&II
Metal Complexes
metal cation is attached to a group of surrounding molecules or ions (ligands) by coordinate covalent bonds
coordinate => ligand donates both electrons
each ligand makes at least one coordinate covalent bond to the metal cation
any atom (from a ligand) that is directly bound to the metal cation is called a donor atom
coordination number (C.N.) = the total number of donor atoms surrounding a metal cation (= total number of coordinate covalent bonds)
TM I-Intro to Complexes

2. Lewis definition of acids and bases
CHM 122
Week 12, I&II
Lewis definition of acids and bases
Base: e- - pair donor
Acid: e- - pair acceptor
In metal complexes, the ligand is always a Lewis base (makes a coordinate covalent bond) and the metal cation is a Lewis acid (accepts the lone pair from ligand).
Note: definition applies to things other than metal complexes (e.g., NH3-BF3)
TM I-Intro to Complexes

3. Examples of Metal Complexes
CHM 122
Week 12, I&II
Examples of Metal Complexes
Ni(H2O)62+
Co(NH3)4Cl2+
Six H2O ligands bound to a Ni2+ cation
Four NH3 ligands and two Cl- ligands bound to a Co3+ cation
TM I-Intro to Complexes

4. More Examples Ni(H2O)4SO4 Ni(CN)42-
CHM 122
Week 12, I&II
More Examples
Ni(H2O)4SO4
Ni(CN)42-
Four H2O ligands and one SO42- ligand bound to a Ni2+ cation
Four CN- ligands bound to a Ni2+ cation
TM I-Intro to Complexes

5. Charge on complex vs. charge on cation
CHM 122
Week 12, I&II
Charge on complex vs. charge on cation
charge on complex = sum of the charges of the metal cation (oxidation #) plus charges on all ligands bound
don’t mix up charge on cation with charge on the (whole) complex! **charge on complex appears as right superscript**
Examples
Co(NH3)4Cl2+
Charge on Co is +3
Charge of each NH3 is 0
Charge of each Cl- is -1
Charge of complex is
+3 + 4(0) + 2(-1) = +1
Ni(CN)42-
Charge on Ni is +2
Charge of each CN- is -1
Charge of complex is
+2 + 4(-1) = -2
TM I-Intro to Complexes

6. Coordination Compounds are Neutral
CHM 122
Week 12, I&II
Coordination Compounds are Neutral
**The species inside the brackets is always a complex ion (metal cation + ligands); the part(s) outside of brackets are counterions.**
if charge on complex is
neutral, the complex itself is a coordination compound
e.g., Pt(NH3)2Cl2
Charge on Pt = +2
Charge on NH3 = 0
Charge on Cl- = -1
not neutral, complex plus counterions can form a neutral salt, also called a coordination compound
e.g., [Ni(NH3)6]Cl2 ; complex is a cation; Cl-’s are counterions
In water, it dissociates into Ni(NH3)62+ and two Cl- ions:
[Ni(NH3)6]Cl2  Ni(NH3) Cl-
e.g., (NH4)4[Fe(CN)6] ; complex an anion; NH4+’s, counterions
(NH4)4[Fe(CN)6]  4 NH4+ + Fe(CN)64-
Charge of complex is
+2 + 2(0) + 2(-1) = 0
TM I-Intro to Complexes

7. TM I-Intro to Complexes
CHM 122
Week 12, I&II
Example
e.g., [Co(NH3)4Cl2]Cl
inside brackets = metal complex
outside= counter ion(s)
# ligands = 6 (four neutral NH3’s; two Cl-’s)
# counterions = 1 (Cl-)
Note! anions can sometimes be ligands and sometimes be counterions!
neutral ligands can never be counterions!
# donor atoms = 6 (four N’s from NH3’s; two Cl’s from Cl-’s)  CN = 6
TM I-Intro to Complexes

8. Common C.N.’s and their Geometries
CHM 122
Week 12, I&II
Common C.N.’s and their Geometries
C.N.:
TM I-Intro to Complexes

9. Example of a tetrahedral metal complex
CHM 122
Week 12, I&II
Example of a tetrahedral metal complex
Ni(CO)42+
Four CO ligands bound to a Ni2+ cation
TM I-Intro to Complexes

10. Ligand Types (See Table 24.3 in Tro [next slide])
CHM 122
Week 12, I&II
Ligand Types (See Table 24.3 in Tro [next slide])
Ligands that bind to a metal cation via:
one donor atom are called monodentate ligands
Cl-, NH3, CN-, H2O (donor atoms are, respectively, Cl, N, C (or N!), O
more than one donor atom are called polydentate ligands
two atoms per ligand – bidentate (en, ox2-, bpy)
three atoms, tridentate
etc
TM I-Intro to Complexes

11. Learn the abbreviations for these two (ox and en) Bidentate
CHM 122
Week 12, I&II
NOTE: All donor atoms must have at least one ____________, which it uses to make a bond to the metal cation.
Monodentate
(donor atom circled)
lone pair
Monodentate
(but two possible donor atoms [only one can bond at a time])
Learn the abbreviations for these two (ox and en)
Bidentate
(two donor atoms; both bond to the same metal cation at the same time)

12. NOTE: Some ligands are…
CHM 122
Week 12, I&II
NOTE: Some ligands are…
neutral
And some ligands are…
negatively charged

13. Ligand Types (Table 20.13, McMurry & Fay; Similar to Tro, 24.2)
CHM 122
Week 12, I&II
Ligand Types (Table 20.13, McMurry & Fay; Similar to Tro, 24.2)
TM I-Intro to Complexes

14. Ligand Types (Tables 20.13 (McMurry) and 24.2 (Tro), continued)
CHM 122
Week 12, I&II
Ligand Types (Tables (McMurry) and 24.2 (Tro), continued)
TM I-Intro to Complexes

15. Example – ethylenediamine (en)
CHM 122
Week 12, I&II
Example – ethylenediamine (en)
en = NH2CH2CH2NH2
each N in one en ligand can
bind to a metal cation!
Co(en)2Cl2+
# ligands = 4 (two en’s, two Cl-’s)
C.N. = 6 (not 4!) because each en ligand makes two coordinate covalent bonds to the Co3+ using two different N atoms per ligand
TM I-Intro to Complexes