Eg ca ca2 ca ca 2 1s 22s 2 2p6 3s 23p 6 4s2 1s2 2s2

e.g., Ca → Ca
2+
Ca → Ca
2+
1s
2
2s
2
2p
6
3s
2
3p
6
4s
2
→ 1s
2
2s
2
2p
6
3s
2
3p
6
[Ar]4s
2
→ [Ne]
3s
2
3p
6
= [Ar]
V → V
2+
1s
2
2s
2
2p
6
3s
2
3p
6
4s
2
3d
3
→
1s
2
2s
2
2p
6
3s
2
3p
6
4s
0
3d
3
= 1s
2
2s
2
2p
6
3s
2
3p
6
3d
3
[Ar]4s
2
3d
3
→ [Ar]3d
3
3d ↓ ↓ ↓ ↓ ↓ ↓
4s ↓↑
. .
3p ↓↑ ↓↑ ↓↑ ↓↑ ↓↑ ↓↑
3s ↓↑ ↓↑
2p ↓↑ ↓↑ ↓↑ ↓↑ ↓↑ ↓↑
2s ↓↑ ↓↑
1s ↓↑ ↓↑
Loss of electrons converts the metal into a cation, which is a Lewis acid. In the
case of groups I and II metal ions the cations are stable (Noble gas structures),
so the Lewis acid is satisfied by a Lewis base serving as a counterion. But in
the case of transition metals, where the metal ion has partially unfilled orbitals,
the Lewis acid is satisfied by Lewis bases bonding with these orbitals, as well
as serving as counterions.

CHEM162-2012 CHAPTER 17 57
ET: Lewis acid: A substance that accepts a pair of electrons to form a covalent bond; Lewis base: a substance
that donates a pair of electrons to form a covalent bond.
ET: A complex ion consists of a central metal cation (Lewis acid), usually a transition metal, to which
ligands* (Lewis bases, e.g., Cl
-
, OH
-
, NH
3
, H
2
O, CN
-
) are tightly attached.
* Ligand (L: ligo, to bind):
A substance (neutral or anionic) having a lone electron pair that bonds to a transition metal cation (i.e., Lewis base-Lewis acid interaction)
Some Formation Constants for Complex Ions
Complex Equilibrium Reaction K
f
[Co(NH
3
)
6
]
3+
Co
3+
+ 6NH
3
←
→
[Co(NH
3
)
6
]
3+
4.5 x 10
33
[Cu(NH
3
)
4
]
2+
Cu
2+
+ 4NH
3
←
→
[Cu(NH
3
)
4
]
2+
1.1 x 10
13
[Fe(CN)
6
]
4-
Fe
2+
+ 6CN
-
←
→
[Fe(CN)
6
]
4-
1 x 10
37
[Fe(CN)
6
]
3-
Fe
3+
+ 6CN
-
←
→
[Fe(CN)
6
]
3-
1 x 10
42
[PbCl
3
]
-
Pb
2+
+ 3Cl
-
←
→
[PbCl
3
]
-
2.4 x 10
1
[Ag(NH
3
)
2
]
+
Ag
+
+ 2NH
3 ←
→
[Ag(NH
3
)
2
]
+
1.6 x 10
7
[AgCl
2
]
-
Ag
+
+ 2Cl
-
←
→
[AgCl
2
]
-
1.2 x 10
8
[Ag(S
2
O
3
)
2
]
3-
Ag
+
+ 2S
2
O
3
2-
←
→
[Ag(S
2
O
3
)
2
]
3-
1.7 x 10
13
[Zn(NH
3
)
4
]
2+
Zn
2+
+ 4NH
3
←
→
[Zn(NH
3
)
4
]
2+
4.1 x 10
8
[Zn(CN)
4
]
2-
Zn
2+
+ 4CN
-
←
→
[Zn(CN)
4
]
2-
1 x 10
18
[Zn(OH)
4
]
2-
Zn
2+
+ 4OH
-
←
→
[Zn(OH)
4
]
2-
4.6 x 10
17
Note:
• Unlike K
sp
where the equilibrium equation is written as dissociation (1
substance → 2), in K
f
the equilibrium equation is written as formation (2
substance → 1).
• K
f
values are large, meaning high ratio of complex ion to individual ions.
• The reverse reactions (breaking up the complex ions) have small K’s.
• Large K
f
values mean strong ligand bonds to the central metal cation.
• The ratio of the ligand amount to the Lewis acid charge is frequently 2:1.

CHEM162-2012 CHAPTER 17 58
Z&Z 15-99
(modified with fictitious data)
ET: Complex ion formation is sequential reactions with ligands on transition metal ions, giving large K’s.
Complex ion:
Species where the transition metal ion is bonded to ligands.
When aqueous KI is added gradually to mercury(II) nitrate, an orange
precipitate forms. Continued addition of KI causes the precipitate to
dissolve. Write balanced equations to explain these observations.
(Hint: Hg
2+
reacts with I
-
to form HgI
4
2-
.) What is the overall
formation constant for HgI
4
2-
? Use K
f1
= 0.2 x 10
4
, K
f2
= 6.5 x 10
10
,
K
f3
= 7.6 x 10
7
and K
f4
= 0.1 x 10
9
.