Chemistry Reference
In-Depth Information
120
110
100
90
80
70
60
19
20
21
22
23
24
25
26
27
28
29
30
Z
FIGURE 3.10
Size of transition 3
d
-metal ions (▫ low spin; ▴ high spin). (Data from J.E.
Huheey, E.A. Keiter, and R.L. Keiter,
Inorganic Chemistry: Principles of Structure and
Reactivity,
4
th
edition [New York: Harper Collins, 1993].)
Hydration enthalpies are closely related to formation enthalpies of octahedral aqua
complexes. Because the water molecule is a weak ligand, the resulting configurations
are of the high-spin type.
The variation of enthalpy of M
2+
ions corresponds to the process:
M
2+
(g) + 6 H
2
O(l) → [M(H
2
O)
6
]
2+
(aq)
(3.35)
Ca
2+
, Mn
2+
, and Zn
2+
have
d
0
,
d
5
, and
d
10
, so CFSE is 0. Other metal ions deviate
from the expected line due to extra CFSE, with two maxima for the configuration
d
3
tion enthalpies of divalent transition metal ions.
3.3.2.6 Lattice Energy
The lattice energy variation for some ionic compounds with C.N. 6, for example,
halogenide MX
2
from CaX
2
to ZnX
2
, is influenced by CFSE (octahedral field). In
Figure 3.12
,
the lattice energies of the difluoride of the 3
d
metal ions are plotted as a
function of the atomic number.
3.3.2.7 Stability of Some Oxidation States
In aqueous solutions, the Co
3+
ion is not stable. It is reduced by water to the Co
2+
ion.
However, if there are strong field ligands present in solution, the Co
3+
ion is stabilized,
due to the CFSE (Oh) that are highest for a high-spin configuration t
2g
6
(−24Dq + 3P).
The oxidation of Co
2+
to Co
3+
is accomplished by configuration changes from low
spin to high spin and it is considered to take place in two stages:
I. Redistribution of electrons in the low spin state
Co
2+
(t
2g
5
e
g
2
) → Co
2+
(t
2g
6
e
g
1
)
(3.36)
