Chapter 5 Pauling “3-Electron Bonds” and
Hypoligated Transition Metal
Complexes
Although it is not well-recognized, transition metal complexes exist for which
Pauling “3-electron bond” theory is relevant. To illustrate this theory, consideration will be given to descriptions of the electronic structures for a number of
octahedral complexes.
5-1 Hypoligated and Hyperligated Transition Metal
Complexes
The low-energy valence-shell atomic orbitals for the transition metals are the five
(inner) (n-1) d orbitals and the ns and three np atomic orbitals. (For atoms of firstrow transition metals Sc,...,Cu, these are the
2
2
3d
x y
,
2
3d z , xy
3d , xz
3d , yz
3d , 4s,
4p x , 4p y and 4p z orbitals; contours for 3d orbitals are displayed in Figure 1-1.)
Pauling
1 has classified transition metal complexes of the general type ML N (M =
transition-metal ion, L = ligand, N = number of ligands) as either hyperligated or
hypoligated according to whether there are sufficient valence-shell orbitals on the
metal ion to form N electron-pair M-L σ-bonds. Thus the isoelectronic ions Co
3+
and Fe
2+ of
3
3 6
[Co(NH ) ]
,
3
6
[CoF ]
and
2
2
6
[Fe(H O) ]
have
6
(3d) valence-shell
configurations. However, magnetic susceptibility measurements for the three
complexes indicate that the distributions of the six electrons amongst the 3d
orbitals of the metal ions must differ. To account for the observed magnetic
moment of zero
1 for
3
3 6
[Co(NH ) ]
, it is necessary to assume that the three 2 g
t
orbitals (d xy , d xz and d yz ) of Co
3+ are doubly-occupied (Figure 5-1 (a)), thereby
generating a low-spin (S = 0 spin) complex. The vacant g
e orbitals ( 2 2
x -y
d
and
Ó Springer International Publishing Switzerland 2016
R.D. Harcourt, Bonding in Electron-Rich Molecules,
Lecture Notes in Chemistry 90, DOI 10.1007/978-3-319-16676-6_5
67
Hypoligated Transition Metal
Complexes
Although it is not well-recognized, transition metal complexes exist for which
Pauling “3-electron bond” theory is relevant. To illustrate this theory, consideration will be given to descriptions of the electronic structures for a number of
octahedral complexes.
5-1 Hypoligated and Hyperligated Transition Metal
Complexes
The low-energy valence-shell atomic orbitals for the transition metals are the five
(inner) (n-1) d orbitals and the ns and three np atomic orbitals. (For atoms of firstrow transition metals Sc,...,Cu, these are the
2
2
3d
x y
,
2
3d z , xy
3d , xz
3d , yz
3d , 4s,
4p x , 4p y and 4p z orbitals; contours for 3d orbitals are displayed in Figure 1-1.)
Pauling
1 has classified transition metal complexes of the general type ML N (M =
transition-metal ion, L = ligand, N = number of ligands) as either hyperligated or
hypoligated according to whether there are sufficient valence-shell orbitals on the
metal ion to form N electron-pair M-L σ-bonds. Thus the isoelectronic ions Co
3+
and Fe
2+ of
3
3 6
[Co(NH ) ]
,
3
6
[CoF ]
and
2
2
6
[Fe(H O) ]
have
6
(3d) valence-shell
configurations. However, magnetic susceptibility measurements for the three
complexes indicate that the distributions of the six electrons amongst the 3d
orbitals of the metal ions must differ. To account for the observed magnetic
moment of zero
1 for
3
3 6
[Co(NH ) ]
, it is necessary to assume that the three 2 g
t
orbitals (d xy , d xz and d yz ) of Co
3+ are doubly-occupied (Figure 5-1 (a)), thereby
generating a low-spin (S = 0 spin) complex. The vacant g
e orbitals ( 2 2
x -y
d
and
Ó Springer International Publishing Switzerland 2016
R.D. Harcourt, Bonding in Electron-Rich Molecules,
Lecture Notes in Chemistry 90, DOI 10.1007/978-3-319-16676-6_5
67