5-1 Hypoligated and Hyperligated Transition Metal Complexes
69
Because low-spin Co
3+ has sufficient valence-shell orbitals to form six electron-pair Co-N bonds,
3
3 6
[Co(NH ) ]
may be classified as a hyperligated
complex
1 .
Isoelectronic
2
2
6
[Fe(H O) ]
and
3
6
[CoF ]
are paramagnetic complexes with
magnetic moments of 5.3 Bohr magneton
1 ; the “spin-only” formula for the
magnetic moment
2
n n
generates a magnetic moment of 4.9 Bohr magneton
when the number of unpaired-electron spins (n) is four. Therefore, each of these
complexes is assumed to have this number of unpaired-electrons with parallel
spins. The 3d-orbital occupations for the Fe
2+ and Co
3+ ions are then those that are
displayed in Figure 5-1(b), and a high-spin (S = 2) complex is thereby generated.
Because each of
2
2
6
[Fe(H O) ]
and
3
6
[CoF ]
has insufficient valence-shell
orbitals to form six electron-pair bonds between the Fe
2+ and Co
3+ and the ligands,
these complexes are classified as hypoligated complexes
1 .
For hypoligated complexes, either of the following valence-bond procedures is
sometimes used to describe the metal-ligand σ-bonding
2 :
(i) The outer
2
2
x -y
4d
and
2
z
4d orbitals are hybridized with the 4s and 4p orbitals
to form six octahedral (
3 2
sp d ) hybrid orbitals, as in Figure 5-1 (b). These
hybrid orbitals may be used for coordination with the six ligands to form six
electron-pair M-L σ-bonds as in valence-bond structures (1) and (2). This
approach may be criticized, because the 4d orbitals lie too high in energy for
them to be utilized in bonding to any significant extent.
(b) The vacant 4s and 4p orbitals, when suitably hybridized (i.e. as x
sp , y
p , and
z
p , y
sp , z
p and x
p , and z
sp , x
p and y
p ), may be used to form four electronpair M-L bonds as in valence-bond structure (3). It is then necessary to invoke
resonance between a set of 15 valence-bond structures of type (3), which differ
in the locations of the four M-L σ-bonds.
In contrast to what pertains for the above valence-bond descriptions of
3
3 6
[Co(NH ) ]
, the simplest molecular orbital descriptions for
3
6
[CoF ]
and
2
2
6
[Fe(H O) ]
use the same set of metal-ion orbitals for bonding, namely the inner
2
2
x y
3d and
2
z
3d orbitals as well as the 4s and 4p orbitals. For
3
3 6
[Co(NH ) ]
the
three non-bonding 2g
t orbitals are doubly occupied, and all molecular orbitals that
are M-L antibonding are vacant. In contrast, two antibonding M-L molecular
orbitals and two 2g
t orbitals are singly-occupied for the
3
6
[CoF ]
complex
3 . The
atomic orbitals for these molecular orbital schemes have also been used in the
above valence-bond description for
3
3 6
[Co(NH ) ]
; they may also be used to
provide a valence-bond description for
3
6
[CoF ]
or
2
2
6
[Fe(H O) ]
if we avail ourselves of Pauling “3-electron bonds
4
”.
69
Because low-spin Co
3+ has sufficient valence-shell orbitals to form six electron-pair Co-N bonds,
3
3 6
[Co(NH ) ]
may be classified as a hyperligated
complex
1 .
Isoelectronic
2
2
6
[Fe(H O) ]
and
3
6
[CoF ]
are paramagnetic complexes with
magnetic moments of 5.3 Bohr magneton
1 ; the “spin-only” formula for the
magnetic moment
2
n n
generates a magnetic moment of 4.9 Bohr magneton
when the number of unpaired-electron spins (n) is four. Therefore, each of these
complexes is assumed to have this number of unpaired-electrons with parallel
spins. The 3d-orbital occupations for the Fe
2+ and Co
3+ ions are then those that are
displayed in Figure 5-1(b), and a high-spin (S = 2) complex is thereby generated.
Because each of
2
2
6
[Fe(H O) ]
and
3
6
[CoF ]
has insufficient valence-shell
orbitals to form six electron-pair bonds between the Fe
2+ and Co
3+ and the ligands,
these complexes are classified as hypoligated complexes
1 .
For hypoligated complexes, either of the following valence-bond procedures is
sometimes used to describe the metal-ligand σ-bonding
2 :
(i) The outer
2
2
x -y
4d
and
2
z
4d orbitals are hybridized with the 4s and 4p orbitals
to form six octahedral (
3 2
sp d ) hybrid orbitals, as in Figure 5-1 (b). These
hybrid orbitals may be used for coordination with the six ligands to form six
electron-pair M-L σ-bonds as in valence-bond structures (1) and (2). This
approach may be criticized, because the 4d orbitals lie too high in energy for
them to be utilized in bonding to any significant extent.
(b) The vacant 4s and 4p orbitals, when suitably hybridized (i.e. as x
sp , y
p , and
z
p , y
sp , z
p and x
p , and z
sp , x
p and y
p ), may be used to form four electronpair M-L bonds as in valence-bond structure (3). It is then necessary to invoke
resonance between a set of 15 valence-bond structures of type (3), which differ
in the locations of the four M-L σ-bonds.
In contrast to what pertains for the above valence-bond descriptions of
3
3 6
[Co(NH ) ]
, the simplest molecular orbital descriptions for
3
6
[CoF ]
and
2
2
6
[Fe(H O) ]
use the same set of metal-ion orbitals for bonding, namely the inner
2
2
x y
3d and
2
z
3d orbitals as well as the 4s and 4p orbitals. For
3
3 6
[Co(NH ) ]
the
three non-bonding 2g
t orbitals are doubly occupied, and all molecular orbitals that
are M-L antibonding are vacant. In contrast, two antibonding M-L molecular
orbitals and two 2g
t orbitals are singly-occupied for the
3
6
[CoF ]
complex
3 . The
atomic orbitals for these molecular orbital schemes have also been used in the
above valence-bond description for
3
3 6
[Co(NH ) ]
; they may also be used to
provide a valence-bond description for
3
6
[CoF ]
or
2
2
6
[Fe(H O) ]
if we avail ourselves of Pauling “3-electron bonds
4
”.