The isolobal analogy
is a concept used in organometallic chemistry to relate the structure of organic
and inorganic molecular fragments and to predict bonding properties of organometallic
compounds.
Molecular fragments are
considered as isolobal "if
i.
the number,
ii.
symmetry properties,
iii.
approximate energy
iv.
shape of the frontier orbitals and
v.
the number of electrons
Isolobal compounds are
somewhat similar to isoelectronic compounds as they posses the same number of valence
electrons and structure. A graphic representation of isolobal structures, with
the isolobal pairs connected through a double-headed arrow with half an orbital
below, is found in Figure 1.
Construction of isolobal fragments
To begin to generate an isolobal
fragment, the molecule needs to follow certain criteria.
- Molecules should satisfy the octet rule and
- all bonding and nonbonding molecular orbitals (MOs) are filled and all antibonding MOs are empty.
For
example methane is a simple molecule from which to form a main group fragment.
The removal of a hydrogen atom from methane generates a methyl radical. The
molecule retains its molecular geometry as the frontier orbital points in the
direction of the missing hydrogen atom. Further removal of hydrogen results in
the formation of a second frontier orbital. This process can be repeated until
only one bond remains to the molecule's central atom. Figure 2 demonstrates
this example of step-by-step generation of isolobal fragments.
Figure 2: Construction of frontier orbitals from methane.
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The isolobal fragments of octahedral
complexes, such as ML6, can be created in a similar way:
- Transition metal complexes should initially satisfy the eighteen electron rule,
- have no net charge, and
- their ligands should be two electron donors (Lewis bases).
For
example, a MoL6 complex
is d6 and neutral. However, removing a ligand to form the first
frontier orbital would result in a MoL-5 complex because Mo has obtained an additional electron
making it d7. To remedy this, Mo can be exchanged for Mn, which
would from a neutral d7 complex in this case, as shown in Figure 3.
This trend can continue until only one ligand is left coordinated to the metal
center.
'Figure 3: Production of a
frontier orbital in an octahedral complex. Since the process is not charge
producing, the metal center must change from d6 Mo to d7
Mn to retain the neutral charge.
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Relationship between tetrahedral and octahedral fragments
MO theory dependence
Any sort of saturated molecule can
be the starting point for generating isolobal fragments.
STEP i.
The molecules' bonding and
nonbonding MOs should be filled and the antibonding MOs empty.
STEP ii.
With each consecutive generation of
an isolobal fragment, electrons are removed from the bonding orbitals and a
frontier orbital is created.
STEP iii.
The frontier orbitals are at a
higher energy level than the bonding and nonbonding MOs.
STEP iv.
Each frontier orbital contains one
electron.
For example, consider Figure 5, which shows the production
of frontier orbitals in tetrahedral and octahedral molecules.
Figure 5: Molecular orbital diagram depiction of frontier orbitals in
methane and a basic ML6 metal complex.
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As seen above, when a fragment is
formed from CH4, one of the sp3 hybrid orbitals involved
in bonding becomes a nonbonding singly occupied frontier orbital. The frontier
orbital’s increased energy level is also shown in the figure. Similarly when
starting with a metal complex such as d6-ML6, the d2sp3
hybrid orbitals are affected. Furthermore the t2g nonbonding metal
orbitals are unaltered.
Ligands
Typical ligands used in the isolobal
analogy are two-electron donors such as phosphines,
halogens or carbonyls. However, other types of ligands can be employed.
If ligands donate multiple pairs of
electrons, they will occupy multiple coordination sites.
For example, the cyclopentadienyl
anion is a six-electron donor, so it occupies three coordination sites.
Polydentate ligands can also be used
in the analogy, such as ethylenediamine, a bidentate ligand, or triethylenetetramine,
a tetradentate ligand.
Figure 6: Example of cyclopentadiene, a multiply-coordinating ligand,
in the isolobal analogy.
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HOW TO SOLVE QUESTIONS ON ISOLOBAL ANALOGY
Ques: Among the Following the
correct statements is:
- CH is isolobal to Co(CO)3
- CH2 is isolobal to Ni(CO)2
- CH is isolobal to Fe(CO)4
- CH2 is isolobal to Mn(CO)4
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