What are Isolobal Species? Discuss their Extension, Significance, and Examples.
Isolobal means similar to lobes (from Greek iso- meaning same, and lobos meaning lobe).
Isolobal species is also known as the isolobal analogy which helps to draw parallels between the structures and bonding of organic and inorganic molecular fragments. In simpler terms, isolobal fragments are "electronically equivalent" in their frontier orbitals, allowing them to be conceptually interchanged in molecules while retaining similar bonding characteristics. The isolobal relationship is often represented by a double-headed arrow with a looped line in the middle. Isolobal species are somewhat similar to isoelectronic species as they possess the same number of valence electrons and structure.
Isolobal analogy concept was proposed by Hoffmann. Two molecular fragments are considered to be an isolobal if they have:
- Same number of frontier orbitals
- Frontier orbitals with similar symmetry
- Frontier orbitals with approximately the same energy
- Same number of electrons in their frontier orbitals
Isolobal Fragments of Main Group Elements
For main group elements, the number of frontier orbitals and the number of electrons in them can often be determined by considering how many electrons are needed to achieve a stable octet configuration (octet rule).
For example: A methyl radical, (CH3*), has 7 valence electrons (4 from carbon, 3 from hydrogen). To achieve an octet, it needs one more electron, residing in a single frontier orbital.
An amino radical, (NH2*), also has 7 valence electrons (5 from nitrogen, 2 from hydrogen) and a single frontier orbital with one electron.
A hydroxyl radical, (OH*), similarly has 7 valence electrons (6 from oxygen, 1 from hydrogen) and a single frontier orbital with one electron. Thus,
CH3 
NH2 OH F Cl
Isolobal Fragments of Transition Elements
For transition metals, the 18-electron rule often serves as a guideline for stability. To find isolobal fragments:
Start with a stable 18-electron complex, such as MLn (where M is a transition metal and L is a two-electron donating ligand like CO, NH3 etc.).
Remove one ligand (L), which formally leaves the metal center with one fewer electron and a vacant coordination site (a frontier orbital).
To maintain charge neutrality in the isolobal fragment, the metal's electron count is adjusted.
For example, if we remove a neutral ligand from a neutral complex, the metal center in the resulting fragment formally gains one electron.
Compare the resulting transition metal fragment with main group fragments based on the number of frontier orbitals and electrons.
For example, a metal complex with electron count of 17 electrons, such as Mn(CO)5, there is one frontier orbital with one electron. This implies that a 17 VE carbonyl fragment is isolobal to a 7 VE organic fragment such as CH3.
Similarly, For a 16 VE fragment such as Fe(CO)4 there are two frontier orbitals with one electron in each of them, and is isolobal to a 6 VE fragment such as CH2.
For a 15 VE fragment such as Co(CO)3 there are three frontier orbitals with overall three valence electrons and is isolobal to a 5 VE fragment such as CH.
Examples of Isolobal Species
CH3 and Mn(CO)5
CH3 has one singly occupied sp3 hybrid orbital.
Mn(CO)5 has 17 valence electrons (7 from Mn + 5 × 2 from CO) and one singly occupied d orbital that can participate in bonding.
Thus, CH3 Mn(CO)5∙
CH2 and Fe(CO)4
CH4 has 8 valence electrons.
Removing two hydrogen atoms to form CH2 leaves 6 valence electrons and two frontier orbitals (often considered as two sp3 hybrid orbitals).
Fe(CO)5 is an 18-electron complex.
Removing one CO ligand gives Fe(CO)4, which has 16 valence electrons (8 from Fe + 4 × 2 from CO) and two frontier orbitals.
Thus, CH2 Fe(CO)4.
CpCo(CO)2 and CpFe(CO)2+
In CpCo(CO)2, Cobalt (Co) has 9 valence electrons, cyclopentadienyl (Cp) typically donates 5 electrons, and each carbonyl (CO) donates 2 electrons,
So, number of electrons = 9 + 5 + 2(2) = 18 electrons.
If we consider the CpCo(CO)2 fragment as having one "dangling" orbital after potentially forming a bond, we can analyze its frontier orbitals.
In CpFe(CO)2+, Iron (Fe) has 8 valence electrons, Cp donates 5, and each CO donates 2,
So, total number of electrons = 8 + 5 + 2(2) = 17 electrons.
The +1 charge reduces the electron count to 16.
Similar to the Co complex, this fragment also has frontier orbitals available for bonding. While the overall electron counts of the complexes are different, the isolobal analogy focuses on the frontier orbitals of the fragments that would be involved in forming a bond. In this case, both fragments can be considered isolobal in certain bonding scenarios.
BH and Fe(CO)3
BH3 has 6 valence electrons.
Removing two hydrogen atoms gives BH with 4 valence electrons and two frontier orbitals.
Fe(CO)5 is an 18-electron complex.
Removing two CO ligands gives Fe(CO)3 with 14 valence electrons (8 from Fe + 3 × 2 from CO) and two frontier orbitals.
Thus, BH Fe(CO)3.
Extensions of the Isolobal Analogy
The concept of isolobal fragments can be extended beyond the examples given so far to include charged species, a variety of ligands other than CO, and organometallic fragments based on structures other than octahedral. Some of the ways of extending the isolobal parallels can be summarized as follows:
1. The isolobal definition may be extended to isoelectronic fragments having the same coordination number. For example,
Mn(CO)5 Re(CO)5 Fe(CO)5+ Cr(CO)5− CH3
2. Gain or loss of electrons from two isolobal fragments yields isolobal fragments. For example
Mn(CO)5 CH3
Cr(CO)5 Mo(CO)5 W(CO)5 CH3+
Note that all the examples shown above are one ligand short of the parent complex. Fe(CO)5 is isolobal with CH3-, for example, because both have filled electron shells and both are one vertex short of the parent polyhedron. By contrast, Fe(CO)5 and CH4 are not isolobal. Both have filled electron shells (18 and 8 electrons, respectively), but CH4 has all vertices of the tetrahedron occupied, whereas Fe(CO)5 has an empty vertex in the octahedron.
3. Other 2-electron donors are treated similarly to CO
Mn(CO)5 Mn(PR3)5 [MnCl5]5− Mn(NCR)5 CH3
4. η5-C5H5 and η6-C6H6 are considered to occupy three coordination sites and to be 6-electron donors
η5-C5H5Fe(CO)2 and η6-C6H6Mn(CO)2 [Fe(CO)5]+ Mn(CO)5
η5-C5H5Mn(CO)2 and η6-C6H6Cr(CO)2 [Mn(CO)5]+ Cr(CO)5
5. Octahedral fragments of formula MLn, (where M has a dx configuration) are isolobal with square-planar fragments of formula MLn - 2 (where M has a dx + 2 configuration and L = 2 -electron donor).
| Octahedral Fragments | Square Fragments | |
|---|---|---|
| MLn | ML(n-1) | |
| Cr(CO)5 | |
[PtCl3]− |
| d6 | d8 | |
| Fe(CO)4 | |
Pt(PR3)2 |
| d8 | d10 |
The fifth of extensions of the isolobal analogy is less obvious than the others.
Significance of the Isolobal Analogy
Isolobal analogy is particularly useful to understand the structures and bonding in complex carbonyl and organometallic compounds by relating them to simpler molecules.
It can be used to predict the stability, reactivity, and other properties of organometallic compounds by drawing parallels with known organic and inorganic compounds.
Isolobal analogy allows to predict the existence and structures of new organometallic compounds by analogy to known organic or inorganic compounds. If a stable organic molecule exists with a certain framework, replacing a fragment with an isolobal organometallic fragment might lead to a stable compound.
Similar isolobal fragments often exhibit similar reactivity patterns.
Isolobal Fragments MCQs
NEET, IIT-JEE, IIT-JAM, GATE, CSIR-NET, DRDO, TIFR
Q1: According to isolobal analogy, the fragment isolobal with Mn(CO)5 is
- CH3
- CH
- Fe(CO)4
- Co(CO)4
Answer: A. CH3
Both Mn(CO)5 (d7) and CH3 have one unpaired electron and same frontier orbital symmetry → isolobal.
Q2: The pair of isolobal fragments is
- Cr(CO)5 and Ni(CO)3
- Fe(CO)4 and CH2
- Co(CO)3 and P
- Mn(CO)5 and BH
Answer: C. Co(CO)3 and P
Both are d9 17-electron fragments with one empty p-orbital → isolobal.
Q3: Which of the following is isolobal with CpFe(CO)2 (Cp = η5-C5H5)?
- CH3
- CH
- CH2
- BH
Answer: A. CH3
CpFe(CO)2 is d7 17e fragment (18+5+4–18 = 17e), same as CH3.
Q4: The fragments isolobal with Os(CO)4 are
- S and Se
- CH2 and SiH2
- O and S
- Fe(CO)4 and Ni(CO)3
Answer: B. CH2 and SiH2
Os(CO)4 (d8) is 16-electron fragment → isolobal with CH2, SiH2.
Q5: Which of the following fragments is isolobal with Ni(η5-C5H5)?
- Co(CO)3
- Fe(CO)4
- Mn(CO)5
- Cr(CO)5
Answer: A. Co(CO)3
CpNi is d9 17e fragment → isolobal with Co(CO)3.
Q6: The set of isolobal pairs among the following is
- (i) Re(CO)5 and CH3 (ii) Pt(PR3)2 and CH2 (iii) CpRh(CO) and CH
- Only (i) and (ii)
- Only (ii) and (iii)
- (i), (ii) and (iii)
- Only (i)
Answer: A. Only (i) and (ii)
CpRh(CO) is 16e (not 17e) → not isolobal with CH.
Q7: Which one is NOT isolobal with Co(CO)4–?
- Fe(CO)42–
- Mn(CO)4–
- BH4–
- CH4
Answer: D. CH4
Co(CO)4– is d10 18e closed-shell fragment → isolobal with CH3+, not CH4.
Q8: The fragment isolobal with Cr(CO)5 is
- CH
- C
- CH2
- carbene (CR2)
Answer: B. C
Cr(CO)5 is d6 16e fragment → isolobal with atomic carbon (or carbido).
Q9: Which of the following can replace Ru(CO)4 in cluster compounds according to isolobal analogy?
- Fe(CO)4
- S
- CH2
- Both (B) and (C)
Answer: D. Both (B) and (C)
Ru(CO)4 (like Fe(CO)4) is 16e d8 → isolobal with CH2, S, Se, etc.
Q10: The isolobal analogue of the fragment [Cp*Ir]2+ (Cp* = η5-C5Me5) is
- CH+
- CH3+
- CH
- C2+
Answer: A. CH+
Cp*Ir2+ is d6 16e fragment → isolobal with CH+.
Q11: According to the isolobal principle, the fragment that is isolobal with the [HFe(CO)4]– unit is
- CH3–
- CH3
- BH3
- CH2
Answer: A. CH3–
Both [HFe(CO)4]– and CH3– are d8 18-electron three-orbital (t2g-like) fragments with one donor lobe.
Q12: Which of the following sets contains only fragments isolobal to each other?
- Mn(CO)5, Re(CO)5, CH3, CpFe(CO)2
- Fe(CO)4, CH2, S, Pt(PR3)2
- Co(CO)3, P, As, Ni(C5H5)
- Cr(CO)5, C, Mo(CO)5, W(CO)5
Answer: B. Fe(CO)4, CH2, S, Pt(PR3)2
All are d8 16-electron fragments with two donor/acceptor orbitals → mutually isolobal.
Q13: The main-group fragment isolobal with the metal fragment [CpMo(CO)3] is
- Cl
- CH3
- CH
- C
Answer: A. Cl
CpMo(CO)3 is a d7 17-electron one-electron donor fragment → isolobal with Cl•, Br•, I•, etc.
Q14: In the cluster [Rh6(CO)16], each Rh(CO)2 vertex unit can be replaced isolobally by
- BH
- CH+
- S2+
- All of the above
Answer: D. All of the above
Each apical/vertex Rh(CO)2 in Rh6(CO)16 is a d9 Rh(0) 16-electron fragment contributing 3 orbitals & 2 electrons to cluster bonding → isolobal with BH, CH+, S+, etc.
Q15: The pair of species that are NOT isolobal is
- Co(CO)4– and BH4–
- Ni(η5-C5H5)2 and ferrocene (as fragments when one Cp is removed)
- Mn(CO)5– and CH3–
- Fe(CO)5 and :C(R)2 (Fischer carbene)
Answer: B. Ni(η5-C5H5)2 and ferrocene (as fragments when one Cp is removed)
Cp2Ni → CpNi+ fragment is d8 16e; Cp2Fe → CpFe+ fragment is d6 16e → different d-count → not isolobal.
Q16: Identify the series showing correct isolobal analogy.
- CH3 and [Fe(CO)5]+
- CH5+ and [Cr(CO)5]–
- CH5+ and Ni(CO)3
- CH+ and CpCo
The correct combinations are:
- A and B only
- A, C and D only
- B and C only
- A and D only
Answer: 2. A, C and D only
A → both 17e d7; C → both 16e d10; D → both 16e d8 (CpCo is 5+9–6 = 16e). B is incorrect because CH5+ (closed shell) ≠ [Cr(CO)5]– (17e).
Q17: The CpM fragment isolobal with the BH fragment is
- CpGe
- CpMn
- CpRu
- CpCo
Answer: D. CpCo
BH contributes 3 orbitals + 2 electrons. CpCo = 5 (Cp) + 9 (Co) – 6 (formal charge 0) = 14 + 3 orbitals from metal → identical frontier orbitals to BH → isolobal.
Q18: According to isolobal analogy, the right set of fragments that might replace Co(CO)3 in [Co4(CO)12] is
- CH, BH and Mn(CO)5
- P, CH, and Ni(η5-C5H5)
- Fe(CO)4, CH2 and SiCH3
- BH, SiCH3 and P
Answer: B. P, CH, and Ni(η5-C5H5)
Co(CO)3 in the cluster is a d9 17-electron fragment. P (pnictogen atom), CH, and CpNi are all 17e fragments with one frontier orbital → isolobal.
Q19: Amongst the following, the pair that is NOT an isolobal pair is
- Mn(CO)5 and CH3
- Fe(CO)4 and O
- Co(CO)3 and R2Si
- Mn(CO)5 and RS
Answer: C. Co(CO)3 and R2Si
Co(CO)3 = 17e d9 (one frontier orbital); R2Si = 16e singlet diradicaloid (two frontier orbitals) → not isolobal.
Q20: The organic species isolobal to [Fe(CO)2(PPh3)]– is
- CH2+
- CH–
- CH3
- CH
Answer: D. CH
[Fe(CO)2(PPh3)]– = Fe(0) d8 + 4 (CO) + 2 (PPh3) + 1 (charge) = 15 valence electrons → 17-electron fragment with one frontier orbital → isolobal with •CH (methyne).