Quintuple Bonds in Metal–Metal Clusters

Complete Study Notes
Quintuple Bonds in Metal–Metal Bonded Compounds & Clusters

1. Introduction to Quintuple Bonds

Quintuple bonds (formal bond order = 5) are the highest recognized metal–metal multiple bonds in stable molecular compounds. They consist of:

• 1 σ bond
• 2 π bonds
• 2 δ bonds

Electronic configuration: σ² π⁴ δ⁴ (10 bonding electrons, 0 antibonding)

Only stable examples are dichromium (Cr≡Cr) and dimolybdenum (Mo≡Mo) complexes with extremely bulky ligands.

2. Orbital Contributions

Bond Type	Orbitals	Number	Strength
σ	dz² – dz²	1	Strong
π	dxz, dyz	2	Strong
δ	dxy, dx²–y²	2	Weaker, but both bonding

All five d-orbitals of each metal are used exclusively for metal–metal bonding → ultra-short distances.

3. Landmark Quintuple-Bonded Compounds

Compound	Year	M–M (Å)	Discoverer
Cr2(Ar′)2 (terphenyl)	2005	1.835	Philip Power
Cr2(Dipp2NacNac)2	2007–2009	1.802–1.807	Various
Mo2(amidinate)2	2008	~2.02	Tsai / Theopold
Cr–Cr (pincer ligands)	2016–2024	1.70–1.75	Modern records

4. Comparison of Metal–Metal Multiple Bonds

Bond Order	Configuration	Typical Metals	M–M Distance (Å)	Stability
4	σ² π⁴ δ²	Re, Mo, W	2.1–2.4	High
4.5	σ² π⁴ δ³	Rare mixed-valence	—	Moderate
5	σ² π⁴ δ⁴	Cr, Mo (bulky ligands)	1.70–2.05	Very air-sensitive

5. Challenges & Significance

• Require ultra-bulky ligands for kinetic protection
• Only Cr and Mo achieve stable quintuple bonds
• No sextuple bonds isolated in molecular species
• Highly reactive toward N₂, CO, alkynes → potential in small-molecule activation

Quintuple bonds represent the current upper limit of molecular metal–metal bonding and continue to challenge and refine bonding theory.

Related Topic: Quadruple Bond

Quintuple Bonds MCQs

MCQs Asked in CSIR-NET, GATE, and IIT-JAM

GATE Chemistry 2022

Q1: The compound with the shortest known metal–metal bond (≈1.75 Å) contains:

1. A quadruple Cr–Cr bond
2. A quintuple Cr–Cr bond
3. A quadruple Mo–Mo bond
4. A quintuple Mo–Mo bond

Correct Answer: B. A quintuple Cr–Cr bond

Explanation: Quintuple-bonded dichromium(I) complexes with bulky ligands exhibit the shortest M–M distances (1.70–1.84 Å), with the current record around 1.75 Å.

IIT-JAM 2021

Q2: The electronic configuration of a metal–metal quintuple bond is:

1. σ² π⁴ δ²
2. σ² π⁴ δ⁴
3. σ² π⁴ δ² δ*²
4. σ² π⁶ δ⁴

Correct Answer: B. σ² π⁴ δ⁴

Explanation: Quintuple bonds have ten bonding electrons: one σ, two π, and two δ bonds → σ² π⁴ δ⁴ configuration.

IIT-JAM 2024

Q3: Which of the following has a formal metal–metal bond order of 3.5?

1. [Re₂Cl₈]^2-
2. [Re₂Cl₈]^-
3. Mo₂(O₂CCH₃)₄
4. Cr₂{Ar}₂ (quintuple bond)

Correct Answer: B. [Re₂Cl₈]^-

Explanation: One-electron oxidation of [Re₂Cl₈]^2- (δ²) gives [Re₂Cl₈]^- (δ¹) → configuration σ²π⁴δ¹ → bond order = (9–0)/2 = 3.5.

CSIR NET June 2022

Q4: The weakest component of a metal–metal quadruple bond is:

1. σ bond
2. π bond
3. δ bond
4. All are equally strong

Correct Answer: C. δ bond

Explanation: δ overlap is poorest due to four-lobe side-on interaction; contributes only ~3–5% of total bond energy but is crucial for short distances and eclipsed geometry.

GATE Chemistry 2021

Q5: Quintuple bonds are stabilized primarily by:

1. Small, electron-withdrawing ligands
2. Bulky, sterically demanding ligands
3. Bridging halide ligands
4. High oxidation state metals

Correct Answer: B. Bulky, sterically demanding ligands

Explanation: Terphenyl, NacNac, and amidinate ligands shield the reactive Cr≡Cr or Mo≡Mo core from dimerization or reaction with solvents/air.

CSIR NET Dec 2023

Q6: Which pair of orbitals contributes to the second δ bond in a quintuple bond?

1. d_xy
2. d_x²–y²
3. d_z²
4. d_xz

Correct Answer: B. d_x²–y²

Explanation: Quintuple bonds use both d_xy and d_x²–y² orbitals for two δ bonds (σ² π⁴ δ⁴).

CSIR NET Dec 2024

Q7: Compounds with metal–metal quintuple bonds are typically:

1. Diamagnetic
2. Paramagnetic with one unpaired electron
3. Paramagnetic with two unpaired electrons
4. Highly colored

Correct Answer: A

σ² π⁴ δ⁴ configuration → all electrons paired → diamagnetic.

IIT-JAM 2021

Q8: The Raman-active M–M stretching frequency is highest for:

1. Re₂Cl₈^2- (quadruple)
2. Mo₂(O₂CR)₄ (quadruple)
3. Cr₂{Ar}₂ (quintuple)
4. Mnv(CO)₁₀ (single)

Correct Answer: C

Quintuple Cr–Cr bonds show very high ν(M–M) ~400–450 cm⁻¹ due to extreme shortness and high bond order.

GATE Chemistry 2022

Q9: The first stable quintuple bond was reported by:

1. F.A. Cotton
2. Philip P. Power
3. Malcolm H. Chisholm
4. Richard R. Schrock

Correct Answer: B

Philip Power’s group reported the first Cr–Cr quintuple bond in 2005 using bulky terphenyl ligands.

CSIR NET June 2024

Q10: In quintuple-bonded Cr₂ complexes, the chromium is formally in oxidation state:

1. +2
2. +1
3. 0
4. +3

Correct Answer: B

Typical formulation Cr₂L₂ with neutral bulky ligands → each Cr is +1 (d⁵ configuration).

CSIR NET Dec 2022

Q11: The second δ bond in a quintuple bond arises from overlap of:

1. d_xy orbitals
2. d_x²-y² orbitals
3. d_xz orbitals
4. d_z² orbitals

Correct Answer: B

Quintuple bond uses both d_xy and d_x²-y² for the two δ components.

GATE Chemistry 2024

Q12: Quintuple bonds are known only for:

1. Re and W
2. Cr and Mo
3. Tc and Ru
4. Os and Rh

Correct Answer: B

Only Cr–Cr and Mo–Mo quintuple bonds have been isolated and structurally characterized.

Complete Interactive Guide to Quintuple Bonds • For JEE Advanced, NEET, Olympiad & Research Students