Dr. Malte Sellin and Professor Ingo Krossing, Albert-Ludwigs-Universität Freiburg, Germany discuss the synthesis, characterization, and bonding of the heterodinuclear cation [MnFe(CO)₁₀]⁺, along with the preparation of the isoelectronic anion [CrMn(CO)₁₀]⁻

 

What did you do?

Dimetal decacarbonyl complexes like Mn₂(CO)₁₀ are fundamental to organometallic chemistry and essential for teaching the subject to chemistry majors. Here, we synthesized and characterized the first heterodinuclear transition metal carbonyl cation, [MnFe(CO)₁₀]⁺. This was done by combining the recently published [Fe(CO)₅]⁺ cation with half an equivalent of Mn₂(CO)₁₀ [1].

As our complex completes the existing series of dimetal decacarbonyl complexes, [MM’(CO)₁₀]ᶻ, all having the same electron count but different charges, we compared this series through experimental analyses and DFT calculations.

 

Why are you interested in this?

Our general interest in transition metal carbonyl cations (TMCCs) is motivated by multiple factors: 

1. Despite their fundamental nature and numerous initial characterizations in the gas-phase or in noble-gas matrices, the number of isolated TMCCs is still quite low. 

2. The challenges in the isolation of novel TMCCs are driving us further in the development of suitable reagents and solvents, which could also help isolate other interesting reactive cations.

3. Additionally, the unique combination of the huge importance of the carbonyl ligand, their structural simplicity, the rather strict 18 valence electron rule, and the ongoing research are making TMCCs ideal ‘textbook compounds’.

As our group had prepared multiple open-shell TMCCs, such as [Fe(CO)₅]⁺ and [Ni(CO)₄]⁺, we were interested in their reactivity towards several binary transition metal complexes. Among multiple combinations, only the [MnFe(CO)₁₀]⁺ complex was stable enough for isolation. We anticipate the title compound will be included in the chemistry curriculum as a ‘textbook compound.’

 

What is new and cool about this work?

The most interesting feature of this complex is the unusual electron distribution between the metal atoms. It can be formulated in two ways: either as a Lewis pair with an 18-valence-electron (VE) and a 16-VE fragment: [(OC)₅Fe⁰ → Mnᴵ(CO)₅]⁺, or as a covalent bond between two 17-VE fragments: [(OC)₅Feᴵ–Mn⁰(CO)₅]⁺.

According to DFT calculations, there is barely any preference for any of these two formulations, meaning that each Lewis-structure represents only half of the reality. So, this compound highlights the need for a balanced ‘gray-scale’ approach rather than a ‘black-and-white’ description.

 

What is the main significance of your results?

The successful generation of [MnFe(CO)₁₀]⁺ shows, that the metalloradical [Fe(CO)₅]⁺ can react as a pseudo-halogen towards suitable substrates, thus making it an iron(I) synthon and also an organometallic building block. While many neutral and anionic transition metal carbonyl clusters have been synthesized by the combination of carbonyl complexes, this principle has been not successfully applied for TMCCs.

 

What is the longer-term vision of your research?

A possible follow-up study on this complex is the investigation of M–M bond cleavage. Depending on the substrate, we expect either heterolytic or homolytic cleavage of this bond.

In the long term, we aim to synthesize coordinately unsaturated TMCCs. The [MnFe(CO)₁₀]⁺ complex investigated here can also be seen as a first step toward weak adducts of the [Mn(CO)₅]⁺ cation. We expect that coordinately unsaturated TMCCs will play an importnt role in in the activation of small molecules in the future.

 

What part of your work was the most challenging?

DFT calculations predicted a certain asymmetry in the [MnFe(CO)₁₀]⁺ cation in the solid state, which would have provided strong evidence for an asymmetric electron distribution in the manganese-iron bond. However, due to the rather small structural differences and disorder in the cation, the geometric consequences of this unique electronic structure could not be determined. As a result, direct proof of the polarized bond was missing.

Consequently, we relied on indirect evidence by combining multiple characterization methods, including Mössbauer, NMR, and IR spectroscopy, as well as the structural characterization of the isoelectronic heterodinuclear complex [CrMn(CO)₁₀]⁻, to obtain sufficient experimental support for this feature.

 

Anything else you would like to add?

We would like to thank our cooperation partners for their engagement.
 

Thank you very much for sharing these insights.

 

The paper they talked about:

• Isolation and Characterization of [MnFe(CO)₁₀]⁺: the Missing Link in the 3d Dimetal Decacarbonyl Series, 
  Malte Sellin, Hendrik Koger, Tobias A. Engesser, Jörg Grunenberg, Ingo Krossing,
  Chem. Eur. J. 2025.
  https://doi.org/10.1002/chem.202500489

 

Malte Sellin is a Post-Doc at the Department of Chemistry at the University of Basel, Switzerland.

Ingo Krossing is Professor at the Institute of Inorganic and Analytical Chemistry at the Albert-Ludwigs-Universität Freiburg, Germany.