Metal-Metal Bonding in Transition Metal Complexes

Metal–metal bonding in transition metal complexes encompasses a spectrum of interactions ranging from single to quintuple bonds, with multiplicity determined by the overlap of metal d orbitals. These bonds underpin unique electronic structures, reactivity profiles and material properties. In complexes with high bond orders, such as quadruple and quintuple bonds, σ, π and δ interactions each contribute to bond strength and directionality. Ligand design and coordination geometry exert a decisive influence on metal–metal distances and electronic delocalisation, enabling fine-tuning of redox potentials, catalytic activity and magnetic behaviour. In paddlewheel frameworks, for example, bridging ligands enforce close metal–metal contacts while modulating the symmetry and energy of frontier orbitals. Heterobimetallic architectures further expand bonding possibilities by combining metals with distinct electron counts and orbital energies, yielding novel redox flexibility and cooperative small-molecule activation. Advances in spectroscopic methods, quantum chemical calculations and in situ reactivity studies have deepened our understanding of how ligand electronics, steric factors and metal identity converge to stabilise high-order bonds, unlock unconventional oxidation states and drive selective transformations of small molecules such as CO₂ and SO₂.

Research from Nature Portfolio

No recent Nature Portfolio content available.

Metal-Metal Bonding in Transition Metal Complexes publication trend

The graph below shows the total number of articles in metal-metal bonding in transition metal complexes across all publications each year (not limited to Nature Index journals).

Technical terms

Bond order: A measure of the number of shared electron pairs between two metal centres, corresponding to σ, π and δ contributions.

Paddlewheel complex: A dinuclear framework in which four bridging ligands link two metal atoms, resembling a paddlewheel shape.

δ bond: A metal–metal interaction arising from the side-on overlap of two d orbitals beyond σ and π bonds.

π metal–ligand interaction: Overlap between metal d orbitals and ligand π systems that can reinforce or attenuate metal–metal bonding.

Dative bond: A coordination bond in which both electrons in the bond originate from the same atom, often from a ligand to a metal.

References

1. Torsion Effects Beyond the δ Bond and the Role of π Metal‐Ligand Interactions. Advanced Science (2024).
2. Enhancing the Air Stability of Dimolybdenum Paddlewheel Complexes: Redox Tuning through Fluorine Substituents. Inorganic Chemistry (2022).
3. CO 2 and SO 2 activation by a Cr–Cr quintuple bond. Chemical Communications (2014).