Source: Corey Burns, Tamara M. Powers, Department of Chemistry, Texas A&M University

Paddlewheel complexes are a class of compounds comprised of two metal ions (1^st, 2^nd, or 3^rd row transition metals) held in proximity by four bridging ligands (most commonly formamidinates or carboxylates) (Figure 1). Varying the identity of the metal ion and the bridging ligand provides access to large families of paddlewheel complexes. The structure of paddlewheel complexes allows for metal-metal bonding, which plays a vital role in the structure and reactivity of these complexes. Due to the diversity of electronic structures that are available to paddlewheel complexes - and the corresponding differences in M-M bonding displayed by these structures - paddlewheel complexes have found application in diverse areas, such as in homogeneous catalysis and as building blocks for metal-organic frameworks (MOFs). Understanding the electronic structure of the M-M bonds in paddlewheel complexes is critical to understanding their structures and thus to application of these complexes in coordination chemistry and catalysis.

Figure 1. General structure of paddlewheel complexes, where M can be a 1^st, 2^nd, or 3^rd row transition metal.

When two transition metals are held in close proximity the d-orbitals overlap, which can result in the formation of M-M bonds. Overlapping d-orbitals can form three types of bonds - σ, π, and δ - depending on the symmetry of the orbitals involved. If we assign the molecular z-axis to be coplanar with the M-M bond, a σ bond is formed by overlap of the d_z² orbitals and π bonds are formed by overlap of the d_xz and d_yz orbitals. δ bonds are generated by overlap of d-orbitals that have two planar nodes (d_xy and d_x²–_y²). As a result, all four lobes of the d-orbital overlap and the corresponding δ bond has two planar nodes (Figure 2). In theory, with the addition of δ bonds, paddlewheel complexes are capable of supporting quintuple bonds, or five bonds between metal atoms.¹ In most complexes, the d_x²–_y² forms strong metal-ligand bonds and does not meaningfully contribute to M-M bonding. Thus, quadruple bonds are the maximum bond order in many complexes.

Figure 2. Visual representation of σ, π, and δ bonding MOs resulting from the linear combination of metal d-orbitals. The d_z² atomic orbitals have the best spatial overlap, followed by the d_xz and d_yz orbitals. The d_xy atomic orbitals have the least amount of spatial overlap.

In this video, we will synthesize the dimolybdenum paddlewheel complex Mo₂(ArNC(H)NAr)₄, where Ar = p-(MeO)C₆H₄, which features a quadruple bond. We will characterize the compound by NMR spectroscopy and use X-ray crystallography to study the M-M bond.