A 2026 study reported the first metal–metal multiply bonded complex in which each metal carries a classical meridionally coordinating pincer ligand: two-electron reduction of the molybdenum(III) pincer tribromide (PArNP)MoBr3 with sodium/mercury amalgam under argon gives the quadruply bonded dimer (κ3-PArNP)BrMo≣MoBr(κ3-PArNP) (Mo–Mo = 2.172 Å). The source work itself leaves open three linked questions that together define one objective — establishing the (pincer)M≣M(pincer) quadruple-bond motif as a general, mechanistically understood, synthetically controllable class of compounds: (i) the pathway by which the dimer forms (a proposed sequence via a one-electron-reduced dibromide that dimerizes remains unconfirmed, and the stronger reductant KC8 diverts the reaction to a monomeric (PArNP)MoBr fragment); (ii) whether analogous dimers can be synthesized with other pincer ligands (PCP, POCOP — computationally predicted to be favorable) and other metals (rhenium, chromium); and (iii) the scope of reactivity and potential catalytic applications of such dimers. No follow-up work addressing any of these questions has been reported in the literature as of 2026; the problem is open.

Background

A metal–metal multiple bond is a direct covalent bond between two transition-metal centers. The highest order found in classical molecular chemistry is the quadruple bond, built from one σ, two π, and one δ interaction between the metals (formal configuration σ2π4δ2\sigma^2\pi^4\delta^2), which arises when each metal has a d4d^4 configuration, e.g. Mo(II), Re(III), or Cr(II). Quadruple bonds were first recognized by Cotton and co-workers in the rhenium(III) dimer anion [Re2Cl8]2−[\mathrm{Re_2Cl_8}]^{2-} (1964), and thousands of examples are now known; over 900 structurally characterized Mo–Mo quadruple bonds were counted in the Cambridge Structural Database as of 2025 (as surveyed in the source paper). Almost all of them are supported by small bridging bidentate ligands — carboxylates, amidinates, halides — that hold the two metals in a lantern or square-prismatic geometry, as in Mo2(O2CR)4 paddlewheel compounds. Ligand frameworks that enforce very low coordination, such as bulky diamantane-like silyldiamides in the three-coordinate Mo–Mo quadruply bonded complex of Tsai et al., or amidinate/boraamidinate series in which the formal Mo–Mo bond order can be stepped from 3 to 5, show how strongly the ligand set controls the bond.

Pincer ligands are tridentate ligands that bind one metal in a plane, meridionally (denoted κ3-mer); the 'classical' families carry terminal phosphine or phosphinite arms on an aryl or amide backbone: PCP (2,6-bis(phosphino)aryl), POCOP (2,6-bis(phosphinito)aryl), and PNP-type amides such as PArNP, the monoanionic bis(2-(diisopropylphosphino)-4-methylphenyl)amide used in the source work. Pincer chemistry is one of the largest and most tunable platforms in organometallic chemistry, but it had almost never intersected direct metal–metal multiple bonds: prior to 2026, only three reports (as summarized in the source paper) described metal–metal bonded complexes with a κ3-mer pincer on each metal, all Pt–Pt dimers with long bonds (≳3.0 Å) and auxiliary bridging ligands, and the single earlier Mo–Mo quadruple bond involving a tridentate ligand had that ligand coordinated to only one of the two metals. Pincer-ligated molybdenum dimers in the literature are typically bridged by dinitrogen (the platform for catalytic N2-to-ammonia chemistry) rather than directly metal–metal bonded, and other pincer dimers, such as a Co2N2 diamond-core compound arising from an unusual rearrangement of a PNP pincer, likewise contain no short unsupported M–M bond.

The source work (preprint January 2026; published in Dalton Transactions, May 2026) closed this gap: two-electron reduction of the Mo(III) tribromide (PArNP)MoBr3 with Na/Hg under argon yields (κ3-PArNP)BrMo≣MoBr(κ3-PArNP), the first metal–metal complex with classical pincer ligands on both metals. Its Mo–Mo distance of 2.172 Å sits at the long edge of the normal 2.06–2.17 Å range for Mo–Mo quadruple bonds, and natural bond orbital (NBO) analysis supports the four bonding components. Two further observations frame the open questions. First, reductant choice is decisive: the stronger reductant KC8 (potassium graphite, a strong solid-state electron-transfer reagent) does not give the dimer but a paramagnetic tetrahydrofuran adduct of the monomeric fragment (PArNP)MoBr. Second, M06 density-functional calculations using isodesmic reactions (idealized reactions that conserve the number of each bond type on both sides, so that the computed energy difference isolates the effect of interest) indicate that the hypothetical dimers [(iPrPCP)MoBr]2 and [(iPrPOCOP)MoBr]2 would have slightly stronger Mo–Mo bonds than the experimental dimer, arguing that the amide-bridged PArNP ligand is not uniquely stabilizing and that dimerization should be thermodynamically favorable for a broader class of pincers — although this computational extrapolation rests on only two ligands and an unvalidated functional choice.

Problem Statement

Establish the (pincer)M≣M(pincer) metal–metal quadruple-bond motif as a general, mechanistically understood, and synthetically controllable class of compounds, starting from the sole known example (κ3-PArNP)BrMo≣MoBr(κ3-PArNP). Three linked, source-faithful sub-questions jointly serve this objective:

  1. Formation mechanism. Determine the actual pathway by which reduction of (PArNP)MoBr3 with Na/Hg under argon gives the quadruply bonded dimer. The proposed sequence — initial one-electron reduction to the dibromide (PArNP)MoBr2 (denoted 1-Br2), dimerization of this fragment, and subsequent reduction to the final dimer — has not been confirmed experimentally or theoretically, and it must also explain the observed reductant-dependent bifurcation: the stronger reductant KC8 instead yields the monomeric paramagnetic (PArNP)MoBr(THF) adduct.

  2. Generality across ligands and metals. Determine whether analogous (pincer)M≣M(pincer) quadruply bonded dimers can be synthesized with other pincer ligands — e.g., the aryl-based PCP and POCOP ligands, for which M06 isodesmic calculations predict Mo–Mo bond strengths slightly greater than in the experimental dimer — and with other metals, notably rhenium and chromium; alternatively, demonstrate rigorously which analogues are inaccessible and why. The named PCP and POCOP ligands and the rhenium and chromium metals are illustrative examples of the required scope rather than an exhaustive list: resolving this sub-question requires a general conclusion about which pincer ligands and metals support the motif, or a rigorous argument that no such generalization is available, and not only the named cases.

  3. Reactivity and catalytic scope. Determine what chemistry the M≣M quadruple bond of such dimers supports, including small-molecule activation (e.g., of N2, CO, or PMe3) and whether these dimers can serve as catalysts or catalyst precursors.

All three sub-questions are mandatory and jointly necessary to resolve the task: the task counts as resolved only when every sub-question is answered rigorously, and a rigorous answer to only some of them does not resolve the task.

References

  1. Souvik Mandal, Ethan Y. Song, Thomas J. Emge, Faraj Hasanayn, Alan Goldman, "A metal-metal quadruply bonded dimer of two pincer-ligated metal centers", ChemRxiv preprint, posted 2026-01-07. DOI: 10.26434/chemrxiv-2026-cxh6t, https://doi.org/10.26434/chemrxiv-2026-cxh6t
  2. Souvik Mandal, Ethan Y. Song, Thomas J. Emge, Faraj Hasanayn, Alan S. Goldman, "A metal–metal quadruply bonded dimer of two pincer-ligated metal centers", Dalton Transactions, 2026, 55(20), 7816–7820. DOI: 10.1039/d6dt00920d, https://doi.org/10.1039/d6dt00920d
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  7. Fitzerald Hujon, R. H. Duncan Lyngdoh, Henry F. Schaefer, R. Bruce King, "From no bond to sextuple bond? CrCr distances in binuclear chromium systems", Inorganica Chimica Acta, 2026, 602, 123296. DOI: 10.1016/j.ica.2026.123296, https://doi.org/10.1016/j.ica.2026.123296
  8. Alison R. Fout, Falguni Basuli, Hongjun Fan, John Tomaszewski, John C. Huffman, Mu-Hyun Baik, Daniel J. Mindiola, "A Co2N2 Diamond-Core Resting State of Cobalt(I): A Three-Coordinate CoI Synthon Invoking an Unusual Pincer-Type Rearrangement", Angewandte Chemie International Edition, 2006, 45(20), 3291–3295. DOI: 10.1002/anie.200504343, https://doi.org/10.1002/anie.200504343