A newly reported N-heterocyclic carbene called BnITr has a percent buried volume above 60%, according to chemists Ludwig Zapf and Eric Rivard. Its benzylated backbone positions the phenyl rings of its N-bound trityl groups close to the metal-binding center, creating unusually strong steric shielding. That bulky shape helped stabilize rare low-coordinate lithium and gallium(I) cations—but an attempt to make a one-coordinate palladium(0) complex instead triggered ligand activation and trityl migration.
What BnITr is, and what makes it unusually bulky
N-heterocyclic carbenes (NHCs) are ligands: molecules that bind to a metal through a carbon atom in a ring. Chemists can tune their steric properties—the space they occupy around a metal—and their electronic properties by changing the ring backbone and the groups attached to its nitrogen atoms.
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BnITr combines a benzylated backbone with trityl groups attached to the nitrogens. The backbone affects how the trityl phenyl rings are oriented: they sit close to the carbene donor center, the site that binds the metal. In the authors’ structural explanation, that arrangement creates a substantial shield around the donor.
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Percent buried volume, written %Vbur, estimates how much of the space around a metal center is occupied by a ligand. Zapf and Rivard report that BnITr exceeds 60%. This is the value reported for their ligand and structural analysis; it should not be treated as a context-free constant independent of the metal complex or calculation method.
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What the ligand enabled—and what it did not
The study reports that BnITr enabled the isolation or stabilization of quasi-monocoordinated lithium and gallium(I) cations. “Quasi-monocoordinated” describes a metal center with one principal coordinating ligand, while allowing that the full chemical environment may be more complicated than a perfectly isolated, one-bond case. Such low coordination is unusual because metals commonly bind to multiple ligands.
The palladium experiment provides an important counterexample to the idea that extra bulk guarantees a desired complex. The researchers attempted to form a one-coordinate Pd(0) complex, but instead observed redox-triggered ligand activation and migration of a trityl group to palladium. The target complex was therefore not obtained as expected.
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How the reported size compares with an earlier NHC
Chemistry World reported a 58.8% buried-volume figure for a biphenyl-wingtip NHC from Michal Szostak’s group in 2025. Zapf and Rivard’s above-60% result is higher, but the figures come from different ligand and complex contexts. They are useful as a literature comparison, not a controlled head-to-head measurement.
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| Ligand or result | Reported %Vbur | Context |
|---|---|---|
| BnITr | Above 60% | Reported by Ludwig Zapf and Eric Rivard in 2026 for their ligand and structural analysis. |
| Biphenyl-wingtip NHC | 58.8% | Reported for Michal Szostak’s group in 2025, as described by Chemistry World. |
“Bulkiest yet” is a state-of-the-art comparison at the time of publication, not a permanent record. A meaningful comparison between bulky NHCs should consider the metal complex and calculation context alongside the percentage, as well as ligand geometry, coordination number, and observed reactivity.
What the study establishes about preparation and evidence
Chemistry World describes BnITr’s synthesis as a two-step route starting from commercially available 2-trityl-benzimidazole. That does not establish that BnITr itself is commercially sold. The paper’s supporting information includes experimental details, NMR and IR spectra, computational details, Cartesian coordinates, and crystallographic data.
The Chemical Science study reports experimental and computational characterization focused on selected silver, gold, and rhodium complexes, in addition to the lithium, gallium(I), and attempted palladium chemistry. The article, “Achieving (quasi)-monocoordination in metal complexes with an exceptionally bulky carbene ligand,” by Ludwig Zapf and Eric Rivard, appeared in Chemical Science in 2026, volume 17, pages 6500–6506 (DOI: 10.1039/D5SC09514J). It was first published on 2 February 2026 as an open-access Edge Article under a Creative Commons Attribution-NonCommercial 3.0 licence.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why the result matters beyond a size record
The significance of BnITr is not simply that its reported buried volume exceeds an earlier comparison. Its design shows how changing a ring backbone can reposition bulky nitrogen substituents around a donor center, and the metal chemistry illustrates both the opportunity and the limit of that shielding: it can support unusual low-coordinate species, but it does not prevent other reactions or ensure that a targeted complex will form.
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