A 2008 density-functional study of the rhodium pinacolborane complexes [CpRh(HBpin)2] and [CpRh(HBpin)(Bpin)2] found heavily elongated but retained B–H bonds (Wiberg bond indices 0.31–0.33 versus 0.96 in free pinacolborane) and assigned them 'stretched sigma-borane' electronic descriptions, while leaving open whether stable structures genuinely intermediate between sigma-borane (retained B–H) and oxidative-addition hydride–boryl formulations exist, which combinations of ancillary ligand identity, the boron ligand set at the metal (borane versus boryl co-ligands), metal identity, and borane Lewis acidity govern the balance between the two extremes, and how the computed hydrogen positions could be validated given that X-ray diffraction locates H atoms imprecisely. No systematic or predictive account of the governing factors exists.

Background

A sigma-borane complex is a coordination compound in which an intact B–H bond of a neutral borane binds side-on to a transition-metal center through its sigma bonding electron pair — the boron analogue of the Kubas-type dihydrogen complex, in which an intact H–H bond coordinates to a metal without cleaving. The competing oxidative-addition description is a metal hydride boryl complex: the B–H bond is formally cleaved into a metal–hydride (M–H) and a metal–boryl (M–B) bond. The two formulations are now understood as the endpoints of a B–H activation continuum, and species near the middle are invoked as intermediates in hydroboration, C–H borylation, and borane dehydrocoupling/dehydrogenation catalysis. Landmark experimental realizations include manganese and rhenium sigma-borane complexes (Schlecht and Hartwig, 2000) and ruthenium borane sigma-complexes that interconvert with hydride species and release hydrogen reversibly (Alcaraz, Grellier and Sabo-Etienne, 2009).

Pinacolborane (HBpin = HB(OCMe2CMe2O)) is a dialkoxyborane widely used in catalytic hydroboration and borylation; Bpin is the pinacolboryl ligand. Cp denotes the cyclopentadienyl ligand (η5\eta^{5}-C5_{5}H5_{5}) and Cp* its pentamethyl derivative. The source study (Pandey, 2008) computed the model complexes [CpRh(HBpin)2_{2}] and [CpRh(HBpin)(Bpin)2_{2}] by density functional theory (DFT) at the BP86/TZ2P level with scalar-relativistic ZORA (zeroth-order regular approximation) treatment of rhodium, as truncated models of Cp* complexes whose heavy-atom frameworks had been determined by X-ray diffraction. The computed Rh–B distances and B–Rh–B angles reproduced the X-ray values closely, while the coordinated B–H bonds emerged strongly elongated relative to free HBpin (computed free B–H 1.190 Å): Wiberg bond indices — a bond-order metric from natural bond orbital (NBO) analysis — fell from 0.96 in free HBpin to 0.31–0.33 in the complexes, the Rh–B and Rh–H bonding NBOs were polarized 65–79% toward rhodium, and the highest occupied molecular orbital retained well-developed B–H bonding character. Pandey described these as 'stretched sigma-borane' complexes: B–H bonds heavily weakened but not cleaved, with substantial metal–hydrogen and metal-to-boron interactions — far along the continuum toward oxidative addition, but still on the sigma-borane side of it. Earlier DFT studies using different ligand models had given conflicting geometries for the same complexes.

Three issues were left open. First, the computations used Cp in place of Cp* and simplified borane models, so the effect of full ligand substitution on the sigma-borane/hydride–boryl balance was unquantified. Second, and more generally, the field lacked knowledge of whether stable structures that are true intermediates between the two extremes exist, and of which combinations of ancillary ligand identity, the boron ligand set at the metal (borane versus boryl co-ligands), metal identity, and borane Lewis acidity govern the relative stabilities of the two electronic extremes. Third, because X-ray diffraction locates hydrogen atoms imprecisely, the computed hydrogen positions and B–H bond characters lacked direct experimental benchmarks (neutron diffraction or high-level spectroscopic references). The diagnostics used in this field to place a structure on the continuum include geometrical metrics (B–H, M–H, M–B distances), NBO/Wiberg bond indices, the topological Atoms-in-Molecules (AIM) analysis of the electron density (which assigns bond critical points between atom pairs), and energy-decomposition analysis (EDA), which partitions metal–ligand bonding into electrostatic, orbital, and Pauli-repulsion terms.

Elsewhere on the continuum, isolated osmium elongated sigma-borane and bis(elongated sigma)-dihydridoborate complexes and computed platinum intermediates show that continuum-middle structures are realizable, while in one rhodium boryl hydride the eta2-B–H arrangement is a transition state; neutron-diffraction hydrogen benchmarks have been established for an iridium sigma-borane and for amine-borane-derived Rh/Ir complexes.

Problem Statement

The problem, raised by the source study for the rhodium–pinacolborane system and extended to metal–borane adducts as a class, is unified by one objective — a predictive account of what determines a metal–borane adduct's position on the σ-borane ↔ hydride–boryl continuum, anchored to hydrogen-locating experiment wherever such data can be obtained. Its three linked parts are each independently answerable sub-questions, and a complete resolution of the objective requires all three; within part (2), both the general placement rules and the specific Cp* truncation case of the source complexes are required:

(1) Existence and isolability of true intermediates. Genuine intermediates between the σ-borane (retained B–H) and oxidative-addition hydride–boryl formulations are established for other metals — isolated osmium elongated σ-borane complexes and computed platinum intermediates — but not for the rhodium–pinacolborane system. Can such an intermediate be isolated (or shown to be a transition state) for the rhodium–pinacolborane complexes at issue — the Cp* analogues of the computed model complexes [CpRh(HBpin)2] and [CpRh(HBpin)(Bpin)2]? A genuine intermediate (an 'elongated σ-borane' in the diagnostic convention developed for third-row metal complexes) has a substantially elongated but not cleaved B–H bond, a significant M–H interaction, and an emerging M–B bond; in AIM terms it displays a triangular M–H–B topology with M–B, M–H and B–H bond critical points, in contrast to an unelongated σ-borane (M–B and B–H critical points only) or a hydride–boryl formulation (no B–H bond path).

(2) Governing factors. Which combinations of ancillary ligand identity (e.g., Cp versus Cp*, phosphine versus pincer), the boron ligand set at the metal (neutral σ-borane versus boryl co-ligands, as in [CpRh(HBpin)2] versus [CpRh(HBpin)(Bpin)2]), metal identity, and borane Lewis acidity (set by the substituents at boron) govern the relative stabilities of the σ-borane and hydride–boryl electronic extremes — that is, what rules predict where on the continuum a given M–(B–H) fragment sits? This includes a truncation question specific to the source complexes: does replacing the Cp model with the experimental Cp* ligand (and full pinacolborane substituents) shift the balance between the stretched σ-borane and hydride–boryl descriptions of the rhodium complexes?

(3) Experimental validation of hydrogen positions. Can the computed hydrogen positions and B–H bond character of the stretched σ-boranes be corroborated by H-locating experiments — neutron diffraction or comparably precise benchmarks — for the Rh–HBpin system specifically, where X-ray diffraction locates H atoms imprecisely?

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