Charge density and NMR unravel Ni-agostic bonding

To pin down the nature of the β-agostic bond in late transition metal complexes, Scherer (Augsburg) and coworkers synthesized [(DCpH)Ni(d^tbpe)]⁺[BF₄]^– (DCp = dicyclopentadiene, d^tbpe = ^tBu₂PCH₂CH₂P^tBu₂), a rigid β-agostic complex. In a combined experimental and theoretical study the charge density and NMR shifts were scrutinized.

To get good agreement with experiment, NMR calculations necessitated explicit spin-orbit coupling with hybrid GGAs in ADF. Diamagnetic (σ^d), paramagnetic (σ^p) and spin-orbit (σ^SOC) contributions to the shielding tensor of early and late transition metal complexes revealed that the ¹H NMR chemical shift of the agostic H is determined mostly by σ^p while the atomic charge and σ^d at H remain rather constant in a variety of model systems studied. The term ‘hydridic shift’ to explain upfield shifts is therefore misleading. The experimental charge density analysis, in perfect agreement with AIM calculations in ADF, helps in further classifying β-agostic interactions. For late transition metal complexes, significant electron density builds up in the M-H bond at the expense of the C-H bond. An MO analysis reveals that the late transition metal β-agostic bond is well described by the classic Dewar-Chatt-Duncanson model for metal-alkenes, augmented with a M←L σ-donation term.

Excellent agreement between the experimental (left) and calculated (middle) topology of -∇²ρ(r) Right: HOMO-7 demonstrating the contribution of Ni←L σ-donation.