Direct measurement of covalent three-center, two-electron M-H-B bonding in Zr and Hf borohydrides using B K-edge XAS

Metal borohydride complexes have long been the subject of intense fundamental interest because of their unconventional metal-ligand bonding that occurs via three-center, two-electron M-H-B bonds. This type of bonding implies significant delocalization of electron density over all three atoms, but the degree of orbital mixing between the metal and boron has been difficult to assess by direct experimental means. Herein, we demonstrate how ligand K-edge X-ray absorption spectroscopy (XAS) conducted at the B K-edge yields evidence of significant covalent M-H-B bonding with Zr and Hf. To accommodate the B K-edge XAS studies, which were conducted under ultra-high vacuum (<10^-8 torr), we prepared a series of new [Zr(RBH₃)₄] and [Hf(RBH₃)₄] complexes with substituents that attenuate volatility (R = benzyl, phenyl, mesityl, 2,4,6-triisopropylphenyl, and anthryl). ¹H and ¹¹B NMR spectroscopy, IR spectroscopy, and single-crystal X-ray diffraction (XRD) studies revealed metal and ligand dependent differences in the BH₃ chemical shifts that correlate to changes in M-B distances and select B-H vibrational stretching modes. The B K-edge XAS spectra of the Zr and Hf complexes yielded a pre-edge feature that was assigned as B 1s → M-H-B π* based on comparison to time-dependent density functional theory (TDDFT) calculations. The pre-edge transitions appear due to covalent mixing between boron and the metal, thereby demonstrating how B K-edge XAS can provide direct evidence of covalent three-center, two electron M-H-B bonding in borohydride complexes using boron as a spectroscopic reporter.