Transition metals as Lewis bases: "Z-type" boron ligands and metal-to-boron dative bonding.

While the vast majority of inorganic chemistry involves electron donation from main-group atoms to metals, an intriguing yet flip-side exists: where Lewis-basic metals donate electron density to Lewis-acidic main-group atoms (most often boron). These so-called "Z-type" ligands, along with other less clear-cut complexes, are examples of this metal-ligand role reversal. This perspective article offers an introduction to metal-to-boron dative bonding, and attempts to correlate spectroscopic and structural data from the complexes.