Reference has already been made to the unpredictability and irregularity of trends in 11 B NMR resonance frequencies (vide supra); however, typically, though it need not of necessity, the vacant 2p z orbital of boron plays a crucial role in the position of chemical shifts in 11 B NMR spectra. Strictly speaking, trivalent boron species resonate at much lower fields than their quadrivalent counterparts. For instance, trimethyl borane as a true trivalent boron species resonates at the lowest field, whereas trimethyl borate as a somewhat quadrivalent boron species with considerable π-bonding character resonates at much higher fields in B NMR. The belief that the determinant factor in the position of chemical shifts in B NMR spectra is the π-donating ability of substituents on boron, implies that diethylamino substituents on boron should bring about upfield shifts for the B signals in their NMR spectra, relative to that from -OMe by virtue of the fact that -NEt 2 is mesmerically more π-donating than -OMe. However, this effect appears to be reversed in aliphatic boron species, as such B(NEt 2 ) 3 resonates at a much lower field in B NMR than that observed for B(OMe) 3 . 9