Tricoordinate Boranes and Trialkylboranes

Borane (BH3) and trialkylboranes have well-dehned resonance values. Trialkylboranes rarely show resonances outside the narrow low field range 80-95 ppm regardless of the structure of the alkyl group. The chemical shifts of alkylboranes of the formula BRH2 or BR2H, however, cannot be as easily predicted and are highly dependent upon the structure of the alkyl groups on the boron atom (Table 2.2). 

Formation of a dimerized species is common in tricoordinated boranes lacking steric hindrance. Their presence leads to a shift towards lower resonance frequencies and the possibility of more convoluted spectra by virtue of additional coupling interactions. Dative interactions with boron, where the two electrons forming the bond originate from another moleeule (the solvent or a basic ligand), also lead to higher field shifts. 

Varying the unsaturated substituents on the boron atom has a larger effect on the b chemical shifts than saturated analogs (Table 2.3). Ligands with n-bonds adjacent to the central boron atom are more prone to donate into 

Similarly, the introduction of heteroatoms like oxygen, nitrogen, or halogens results in lower chemical shifts when compared to alkylboranes. The reason for this resides in the inter- or intramolecular donation of the lone pairs of the heteroatom into the p orbital of the central boron. This has the same effect as unsaturated systems, leading to upheld shifts of B signals. 

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