Characterization of the Ligand Properties of Donor-stabilized Pnictogenyltrielanes

A general synthesis and the characterization of novel alkyl-substituted NHC-stabilized pnictogenylboranes NHC ⋅ BH₂ ER₂ (NHC=N-heterocyclic carbene, E=P, As; R₂ =Me₂ , Ph₂ , ^t BuH, Cy₂ , (SiMe₃ )₂ ) are reported. These compounds were reacted with Ni(CO)₄ to the corresponding complexes of the type [(NHC ⋅ BH₂ ER₂ )Ni(CO)₃ ] to determine their donor strength by Tolman Electronic Parameters (TEPs) and their steric demand as ligands compared to classical phosphines, superbasic phosphines and other commonly applied donor systems. The results show that the NHC-stabilized pnictogenyltrielanes can be considered as being highly basic, while their steric influence depends strongly on the organic residues as well as the donor attached to the {BH₂ } moiety. Although weaker than commonly used superbasic phosphines, the donor strength of pnictogenyltrielanes in general can be classified as of similar strength as NHCs. The steric and electronic properties can easily be modified by alkyl substitution as evident from the TEP trends.