Phosphanylborohydrides: First Assessment of the Relative Lewis Basicities of [BH ₃ PPh ₂ ] ^– , CH ₃ PPh ₂ , and HPPh ₂

The compounds K[PPh 2 ], HPPh 2 , CH 3 PPh 2 , BH 3 (H)PPh 2 , BBr 3 (H)PPh 2 , BH 3 (CH 3 )PPh 2 , BBr 3 (CH 3 )PPh 2 , [H 2 PPh 2 ]I, [CH 3 (H)PPh 2 ]I, [(CH 3 ) 2 PPh 2 ]I, and K[(BH 3 ) 2 PPh 2 ] have been investigated by NMR spectroscopy. In addition, X-ray crystal structures have been determined for K(18-crown-6)[PPh 2 ], BBr 3 (H)PPh 2 , BBr 3 (CH 3 )PPh 2 , [H 2 PPh 2 ]I, [CH 3 (H)PPh 2 ]I, [(CH 3 ) 2 PPh 2 ]I, and K(18-crown-6)[(BH 3 ) 2 PPh 2 ]. An evaluation of coupling constants (e.g. 1 J PCi ; C i = ipso carbon atom of a phenyl ring) augmented by an inspection of key structural parameters (e.g. the angles C i -P-C i ') leads to the conclusion that dative bonds originating from ligand [BH 3 PPh 2 ] - possess a significantly higher p character than dative bonds involving the ligands HPPh 2 and CH 3 PPh 2 . The 1 J PB values obtained for BH 3 (H)PPh 2 , BH 3 (CH 3 )PPh 2 , and [(BH 3 ) 2 PPh 2 ] - suggest [BH 3 PPh 2 ] - to form the strongest [BH 3 ] adduct of all three compounds which is in agreement with the results of displacement reactions employing the couples CH 3 PPh 2 / [(BH 3 ) 2 PPh 2 ] - and [BH 3 PPh 2 ] - /BH 3 (CH 3 )PPh 2 .