Phosphido-bis(borane) complexes of the alkaline earth metals

The reaction between two equivalents of {(Me 3 Si) 2 CH}(Ph)PH(BH 3 ) ( 1 ) and Bu 2 Mg, followed by two equivalents of BH 3 ·SMe 2 , gives the corresponding phosphido-bis(borane) complex, which may be crystallised as two distinct chemical species: the complex [{(Me 3 Si) 2 CH}(Ph)P(BH 3 ) 2 ] 2 Mg(THF) 4 ·THF ( 2a ), and two different THF solvates (1 : 1 and 1 : 2) of the solvent-separated ion triples [{(Me 3 Si) 2 CH}(Ph)P(BH 3 ) 2 ] 2 [Mg(THF) 6 ]·THF ( 2b ) and [{(Me 3 Si) 2 CH}(Ph)P(BH 3 ) 2 ] 2 [Mg(THF) 6 ]·2THF ( 2c ). Similar reactions between two equivalents of 1 and either (4- t BuC 6 H 4 CH 2 ) 2 Ca(THF) 4 or [(Me 3 Si) 2 CH] 2 Sr(THF) 2 , followed by two equivalents of BH 3 ·SMe 2 , give the heavier alkali metal complexes [{(Me 3 Si) 2 CH}(Ph)P(BH 3 ) 2 ] 2 M(THF) 4 [M = Ca ( 3 ), Sr ( 4 )]. Surprisingly, compounds 2a , 3 and 4 adopt almost identical structures in the solid state, which differ only in the geometrical arrangement of the phosphido-bis(borane) ligands and the hapticity of the borane groups. The phosphido-bis(borane) complexes 2-4 crystallise with remarkably similar structures, in spite of the progressively increasing ionic radii of the metal centres.