The electronic and steric effects in the stoichiometric dehydrocoupling of secondary and primary phosphine-boranes H3B center dot PR2H [R = 3,5-(CF3)(2)C6H3; p-(CF3)C6H4; p-(OMe)C6H4; adamantyl, Ad] and H3B center dot PCyH2 to form the metal-bound linear diboraphosphines H3B center dot PR2BH2 center dot PR2H and H3B center dot PRHBH2 center dot PRH2, respectively, are reported. Reaction of [Rh(L)(eta(6)-FC6H5)][BAr4F] [L = Ph2P(CH2)(3)PPh2, Ar-F = 3,5-(CF3)(2)C6H3] with 2 equiv of H3B center dot PR2H affords [Rh(L)(H)(sigma,eta-PR2BH3)(eta(1)-H3B center dot PR2H)]center dot[BAr4F]. These complexes undergo dehydrocoupling to give the diboraphosphine complexes [Rh(L)(H)(sigma,eta(2)-PR2 center dot BH2PR2 center dot BH3)][BAr4F]. With electron-withdrawing groups on the phosphine-borane there is the parallel formation of the products of B-P cleavage, [Rh(L)(PR2H)(2)] [BAr4F], while with electron-donating groups no parallel product is formed. For the bulky, electron rich, H3B center dot P(Ad)(2)H no dehydrocoupling is observed, but an intermediate Rh(I) sigma phosphine-borane complex is formed, [Rh(L){eta(2)-H3B center dot P(Ad)(2)H)][BAr4F], that undergoes B-P bond cleavage to give [Rh(L){eta(1)-H3B center dot P(Ad)(2)H}{P(Ad)(2)H}][BAr4F]. The relative rates of dehydrocoupling of H3B center dot PR2H (R = aryl) show that increasingly electron-withdrawing substituents result in faster dehydrocoupling, but also suffer from the formation of the parallel product resulting from P-B bond cleavage. H3B center dot PCyH2 undergoes a similar dehydrocoupling process, and gives a mixture of stereoisomers of the resulting metal-bound diboraphosphine that arise from activation of the prochiral P-H bonds, with one stereoisomer favored. This diastereomeric mixture may also be biased by use of a chiral phosphine ligand. The selectivity and efficiencies of resulting catalytic dehydrocoupling processes are also briefly discussed.