The coordination chemistry of the neutral diphosphines, R2P(CH2)(2)PR2 (R = Me or Et) and o-C6H4(PR'(2))(2) (R' = Me or Ph), and the diarsine, o-C6H4(AsMe2)(2), toward the Lewis acidic BX3 (X = F, Cl, Br, and I) fragments is reported, including several rare complexes incorporating BF3 and BF2+. The studies have revealed that the flexible dimethylene linked diphosphines form[(BX3)(2){mu-R2P(CH2)(2)PR2}] exclusively, confirmed by multinuclear NMR (H-1, B-11, F-19{H-1}, and P-31{H-1}) and IR spectroscopy and microanalytical data. Crystallographic determinations of the four BX3 complexes with Et2P(CH2)(2)PEt2 confirm the 2:1 stoichiometry and, taken together with the spectroscopic data, reveal that the Lewis acid behavior of the BX3 fragment toward phosphine ligands increases in the order F << Cl similar to Br < I. The first diphosphine- and diarsine-coordinated dihaloboronium cations, [BX2{o-C6H4(EMe2)(2)](+) (E = P, As), are obtained using the rigid, preorganized o-phenylene linkages. These complexes are characterized similarly, and the data indicate that the complexes with o-C6H4(AsMe2)(2) are much more labile and readily decomposed than the phosphine analogues. X-ray crystallographic studies on [BX2{o-C6H4(PMe2)(2)}][BX4] (X = Cl, Br), [BI2{o-C6H4(PMe2)(2)}][I-3], and [BCl2{o-C6H4(AsMe2)(2)}][BCl4] confirm the presence of distorted tetrahedral coordination at boron through a chelating diphosphine or diarsine and two X ligands, with d(B-P) revealing a similar increase in Lewis acidity down group 17. Comparison of d(B-P) and d(B-As) reveals an increase of ca. 0.08 angstrom from P to As. Reaction of BCl3 with the diphosphine dioxide Ph2P(O)CH2P(O)Ph-2 gives the ligand-bridged dimer [(BCl3)(2){Ph2P(O)CH2P(O)Ph-2}], while using either BF3 gas or [BF3(SMe2)] gives a mixture containing both [(BF3)(2){mu-Ph2P(O)CH2P(O)Ph-2}] and the unexpected difluoroboronium salt, [BF2{Ph2P(O)CH2P(O)Ph-2)][B2F7] containing a chelating phosphine oxide. The structure of the latter was confirmed crystallographically.