Structural properties of the B+-H-2 electrostatic complex are investigated through its rotationally resolved infrared spectrum in the H-H stretch region (3905-3975 cm(-1)). The spectrum, which was obtained by monitoring B+ photofragments while the IR wavelength was scanned, is consistent with the complex having a T-shaped structure and a vibrationally averaged intermolecular separation of 2.26 angstrom, which decreases by 0.04 angstrom when the H-2 subunit is vibraitionally excited. The H-H stretch transition of B+-H-2 is red-shifted by 220.6 +/- 1.5 cm(-1) from that of the free H-2 molecule, much more than for other dihydrogen complexes with comparable binding energies. Properties of B+-H-2 and the related Li+-H-2, Na+-H-2, and Al+-H-2 complexes are explored through ab initio calculations at the MP2/aug-cc-pVTZ level. The unusually large red-shift for B+-H-2 is explained as due to electron donation from the H-2 sigma(g) bonding orbital to the unoccupied 2p(z) orbital on the B+ ion.