Ab initio calculations at the MP2/aug'-cc-pVTZ level have been carried out to investigate the structures and binding energies of cationic complexes involving protonated sp, sp(2), and sp(3) phosphorus bases as proton donor ions and the sp-hybridized phosphorus bases H-C P and H3C-C P as proton acceptors. These proton-bound complexes exhibit a variety of structural motifs, but all are stabilized by interactions that occur through the T cloud of the acceptor base. The binding energies of these complexes range from 6 to 15 kcal/mol. Corresponding complexes with H3C-C P as the proton acceptor are more stable than those with H-C P as the acceptor, a reflection of the greater basicity of H3C-C P. In most complexes with sp(2)- or sp(3)-hybridized P-H donor ions, the P-H bond lengthens and the P-H stretching frequency is red-shifted relative to the corresponding monomers. Complex formation also leads to a lengthening of the C P bond and a red shift of he C P stretching vibration. The two-bond coupling constants (2 pi h)J(P-P) and (2 pi h)J(P-C) are significantly smaller than (2h)J(p-P) and (2hJ)(P-C) for complexes in which hydrogen bonding occurs through lone pairs of electrons on P or C. This reflects the absence of significant s electron density in the hydrogen-bonding regions of these pi complexes.