We investigate the atomic structure of B-P pairs in bulk Si through first-principles pseudopotential calculations, and find that a B-P pair is energetically most stable when P is positioned at a substitutional site in the second nearest neighborhood of B. We also examine the energetics of various I$_s$-B-P complexes that consist of B, P, and Si self-interstitial (I$_s$). In this complex, P also prefers a substitutional site in the second nearest neighborhood of B; however, its actual position depends on the charge state of the complex. We find that the formation of the I$_s$-B-P complex is energetically more favorable than for the I$_s$-B complex, which is a major diffusing species in B-doped material.