Ab initio calculations are presented for several species containing a silicon-phosphorus bond. The types of bonding studied include "normal" single, double, and triple bonds, as well as an ylide-like structure. The latter is found to be much less strongly bound than the carbon analogue, with a smaller stretching force constant than that in silylphosphine. The insertions of silylene into the phosphine bond and of phosphinosilylene into H2 are discussed, with the former being illustrated using localized molecular orbitals along the intrinsic reaction coordinate (IRC). Silylene to silene isomerizations in both the closed-shell singlet and the lowest triplet states of SiPH3 are analyzed in the same manner.