A computational study on energies and geometries of a representative set of thiaphosphirane derivatives 1a-e and their W(CO)(5) (W) and BH3 (B) complexes is reported. A particular focus was put on ring-opening reactions of kappa P- (2) and kappa S-complex isomers (3). Concerning the ring strain energy, a general trend was observed for compounds 1a,d, 2Wa,d, and 2Ba,d: (i) substituted rings are less strained than the parent compounds, and (ii) kappa P-complexation with a W(CO)(5) group (2Wa,d) significantly increases the ring strain (5.63 and 4.38 kcal/mol) which is exceeded in the case of kappa P-BH3 complexation (2Ba,d) (7.14 and 7.22 kcal/mol). To unveil the thermal endocyclic bond weakness, a variety of bond strength related descriptors such as bond distance, relaxed force constants k(0), Bader's quantitative theory of atoms-in-molecules parameters such as the electron density rho(r) and its Laplacian at bond critical points, and several bond order quantities (Wiberg bond index, Mayer bond order, and Lowdin bond order) were calculated. Heterolytic ring-opening reactions were investigated, revealing some general trends: (i) the strongest donor substituent at carbon significantly lowers relative energies for both the P-C and C-S bond cleavage products as well as the corresponding transition states, (ii) kappa P-complexes are more stable than the corresponding kappa S-complexes, for cyclic and acyclic species, and (iii) P-to-S haptotropic shifts in P-C bond cleavage products are disfavored processes, whereas it is more favored for C-S bond cleavage products. Other rearrangement products, being within energetic reach, were located on the potential energy surface. Two deserve particular mention as one stems from a combined H-2 elimination and C-S bond cleavage of 2Bb and the other represents a first case of peribicyclic reaction leading to 7B'.