Correlated molecular orbital theory at the coupled cluster CCSD(T) level with augmented correlation consistent basis sets has been used to predict the structure and energetic properties of the isomers of [Si,N,S] and [Si,P,S]. The predicted ground states are linear (SNSi)-S-2 and cyclic (SPSi)-S-2. The other two isomers are predicted to be similar to 20 to 50 kcal/mol less stable than the ground state. The excess spin is mainly on S for (SNSi)-S-2 and on P for (SPSi)-S-2. The calculated total atomization energies with the CBS limits derived from different methods differ by similar to 2 kcal/mol. The results provide the best available heats of formation for these species. The bond dissociation energies (BDEs) in (SNSi)-S-2 are comparable to those in the corresponding diatomic molecules. For cyclic (SPSi)-S-2, the formation of P-4 + (SSi)-S-2 requires less energy than the other bond dissociation processes. The BDEs in the higher energy isomers are substantially smaller than the corresponding diatomic species.