The hybrid density functional (B3LYP) calculations have been carried out to study the mechanism of acetylene thioboration reactions catalyzed by Pd(0)/Pt(0) complexes, and the results are compared to those for the diboration reactions. The main findings can be summarized as follows. (i) No oxidative-addition product of the S−B bond to the Pd(PH3)2 was found, therefore, the oxidative-addition mechanism proposed by Suzuki et al. is not the proper one for the thioboration reaction of alkynes. (ii) We propose a new mechanism involving the following steps: (a) acetylene coordination to Pd(PH3)2, (b) dissociation of a phosphine ligand, (c) addition of the S−B bond to the metal center via a metathesis-like transition state, (d) isomerization of the resultant complex, accompanied by recoordination of the phosphine ligand, and (e) the reductive elimination of the alkenyl−thioboron product, R‘S−CHCH−B(OR)2. The rate-determining stage is found to be the addition of the S−B bond to the metal center via a metathesis-...