Why do Pt(PR3)2 complexes catalyze the alkyne diboration reaction, but their palladium analogues do not?: A density functional study

The B3LYP hybrid density functional method has been applied to study theoretically the mechanism of the Pd(0)-catalyzed alkyne diboration reaction. It has been found that this reaction proceeds via the same mechanism as the Pt(0)-catalyzed diboration reaction and involves the following steps: (i) coordination of diborane R2B-BR2 to the Pd(0) complex, (ii) oxidative addition of the B-B bond to Pd, (iii) dissociation of one phosphine ligand, (iv) coordination of acetylene, (v) insertion of acetylene into one of the Pd-B bonds, (vi) isomerization of the resultant complex accompanied by recoordination of a phosphine ligand, and (vii) reductive elimination of the alkenyl-diboron products. However, the Pd(0) complex cannot catalyze the alkyne diboration reaction, while its Pt(0) analogue can. The main reason for this difference is found to be in the oxidative addition process of the B-B bond to M(PH3)(2) This step takes place for M = Pt with a 14.0 kcal/mol activation barrier and is exothermic, but it does not take place for NI = Pd, where the addition product is not stable due to a very small reverse barrier. The origin of this inactivity of Pd(0) is the d(10)-->sd(9) promotion energy, which is not required for Pt(0) with the sd(9) ground state.