Experimental and theoretical studies on the course of CO insertion into Pt-C and Pd-C bonds in neutral and cationic complexes, [MR(Cl){P(CH₃)₃}₂] and [MR{P(CH₃)₃}₂(s)]⁺BF ₄ ^- (M=Pt, Pd, R=CH₃, C₆H₅, s=coordinated solvent)

Behavior of neutral and cationic cis- and trans-monoorganoplatinum complexes has been examined and compared with that of the corresponding monoorganopalladium complexes. The cis- and trans-monoorganoplatinum complexes having two trimethylphosphine ligands, [PtR(acetone){P(CH₃)₃}₂]⁺BF ₄ ^- (R=CH₃ and C₆H₅) (4 and 5), have been prepared by removal of the chloride ligand in neutral complexes, [PdR(Cl){P(CH₃)₃}₂] (2 and 3) with one equivalent of AgBF₄. The cationic platinum complexes retaining the cis configuration of the parent neutral complexes, when prepared at low temperature, were found to be isomerized into the trans complexes 5 above -10°C for the methyl complex 5a and above -30°C for the phenyl complex 5b. Treatment of the cationic complexes 4 and 5 with CO gave no CO insertion product but afforded only the CO-coordinated trans-monoorganoplatinum complexes. The reluctance of the organoplatinum complexes toward CO insertion stands in contrast with the ease for the CO insertion of the corresponding organopalladium complexes. For clarifying the reasons of the marked difference between the behavior of the two Group 10 metal complexes the ab initio molecular orbital calculations with the MP2 level have been performed. We have found the transition states for the cis to trans isomerization of the three-coordinated cationic monoalkylplatinum complexes and for the CO insertion into the Pt-CH₃ bond. The calculated barriers for the isomerization and CO insertion in the cationic organoplatinum complex are higher than those for the corresponding organopalladium complex in agreement with the experimental results showing the poor reactivity of the solvent-coordinated cationic organoplatinum complexes toward CO insertion compared to the organopalladium complexes. The reason for the difference in the reactivity between the palladium and platinum complexes can be ascribed to the relativistic effect of the platinum complexes.