η³-Propargyl/Allenyl Complexes of Platinum and Palladium of the Type [(PPh₃)₂M(η³-CH₂CCR)]⁺

A study is reported on the synthesis, spectroscopic properties, molecular structure, and reaction chemistry of η³-propargyl/allenyl complexes of platinum and palladium of the type [(PPh₃)₂M(η³-CH₂CCR)]⁺ (M = Pt, R = Ph (1a), Me (1b); M = Pd, R = Ph (2a), Me (2b)). Complexes 1a,b were obtained by each of the following methods:  (i) reaction of trans-(PPh₃)₂PtBr(η¹-CH₂C⋮CPh) with AgO₃SCF₃; (ii) treatment of (PPh₃)₂Pt(η²-PhC⋮CCH₂OMe) with BF₃·OEt₂; (iii) addition of RC⋮CCH₂OS(O)₂C₆H₄Me-p to (PPh₃)₂Pt(η²-C₂H₄) in solution. Complexes 2a,b resulted upon (i) abstraction of Br^- from trans-(PPh₃)₂PdBr(η¹-CH₂C⋮CR)/trans-(PPh₃)₂PdBr(η¹-C(R)CCH₂) with AgBF₄ and (ii) treatment of Pd₂(dba)₃·CHCl₃ with PPh₃ and RC⋮CCH₂OS(O)₂C₆H₄Me-p. The structures of 1a(OTf)·CH₂Cl₂ (Tf = O₂SCF₃) and 2a(BF₄) were determined by X-ray diffraction techniques. Both cationic metal complexes show remarkably similar structures, the salient features being the attachment to the metal of all three propargyl/allenyl carbon atoms of the nonlinear C₃ fragment (C(1)−C(2)−C(3) bond angles:  152.2(9)° for 1a, 154.7(5)° for 2a) and the essentially coplanar arrangement of the three carbon, two phosphorus, and platinum or palladium atoms. 1a,b react readily with MeOH and Et₂NH to afford the corresponding complexes [(PPh₃)₂Pt(η³-CH₂C(X)CHR)]⁺ (X = OMe, NEt₂); 2a,b behave analogously, except that the reaction with MeOH requires trace amounts of NaOMe or NEt₃. ¹H and ¹³C NMR spectroscopic data suggest that the reaction products are best formulated as resonance hybrids of η³-allyl and metallacyclic structures.