Insertion Reactions of Square‐Planar Diorganoplatinum. 2. Stereoselective Carbonylation of trans ‐Pt(R)(R′)(PPh 3 ) 2 Leading to cis ‐Pt(R)(COR′)(PPh 3 ) 2 and Isomers of Pt(PPh 3 )(COKCOR′)(R)

Synthesis of square‐planar trans dialkyl and alkyl aryl complexes of platinum(II), trans ‐Pt(R)(R′)(PPh 3 ) 2 (R = Me R′ = Et ( 2a ); R = Me R′ = Ph ( 2b ); R = R′ = Et ( 2c ); R = Et R′ = Ph ( 2d )) was established employing transmelaiation of trans ‐Pt(R)(I)(PPh 3 ) 2 with Grignard reagents R′MgX. Complexes 2a‐2c underwent stereoselective carbonylation to produce cis ‐Pt(R)(COR′)(PPh 3 ) 2 (R = Me R′ = Et ( 3a ); R = Me R′ = Ph ( 3b ); R = R′ = Et ( 3c )). These facile transformations were exclusive and quantitative. The reactivity of carbonylation followed the order Et > Ph > Me. A similar reaction of 2d with CO yielded EtC(O)Ph instead. The cis acyl alkyl or acyl aryl complexes 3a‐3c suffered substitution of CO for a PPn 3 , leading to two isomers SP ‐4‐3‐ and SP ‐4‐4‐Pt(PPb 3 )(CO)(COR′)(R) (R = Me R′ = Et ( 4a, 4a′ ); R ‐ Me R′ = Ph ( 4b, 4b′ ); R = R′ = Et ( 4c, 4c′ )), respectively. The reverse reactions of 4 to 3 readily occurred when external PPh 3 was provided. The stereoselectivity of carbonylation of trans ‐Pt(R)(R′)(PPh 3 ) 2 is explained by a mechanism in which reversible displacement of a PPh 3 by CO in the reactant precedes to form a four‐coordinate dialkyl carbonyl intermediate Pt(PPh 3 )(CO)(R)(R′). The ensuing alkyl (or aryl) migration from metal to the carbonyl carbon achieves the cis acyl alkyl (or aryl) configuration. Recoordination of a PPh 3 completes the reaction. X‐ray structures of 2b, 2c, 3b and 4b as single crystals are provided.