Transmetalation between Pd(Br)(Ph-A)(PCyp(3))(2) (Ph = phenyl, Cyp = cyclopentyl) and Cu((CHMePhB)-H-a)(NHC) (NHC = 1,3-bis(2,6-dlisopropylphenyl)-imidazolidin-2-ylidene) is an important elementary step in recently reported catalytic cross-coupling reaction by Pd/Cu cooperative system. DFT study discloses that the transmetalation occurs with inversion of the stereochemistry of the (CHMePhB)-H-a group. In its transition state, the (CHMePhB)-H-a group has almost planar structure around the C-a atom. That planar geometry is stabilized by conjugation between the pi* orbital of the Ph-B and the 2p orbital of the C-a. Another important factor is activation entropy (Delta S degrees(double dagger)); retention transmetalation occurs through Br-bridging transition state, which is less flexible than that of the inversion transmetalation because of the Br-bridging structure, leading to a smaller activation entropy in the retention transition state than in the inversion transition state. For (CHMeEt)-H-a group, transmetalation occurs in a retention manner. In the planar (CHMeEt)-H-a group of the inversion transition state, the C-a 2p orbital cannot find a conjugation partner because of the absence of pi-electron system in the (CHMeEt)-H-a. Transmetalation of (CHMe)-H-a(CH=CH2) occurs in a retention manner because the vinyl pi* is less effective for the conjugation with the C-a 2p because of its higher orbital energy than the Ph pi*. The introduction of electron withdrawing substituent on the Ph-B is favorable for inversion transmetalation. These results suggest that the stereochemistry of the C-a atom in transmetalation can be controlled by electronic effect of the (CHMeR)-H-a (R = phenyl, vinyl, or alkyl) and sizes of the substituent and ligand. [GRAPHICS]