A reverse version of this effect in Grubbs second-generation catalyst is caused by the clicking of a carbene rotamer. In the Heck reaction, this effect is responsible for the regioselectivity of the carbopallidation step due to presence of a halogen in the catalyst. When a substituent has strong backdonation, this effect occurs through stabilization of a five-member intermediate in two-step associative reactions. CO and C2H4 are particularly strong in this effect, which 
A reverse version of this effect in Grubbs second-generation catalyst is caused by the clicking of a carbene rotamer. In the Heck reaction, this effect is responsible for the regioselectivity of the carbopallidation step due to presence of a halogen in the catalyst. When a substituent has strong backdonation, this effect occurs through stabilization of a five-member intermediate in two-step associative reactions. CO and C2H4 are particularly strong in this effect, which occurs in species where the s-orbital contributes more to the metal bond than p-orbitals. It's particularly prominent in square planar complexes, and the classic example of this effect occurs during the second ammonia addition in the Dhara synthesis of cisplatin. In this effect, a ligand increases the reactivity of the bond across from it in a complex. FTP, name this effect, which in octahedral complexes, competes with the cis effect.
A reverse version of this effect in Grubbs second-generation catalyst is caused by the clicking of a carbene rotamer.
trans effect