Many mechanisms exist reflecting the myriad types of cross-couplings, including those that do not require metal catalysts.  Often, however, cross-coupling refers to a metal-catalyzed reaction of a nucleophilic partner with an electrophilic partner.
In such cases, the mechanism generally involves reductive elimination of R-R from LMR(R) (L = spectator ligand). This intermediate LMR(R) is formed in a two step process from a low valence precursor LM. The oxidative addition of an organic halide (RX) to LM gives LMR(X). Subsequently, the second partner undergoes transmetallation with a source of R. The final step is reductive elimination of the two coupling fragments to regenerate the catalyst and give the organic product. Unsaturated substrates, such as C(sp)−X and C(sp)−X bonds, couple more easily, in part because they add readily to the catalyst.