The first 1,2-additions across a metal-boron double bond are reported, one a definitive chlorogallation of a Fe=B bond, the other a partial chlorogallation of a Mo=B bond that leads to a highly unusual, planar Mo-B-Ga-Cl rhombus. The two reactions occur with opposite regiochemistry, with the Ga atom bound to the Fe atom in the former and to both the Mo and B atoms in the latter. The bonding in the Mo adduct and the reasons for the differing regiochemistry of the reaction are explored computationally.