Experimental and computational efforts are reported which illuminate the mechanism of a novel boron version of the widespread Negishi coupling reaction that offers a new protocol for the formation of aryl/acyl C−B bonds using a bulky boryl fragment. The role of nucleophilic borylzinc reagents in the reduction of the PdII pre‐catalysts to Pd0 active species has been demonstrated. The non‐innocent behavior of the PPh3 ligands of the [Pd(PPh3)2Cl2] pre‐catalyst under activation conditions has been probed both experimentally and computationally, revealing the formation of a trimetallic Pd species bearing bridging phosphide (PPh2−) ligands. Our studies also reveal the monoligated formulation of the Pd0 active species, which led us to synthesize related (η3‐indenyl)Pd‐monophosphine catalysts which show improved catalytic performances under mild conditions. A complete mechanistic proposal to aid future catalyst developments is provided.