A Comprehensive\nMultireference Study of Excited-State\nNi–Br Bond Homolysis in (dtbbpy)Ni<sup>II</sup>(aryl)(Br)

The\nmechanism of visible light-driven Ni–C(aryl)\nbond homolysis\nin (2,2′-bipyridine)Ni<sup>II</sup>(aryl)(halide) complexes,\nwhich play a crucial role in metallaphotoredox catalysis for cross-coupling\nreactions, has been well studied. Differently, the theoretical understanding\nof Ni–halide bond homolysis remains limited. In this study,\nwe introduce a novel electronic structural framework to elucidate\nthe mechanisms underlying photoinduced Ni–Br bond rupture in\nthe (dtbbpy)Ni<sup>II</sup>(aryl)(Br) complex. Using multireference <i>ab initio</i> calculations, we characterized the excited state\npotential energy surfaces corresponding to metal-to-ligand charge\ntransfer (MLCT) and ligand-to-metal charge transfer (LMCT). Our calculations\nreveal that the Ni–Br dissociation, triggered by an external\nphotocatalyst, begins with the promotion of Ni(II) to a <sup>1</sup>MLCT excited state. This state undergoes intersystem crossing with\nrepulsive triplet surfaces corresponding to the <sup>3</sup>MLCT and\nBr-to-Ni <sup>3</sup>LMCT states, resulting in Ni–Br bond breaking\nvia the Dexter energy transfer mechanism. In the absence of a photocatalyst,\nthe photoexcited Ni(II) favors Ni–C(aryl) homolysis, whereas\nthe presence of a photocatalyst promotes Ni–Br dissociation.\nThe Ni(III) species, resulting from the oxidation of Ni(II) by the\nphotocatalyst, was found to be unproductive toward Ni–Br or\nNi–C(aryl) activation.