Catalytic ethylene dimerization is an important chemical reaction that suffers from a lack of selectivity for the desired product, 1-butene. Metal-organic frameworks (MOFs) bearing Ni-based catalytic sites have been shown to yield record selectivity for 1-butene. Early efforts to understand this selectivity revealed that chain propagation and 2-butene formation are competitive with 1-butene, seemingly at odds with experimental evidence that these products are disfavored. Here, we present an alternative mechanism for selective 1-butene formation in the highest performing MOF, Ni(II)-MFU-4l. Our study reveals competing electronic spin configuration pathways that intersect along the reaction coordinate. Intersystem crossing provides an explanation for the selective formation of 1-butene in the MOF. Furthermore, we explore intersystem crossing as a unique design principle for MOF catalyst design and highlight a departure from conventional molecular catalyst design paradigms.