The history of boron hydrides has so far evaded historical scholarship, despite the fact that they are crucial to understand the changing conceptions and models regarding the nature(s) of the chemical bond. In this presentation, I argue that boron hydrides were considered by chemists as the ultimate test for any theoretical account of the chemical bond and that wrong metaphysical assumptions originated and guided their evolution, eventually leading them to industrialization. For historians they provide a fertile ground to observe the plasticity of ideas and concepts, in their ability to be adapted and appropriated by new theoretical contexts. They further provide a paradigmatic case-study for the operative power of metaphysical assumptions in science. The structural debate alone sustained and guided research on boron hydrides. This was largely the consequence of an essential and irreducible incompatibility with Lewis’ covalent bond (1916), which led many of the most prominent chemists to attempt a solution for the structure and nature of bonds in these molecules. Such was the case of Nevil V. Sidgwick, Samuel Sugden, Maurice L. Huggins, Robert Robinson, Thomas M. Lowry, Egon Wiberg, Linus Pauling, Gilbert N. Lewis, Robert S. Mulliken, Simon H. Bauer, H. C. Longuet-Higgins or Newton Lipscomb. Most of these proposals were based on the wrong assumption of an essential analogy between boron and carbon chemistries, backed by an omnipresent principle of unity that seized minds and hearts and restricted the debate over the structure of boron hydrides for a surprisingly extended period of time. These metaphysical assumptions, to which I have called “the carbon spell”, would eventually prove their operative power in the development of boron chemistry. A surprising array of models and theories was put forward in the period under consideration. Among these, I address those of Egon Wiberg, Linus Pauling, Robert Mulliken and Simon H. Bauer. By doing so, I show how the dialectics between analytical chemistry, the emerging physical methods, and the theoretical quantum accounts of Linus Pauling and Robert Mulliken shaped the debate from 1930 to1941, and how the advocates of both sides were able to seize the very same empirical data to champion their theory. I will then analyze how disciplinary commitments, interdisciplinary relations and metaphysical assumptions, eventually prevented to break the carbon spell and discover (or construct) significantly sooner the present-day model for boron hydrides.