The nitrogenase enzyme catalyzes the key reductive step of dinitrogen to ammonia in the global biological nitrogen. The Mo-dependent nitrogenase contains two component proteins that are designated the Fe protein and the MoFe protein. The Fe protein acts as an agent for electron transfer by delivering the necessary reducing equivalents to the MoFe protein, the location of substrate activation and reduction. The enzyme relies on three high nuclearity metal-sulfur complexes to carry out this complicated process: an Fe4S4 cubane unit in the Fe protein; and a P-cluster and iron-molybdenum cofactor (FeMo-cofactor) in the MoFe protein. The macromolecular structure determination of the MoFe protein has revealed a [MoFe7S9] cluster composed of two sulfur-voided [M4S3] cuboidal fragments linked by three µ2-sulfur bridges. Higher-resolution (2.0-Å) protein crystal structures have subsequently revealed the P-cluster to be a symmetric [Fe8S7] structure comprising, in the native state (PN), two [Fe4S3] cuboidal halves, vertex-fused through a µ6-sulfide and further interconnected by two µ2-cysteinate bridges. All these cluster structures are remarkable from a synthetic perspective, and we are exploring new chemistry to better understand the properties of these cluster units.