The rules that govern spin exchange interaction in pristine graphene nanostructures are constrained by the bipartite character of the lattice, so that the sign of the exchange is determined by whether magnetic moments are on the same sublattice or not. The synthesis of graphene ribbons with perfect zigzag edges and a fluoranthene group with a pentagon ring, a defect that breaks the bipartite nature of the honeycomb lattice, has been recently demonstrated. Here we address how the electronic and spin properties of these structures are modified by such defects, both for indirect exchange interactions as well as the emergent edge magnetism, studied both with density functional theory and mean-field Hubbard model calculations. In all instances we find that the local breakdown of the bipartite nature at the defect reverts the sign of the otherwise ferromagnetic correlations along the edge, introducing a locally antiferromagnetic intraedge coupling and, for narrow ribbons, also revert the antiferromagnetic interedge interactions that are normally found in pristine ribbons. Our findings show that these pentagon defects are a resource that permits us to engineer the spin exchange interactions in graphene-based nanostructures. DOI: 10.1103/PhysRevB.94.094414 A central concept in the vast field of carbon-based nanostructures is the fact that their electronic properties can change dramatically depending on their atomic structure. Thus, graphite, graphene, nanotubes, and fullerenes all share the same atomic scale building blocks, carbon atoms with sp A bipartite lattice can be split in two interpenetrating sublattices, A and B, such that first neighbors of A sites are always B sites, and vice versa. Whereas in two-dimensional (2D) graphene the wave functions have the same weight on both sublattices, in structures where there are more atoms of one type than the other, such as zigzag edges, there are zero modes whose wave function is 100% sublattice polarized [2,8]. These states play a crucial role in our understanding of one of the most exciting theory predictions regarding graphene so far, namely, the existence of local moments with ferromagnetic correlations in sublattice imbalanced graphene structures, such as zigzag edges [7,8,[10][11][12][13], graphene functionalized with hydrogen [13][14][15], and a variety of planar aromatic hydrocarbons [16,17]. Whereas a direct experimental local probe of the magnetization is still missing, indirect experimental evidence in full agreement with density functional theory (DFT) and model Hamiltonian calculations [18][19][20][21] supports the existence of sublattice polarized states that most likely host unpaired electrons.Interestingly, the chemical approach recently reported by Ruffieux et al. [19,20] has produced both ribbons with large sections of pristine zigzag edges as well as edges decorated with a fluoranthene group (FG), 1 as those shown in Figs. 1(a) 1 The FG is defined by analogy with the fluoranthene molecule, which structurally comprises one napthalene group and one benz...