Crystals of 2 suitable for scXRD were obtained by slow vapor diffusion of hexane into a toluene solution of the complex. The solid-state structure of 2 (Figure ) revealed the presence of a bridging OtBu ligand consistent with the NMR spectra. The C1-C2 bond length of 1.493(4)Å is consistent with a C-C single bond and of similar length as the C-C bonds of the methylene linkers in 1. The C9-C10 bond length of 1.375(4) in 2 is significantly shorter than the C1-C2 bond length, indicative of double bond character, which is consistent with a partially dearomatized naphthyridine backbone. This is further supported by the observed P1-C1-C2 and P2-C10-C9 angles which are 114.0(2)° and 120.1(2)° and hence consistent with sp and sp 2 hybridization of the bridging carbon respectively. The Co1-O-Co2 angle of 106.46 (8)° is indicative of the tert-butoxide ligand binding in an LX-type fashion to the cobalt centers, similar to the bridging chloride in 1. The Co-Co distance in 2 is 0.8128(18) Å shorter than in 1 since both cobalt atoms are in the PNNP binding pocket in this structure. Compared to other PNNP complexes however, the M-M distance of 3.0710(5) in 2 is still rather large. This seems to be due to the bridging tert-butoxide ligand that is bound in an LX-type fashion, since the relatively large ideal M-L-M angle of 109.5° in such a binding mode forces a larger M-M separation than what would be expected for a 3-center 2-electon (3c-2e) bound bridging ligand for example. This was also observed for the M-M distance in similar PNNPCu 2 OtBu complexes (2.9259(6) -3.0467(2)Å), which feature significantly larger M-M distances compared to dicopper PNNP complexes with 3c-2e bound bridging ligands (2.3870(6) -2.5122(6) Å). The cobalt centers in 2 display distorted tetrahedral geometries and are located slightly above and below the naphthyridine plane, respectively. This is enabled by the flexibility in the phosphine arms, which accommodate the preferred bond angles in combination with the LX-type binding of the tert-butoxide ligand. The solid-state structure is consistent with the lack of symmetry observed in the 1 H NMR spectra. The chloride atoms that point in different directions on Co1 and Co2 break the C s symmetry, and the C 2 symmetry of the ligand is broken by the partially dearomatized backbone. This accounts for the fact that each tBu-group and backbone proton is magnetically inequivalent.