Taking inspiration from the related copper complexes, we reacted 2 with a silane, with the goal to substitute the tert-butoxide for a hydride ligand. Complex 2 was reacted with excess diethylsilane at 90°C, during which the color changed from red/brown to dark brown/black. From this mixture, complex 3 was isolated as an air and moisture sensitive black solid (Scheme 3). Alternatively, 3 can be synthesized from 1 by reacting it with diethylsilane at 110°C, which is a more robust method. In this case, 3 was isolated as a black crystalline solid in 76% yield. Surprisingly, complex 3 was diamagnetic in contrast to 1 and 2. The 31 P NMR spectrum of 3 in C 6 D 6 at 298K features one broad resonance at 117 ppm indicative of a C 2V symmetric complex. The 1 H NMR of 3 in C 6 D 6 at 298K of 3 shows a broad doublet at -13.74 ppm that integrates to 4H, indicative of four hydrides coupling to the phosphines of the PNNP ligand. The two doublets at 7.00 and 6.37 ppm originating from the naphthyridine backbone and one doublet at 2.75 ppm from the methylene linkers also indicate a C 2V symmetric complex. In the aliphatic region there is one doublet at 1.30 ppm coupling to phosphorus ( 3 J H-P = 11.9 Hz) for the tBu-groups and three signals (m, 1.47 ppm, 10 H; t, 1.10 ppm, 6H; q, 0.80 ppm, 4H) for the ethyl groups on the silicon atoms, indicating two magnetically inequivalent ethyl groups. Complex 2 also shows a Si resonance at 136 ppm , a region in which typically (base stabilized) silylenes are found. Crystals of 3 suitable for X-ray diffraction were obtained by slow vapor diffusion of hexane into a benzene solution of the complex. The solid-state structure of 3 (Figure ) features two silicon atoms bridging the two cobalt centers, however, not symmetrically. The Si1-Co2 and Si2-Co1 distances are significantly shorter by ~0.10Å than the Si1-Co1 and Si2-Co2 distances. This asymmetry is also visible in the hydride ligands that were located in the difference Fourier maps, and whose positions are consistent with the DFT optimized structure. Two hydrides (H1M and H3M) are bridging between cobalt and silicon and two hydrides (H2M and H4M) are bound terminally to cobalt, despite there being only one hydride signal in NMR. This difference between the solid-state and solution can be rationalized though a rapid equilibrium between the two sets of hydrides (Scheme 4) on the NMR timescale, due to which their resonances average out. This also explains why in NMR a C 2v symmetric species is observed in contrast to the approximate C 2 symmetry observed in the solid state. We attempted to observe the decoalescence of the hydride peaks using VT NMR, however, even at -70°C no decoalescence was observed. The feasibility of this equilibrium was also evaluated with DFT calculations, which showed that the energy barrier associated with this equilibrium is 4.6 kcal/mol, indicating that this process should be very rapid both at room temperature and at -70°C. The silicon atoms in 3 bind the complex in such a way that one of the ethyl groups points towards the backbone and one in the direction of the hydrides (H2M and H4M). This explains the observation of two magnetically inequivalent ethyl groups in the 1 H NMR spectrum as discussed previously. It also shows that the orientation of the ethyl groups is retained in solution, implying the absence of fast equilibria equilibrating the ethyl groups on the NMR timescale. This supports the idea that only the hydrides move due to an equilibrium as proposed in Scheme 4, as opposed to an equilibration mechanism which would involve dissociation of the silicon groups.