The Co centers in D10 are also in trigonal bipyramidal geometry with axial H (1.65 Å) and Si (2.27 Å) ligands and equatorial C (1.95 Å) and two hydrides (1.49 Å and 1.58 Å) (Figures ). Compared to N4, distances to the equatorial hydride ligands are much shorter and overlap significantly more with the Co 3𝑑 𝑥 2 -𝑦 2 orbital (rather than 3𝑑 𝑥𝑦 in N4) due to geometric differences between bridging hydrides (N4) and bridging silanes (D10) (Figures ). Thus in D10, the Co 3𝑑 𝑥 2 -𝑦 2 orbitals are strongly destabilized by overlap with hydrides and remain vacant, preventing Co-Co σ-bond formation. Despite the absence of the Co-Co σ-bond, D10 is diamagnetic, indicating that all electrons are paired. Computed valence MO energies show that the HOMO-LUMO gap is 1.0 eV larger for D10 than N4, indicating a much stronger ligand field.