Spin state difference between septet 1 and 2 and singlet 3 and 4 can be attributed to three factors: (1) difference in type of ligands between Cl -/tBuO -versus Si(C2H5)2/H -, (2) pseudotetrahedral versus pseudo-octahedral geometry, and (3) shorter inter-Co distance leading to Co-Co σ-interaction. The magnitudes of these factors in determining spin state were assessed by considering energetic differences arising from changes in ligand identity and coordination geometry (full discussion in supporting information, Figure ). In these compounds, spin state is most strongly affected by the binding of hydride ligands in 3 and 4, while the change in coordination geometry plays a smaller but significant role, consistent with expectations from ligand field theory.