Figure shows the proportion of ground state spin of TM complexes for each metal and OS. Some metals in a given OS consistently exhibit the same ground state spin. As expected, if the number of d electrons is less than 4 or more than 8, the number of unpaired d electrons is the determining factor. For instance, all Cr(III) complexes (d 3 configuration) have a quartet ground state. Alternatively, zero-valent complexes such as Cr(0), Fe(0), or Ni(0) centers as well as and Mn(I) complexes possess a singlet ground state. These complexes typically bind to strong-field ligands such as carbonyls or substituted phosphines, which cause a substantial energy separation between d orbitals and favor lower spin states. Intriguingly, one Fe(0) complex (CSD refcode: NUNWUP ) deviates from this trend and is characterized by a triplet ground state (Figure We further examine the distribution of distances between the metal center and its coordinating atoms as the metal-ligand bond lengths in HS states are generally longer compared to those in the LS states due to the population of anti-bonding orbitals. Within this context, Taylor et al., assigned ground state spins of mononuclear octahedral Fe(II)/Fe(III) complexes based on heuristic cut-off values for metal-ligand bond lengths. We here introduce a more general indicator applicable to various metal centers and coordination geometries. We define the relative metal radius, denoted as r rel , as