A set of additive covalent radii is fitted to a combination of experimental data, accurate ab initio computations, and more approximate DFT MM' bond lengths, R R, with 12 valence electrons in approximately (1 σ) 2 (2 σ) 2 π 4 δ 4 (1) ² (2) ² ⁴ ⁴ molecular orbitals. These r 6 r₆ are typically 10-20 pm shorter than the previous triple-bond radii r 3 r₃. Values for the elements Ti-Fe, Zr-Ru, Hf-Os, and Th-Np are fitted. Extrapolated values for suitable cases in Group 3 (including rare earths) and Group 9 plus Pu are also proposed.
Pyykkö et al. (Wed,) studied this question.