Approaching Sixfold Chemical Bonding: Metal Dimers MM' Involving Group 4-8 Transition Metals and the Actinides Th-Np. Additive Covalent Radii r 6 r₆.

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 (1σ)² (2σ)² π ⁴ δ 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.