Additive radii fitted for sixfold metal–metal bonding

What the study found: The study fitted a new set of additive covalent radii, called r6, for metal dimers involving group 4–8 transition metals and the actinides Th–Np. These radii are associated with 12 valence electrons in the orbital pattern (1σ)^2 (2σ)^2 π^4 δ^4, which the title describes as approaching sixfold chemical bonding.
Why the authors say this matters: The authors present these radii as a way to describe and compare metal–metal bond lengths across a range of elements. The study suggests this adds a bonding-based size measure for cases where unusually strong metal–metal bonding is relevant.
What the researchers tested: The researchers fitted additive covalent radii to a combination of experimental data, accurate ab initio computations, and more approximate density functional theory (DFT) metal–metal bond lengths. They report fitted values for Ti–Fe, Zr–Ru, Hf–Os, and Th–Np, and also propose extrapolated values for some Group 3 elements, including rare earths, plus Group 9 and Pu.
What worked and what didn't: The fitted r6 values were typically 10–20 pm shorter than the earlier triple-bond radii r3. The abstract does not report any failures, but it does indicate that some values were fitted directly and others were extrapolated.
What to keep in mind: The available summary does not describe experimental procedures in detail, uncertainty estimates, or limitations beyond the fact that some values were extrapolated. The abstract also does not state which of the fitted or extrapolated values are more reliable than others.

Key points

• A new additive covalent radius set, r6, was fitted for selected metal dimers.
• The fitted cases include Ti–Fe, Zr–Ru, Hf–Os, and Th–Np.
• The r6 values are typically 10–20 pm shorter than the earlier triple-bond radii r3.
• The study links these radii to 12-valence-electron orbital patterns of (1σ)^2 (2σ)^2 π^4 δ^4.
• Extrapolated values are proposed for some Group 3 elements, Group 9, and Pu.