Group 6 diatomic bonding is all relative


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Scientists in China say that the trends governing the periodic table may need a rethink as relativistic effects in group 6 diatomic molecules contradicts some of them.

The periodic table is a powerful resource that describes the mass, electronic configuration and properties of elements. Quantum mechanics has helped reaffirm this by providing a tool to calculate electron configurations and explain periodic behaviour.

But the theory wavers in some circumstances, be it through failing to explain why mercury is a liquid or the colour of gold. Relativity has helped fill in these gaps by accounting for orbital instabilities in the heavier elements through the speed of inner orbital electrons.

This effect may also explain why the bond multiplicity of certain groups of heavy diatomic molecules is non-periodic, according to Yi-Lei Wang and colleagues at Tsinghua University.  Using density functional theory, the team calculated the bond order for each group 6 diatomic molecule. Diatomic chromium, molybdenum and tungsten each have a bond order of six, but this falls to 4 for seaborgium.

The team attribute this to heightened stability in the 7s orbital of seaborgium, which may lower its oxidation state in metal complexes relative to the rest of group 6. Relativistic effects also account for the increase in bond length for diatomic seaborgium.

References

Y-Lei Wang et al, J. Am. Chem. Soc., 2016, DOI: 10.1021/jacs.5b11793


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