Thorium(iv)–antimony complexes exhibiting single, double, and triple polar covalent metal–metal bonds

There is continued burgeoning interest in metal–metal multiple bonding to further our understanding of chemical bonding across the periodic table. However, although polar covalent metal–metal multiple bonding is well known for the d and p blocks, it is relatively underdeveloped for actinides. Homometallic examples are found in spectroscopic or fullerene-confined species, and heterometallic variants exhibiting a polar covalent σ bond supplemented by up to two dative π bonds are more prevalent. Hence, securing polar covalent actinide double and triple metal–metal bonds under normal experimental conditions has been a fundamental target. Here we exploit the protonolysis and dehydrocoupling chemistry of the parent dihydrogen-antimonide anion, to report one-, two- and three-fold thorium–antimony bonds, thus introducing polar covalent actinide–metal multiple bonding under normal experimental conditions between some of the heaviest ions in the periodic table with little or no bulky-substituent protection at the antimony centre. This provides fundamental insights into heavy element multiple bonding, in particular the tension between orbital-energy-driven and overlap-driven covalency for the actinides in a relativistic regime. Actinide–metal multiple bonds are relatively rare, with isolable examples under normal experimental conditions typically restricted to complexes containing a polar covalent σ bond supplemented by up to two dative π bonds. Now complexes featuring polar covalent double and triple bonds between thorium and antimony have been synthesized.