Actinide Pnictinidene Chemistry: A Terminal Thorium Parent-Arsinidene Complex Stabilised by a Super-Bulky Triamidoamine Ligand.

We report the direct synthesis of the terminal pnictidenes [An(Tren TCHS )(PnH)][M(2,2,2-cryptand)] (Tren TCHS ={N(CH 2 CH 2 NSiCy 3 ) 3 } 3- ; An/Pn/M=Th/P/Na 5, Th/As/K 6, U/P/Na 7, U/As/K 8) and their conversion to the pnictides [An(Tren TCHS )(PnH 2 )] (An/Pn=Th/P 9, Th/As 10, U/P 11, U/As 12). Use of the super-bulky Tren TCHS ligand was essential to accessing complete families, and 6 is an unprecedented example of a terminal thorium-arsinidene complex and only the second structurally authenticated parent terminal arsinidene complex of any metal. Comparison of the terminal Th=AsH unit of 6 to the bridging ThAs(H)K linkage in structurally analogous [Th(Tren TIPS ){μ-As(H)K(15-crown-5)}] (Tren TIPS ={N(CH 2 CH 2 NSiPr i 3 ) 3 } 3- ) reveals a stronger Th-As bond in the former compared to the latter, and a large response overall to the nature of the Th=AsH bonding upon removal of the electrostatically-bound K-ion; the σ-bond changes little but the π-bond is significantly perturbed.