The synthesis, structure, and reactivity of a thorium metallacyclopentadiene were comprehensively studied. The reduction of (eta(5)-C5Me5)(2)ThCl2 (1) with potassium graphite (KC8) in the presence of diphenylacetylene (PhC CPh) yields the first thorium metallacyclopentadiene complex (eta(5)-C5Me5)(2)Th(eta(2)-C4Ph4) (2). Density functional theory (DFT) studies suggest that the 5f orbitals are involved in the bonding of the metallacyclopentadiene Th-(eta(2)-C=C-C=C) moiety. Compared to the thorium metallacyclopropene, complex 2 shows a distinctively different reactivity towards diazoalkanes and organic azides such as Me3SiCHN2, 9-diazofluorene and p-tolylN(3), leading to the formation of the six-membered hydrazido complexes (eta(5)-C5Me5)(2)Th[N(N=CHSiMe3)(C4Ph4)] (3) and (eta(5)-C5Me5)(2)Th[N(N=C(C12H8))(C4Ph4)] (4) and the seven-membered bis(triazenido) complex (eta(5)-C5Me5)(2)Th[N(N=N(p-tolyl))(C4Ph4) N(N=N(p-tolyl))] (5), respectively.
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