Treatment of ThCl4(DME)(2) or UCl4 with 1 equiv of dilithiumbis(iminophosphorano) methandiide, [Li2C(Ph2P=NSiMe3)(2)] (1), afforded the chloro actinide carbene complexes [Cl2M(C(Ph2P=NSiMe3)(2))] (2 (M = Th) and 3 (M = U)) in situ. Stable PCP metal-carbene complexes [Cp2Th(C(Ph2P=NSiMe3)(2))] (4), [Cp2U(C(Ph2P=NSiMe3)(2))] (5), [TpU(C(Ph2P=NSiMe3)(2))Cl] (6), and [TpU(C(Ph2P=NSiMe3)(2))Cl] (7) were generated from 2 or 3 by further reaction with 2 equiv of thallium(I) cyclopentadienide (CpTI) in THF to yield 4 or 5 or with 1 equiv of potassium hydrotris(pyrazol-1-yl) borate (TpK) also in THF to give 6 or 7, respectively. The derivative complexes were isolated, and their crystal structures were determined by X-ray diffraction. All of these U (or Th)-carbene complexes (4-7) possess a very short M (Th or U)=carbene bond with evidence for multiple bond character. Gaussian 03 DFT calculations indicate that the M=C double bond is constructed by interaction of the 5f and 6d orbitals of the actinide metal with carbene 2p orbitals of both pi and sigma character. Complex 3 reacted with acetonitrile or benzonitrile to cyclo-add C N to the U=carbon double bond thereby forming a new C-C bond in a new chelated quadridentate ligand in the bridged dimetallic complexes (9 and 10). A single carbon-U bond is retained. The newly coordinated uranium complex dimerizes with one equivalent of unconverted 3 using two chlorides and the newly formed imine derived from the nitrile as three connecting. bridges. In addition, a new crystal structure of [CpUCl3(THF)(2)] (8) was determined by X-ray diffraction.