A mechanistic study of intramolecular hydroamination/cyclization catalyzed by tetravalent organoactinide and organozirconium complexes is presented. A series of selectively substituted constrained geometry complexes, (CGC)M(NR2)Cl (CGC = [Me2Si(eta(5)-Me4C5)((BuN)-Bu-t)](2-); M = Th, 1-Cl; U, 2-Cl; R = SiMe3; M = Zr, R = Me, 3-Cl) and (CGC)An(NMe2)OAr (An = Th, 1-OAr; An = U, 2-OAr), has been prepared via in situ protodeamination (complexes 1-2) or salt metathesis (3-Cl) in high purity and excellent yield and is found to be active precatalysts for intramolecular primary and secondary aminoalkyne and aminoalkene hydroamination/cyclization. Substrate reactivity trends, rate laws, and activation parameters for cyclizations mediated by these complexes are virtually identical to those of more conventional (CGC)MR2 (M = Th, R = NMe2, 1; M = U, R = NMe2, 2; M = Zr, R = Me, 3), (Me2SiCp"(2))UBn2 (Cp" = eta(5)-Me4C5; Bn = CH2Ph, 4), Cp'(2)AnR(2) (Cp' = eta(5)-Me5C5; R = CH2SiMe3; An = Th, 5, U, 6), and analogous organolanthanide complexes. Deuterium KIEs measured at 25 degrees C in C6D6 for aminoalkene D2NCH2C(CH3)(2)CH2CHCH2 (11-d(2)) with precatalysts 2 and 2-Cl indicate that k(H)/k(D) = 3.3(5) and 2.6(4), respectively. Together, the data provide strong evidence in these systems for turnover-limiting CC insertion into an MN(H)R sigma-bond in the transition state. Related complexes (Me2SiCp"(2))U(Bn)(Cl) (4-Cl) and Cp'(2)An(R)(Cl) (R = CH2(SiMe3); An = Th, 5-Cl; An = U, 6-Cl) are also found to be effective precatalysts for this transformation. Additional arguments supporting M-N(H)R intermediates vs M=NR intermediates are presented.