The zirconium and hafnium porphyrin imido complexes (TTP)M = NAriPr [TTP = meso-tetra-p-tolylporphyrinato dianion, M = Zr (1), Hf (2), Ar-iPr = 2,6-diisopropylphenyl] were synthesized from (TTP)MCl2 and 2 equiv of LiNHAriPr. The zirconium imido complex, (TTP)Zr=NAriPr, was also obtained from the preformed imido complex Zr(NAriPr)Cl-2(THF)(2) and (TTP)Li-2(THF)(2). Treatment of (TTP)HfCl2 with excess LiNH(p-MeC6H4) resulted in the formation of a bis(amido) complex, (TTP)Hf(NH-p-MeC6H4)(2) (3), instead of an imido complex. In the presence of excess aniline, 2 formed an equilibrium mixture of bis(amido) compounds, (TTP)Hf(NHPh)(NHAriPr) and (TTP)Hf(NHPh)(2). The nucleophilic character of the imido moiety is exhibited by its reaction with (BuNCO)-Bu-t, producing isolable N,O-bound ureato metallacycles. The kinetic product obtained with zirconium, (TTP)Zr(eta(2)-(NArC)-C-iPr(=(NBu)-Bu-t)O) (4a), isomerized to (TTP)Zr(eta(2)-(NBuC)-Bu-t(=NAriPr)O) (4b) in solution. Upon being heated to 80 degrees C, 3a produced the carbodiimide (ArN)-N-iPr=C=(NBu)-Bu-t and a transient Zr(IV) oxo complex. The analogous hafnium complex (TTP)Hf(eta(2)-(NArC)-C-iPr(=(NBu)-Bu-t)O) (5a) did not eject the carbodiimide upon heating to 110 degrees C but isomerized to (TTP)Hf(eta(2)-(NBuC)-Bu-t(=NAriPr)O) (5b). To support the formulation of 4a and 5a as N,O bound, the complex (TTP)Hf(eta(2)-(NArC)-C-iPr(=NAriPr)O) (6) was studied by variable-temperature NMR spectroscopy. The corresponding thio- and selenoureato metallacycles were not isolable in the reaction between 1 and 2 with (BuNCS)-Bu-t and (BuNCSe)-Bu-t. Concomitant formation of the metallacycle with decomposition to the carbodiimide, (ArN)-N-iPr=C=(NBu)-Bu-t, reflects the lower C-Ch bond strength in the proposed N,Ch-bound metallacycles. Treatment of 2 with 1,3-diisopropylcarbodiimide resulted in the eta(2)-guanidino complex (TTP)Hf(eta(2)-(NArC)-C-iPr(=(NPr)-Pr-i)(NPr)-Pr-i) (7a), which isomerized to the less sterically crowded isomer (TTP)Hf(eta(2)-(NPrC)-Pr-i(=NAriPr)(NPr)-Pr-i) (7b). Complexes 1, 2, 4a, 4b, and 7a were characterized by X-ray crystallography. The monomeric terminal imido compounds, 1 and 2, are isomorphous: M-N-imido distances of 1.863(2) Angstrom (Zr) and 1.859(2) Angstrom (Hf); M-N-imido-C angles of 172.5(2)degrees (Zr) and 173.4(2)degrees (Hf). The structures of the ureato complexes 4a and Ib and the guanidino complex 7a exhibit typical alkoxido and amido bond distances (Zr-N = 2.1096(13) Angstrom (ils), 2.137(3) Angstrom (4b); Zr-O = 2.0677(12) Angstrom (4a). 2.066(3) Angstrom (4b); Hf-N 2.087(2) Angstrom, 2.151(2) Angstrom (7a)).