The zirconium and hafnium porphyrin imido complexes (TTP)MdNAr i Pr [TTP ) meso-tetra-p-tolylporphyrinato dianion, M ) Zr (1), Hf (2), Ar i Pr ) 2,6-diisopropylphenyl] were synthesized from (TTP)MCl 2 and 2 equiv of LiNHAr i Pr . The zirconium imido complex, (TTP)ZrdNAr i Pr , was also obtained from the preformed imido complex Zr(NAr i Pr )Cl 2 (THF) 2 and (TTP)Li 2 (THF) 2 . Treatment of (TTP)HfCl 2 with excess LiNH(p-MeC 6 H 4 ) resulted in the formation of a bis(amido) complex, (TTP)Hf(NH-p-MeC 6 H 4 ) 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)(NHAr i Pr ) and (TTP)Hf(NHPh) 2 . The nucleophilic character of the imido moiety is exhibited by its reaction with t BuNCO, producing isolable N,O-bound ureato metallacycles. The kinetic product obtained with zirconium, (TTP)Zr(η 2 -NAr i Pr C(dN t Bu)O) (4a), isomerized to (TTP)Zr(η 2 -N t BuC(dNAr i Pr )O) (4b) in solution. Upon being heated to 80 °C, 4a produced the carbodiimide Ar i Pr NdCdN t Bu and a transient Zr(IV) oxo complex. The analogous hafnium complex (TTP)Hf(η 2 -NAr i Pr C(dN t Bu)O) (5a) did not eject the carbodiimide upon heating to 110 °C but isomerized to (TTP)Hf(η 2 -N t BuC(dNAr i Pr )O) (5b). To support the formulation of 4a and 5a as N,O bound, the complex (TTP)Hf(η 2 -NAr i Pr C(dNAr i Pr )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 t BuNCS and t BuNCSe. Concomitant formation of the metallacycle with decomposition to the carbodiimide, Ar i Pr NdCdN t Bu, reflects the lower C-Ch bond strength in the proposed N,Ch-bound metallacycles. Treatment of 2 with 1,3diisopropylcarbodiimide resulted in the η 2 -guanidino complex (TTP)Hf(η 2 -NAr i Pr C(dN i Pr)N i Pr) (7a), which isomerized to the less sterically crowded isomer (TTP)Hf(η 2 -N i PrC(dNAr i Pr )N i Pr) (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) Å (Zr) and 1.859(2) Å (Hf); M-N imido -C angles of 172.5(2)°( Zr) and 173.4(2)°(Hf). The structures of the ureato complexes 4a and 4b and the guanidino complex 7a exhibit typical alkoxido and amido bond distances (Zr-N ) 2.1096(13) Å (4a), 2.137(3) Å (4b); Zr-O ) 2.0677(12) Å (4a), 2.066(3) Å (4b); Hf-N ) 2.087(2) Å, 2.151(2) Å (7a)).