Mono- and Dianionic Guanidinate Ligands. Reactivity of [iPrNC(NiPr)₂]Ta(NMe₂)₃ and [(iPrNH)C(NiPr)₂]TaCl(NMe₂)₃ with Me₃SiCl and ArNC (Ar = 2,6-Me₂C₆H₄)

Reactivities of the dianionic guanidinate complex of [ i PrN C(N i Pr) 2 ]Ta(NMe 2 ) 3 ( 1 ) and the monoanionic guanidinate complex Ta(NMe 2 ) 3 Cl[( i PrN) 2 CN(H) i Pr] ( 4 ) have been investigated. Reaction of 1 with Me 3 SiCl produced compound 2, Ta(NMe 2 ) 3 Cl[( i PrN) 2 CN(SiMe 3 ) i Pr], which is proposed to arise from the addition of the Si−Cl bond across a Ta−N(guanidinate) bond of the starting material rather than one of the Ta−NMe 2 bonds. Complex 2 is the analogue of 4 in which H has been replaced by SiMe 3 . Derivatization of 2 was achieved by reaction with PhCH 2 MgCl to produce Ta(NMe 2 ) 3 (CH 2 Ph)[( i PrN) 2 CN(SiMe 3 ) i Pr] ( 3 ). The single-crystal X-ray-determined structural features of 3 are reported and support the spectroscopic characterization of 3, and indirectly that of 2, by revealing a bidentate tetrasubstituted guanidinate monoanion and an η 1 -benzyl group. Complexes 1 and 4 insert 2,6-dimethylphenyl isocyanide (ArNC) to yield the structurally characterized complex Ta(NMe 2 )[C(N i Pr) 3 ][η 2 -(Me 2 N)C N(2,6-Me 2 C 6 H 3 )] 2 ( 5 ), which possesses one dianionic bidentate guanidinate ligand and two η 2 -iminocarbamoyl ligands derived from the insertion of ArNC into two of the Ta−NMe 2 bonds of the starting materials. The formally seven-coordinate Ta(V) center of 5 can be viewed as pseudo trigonal bipyramidal, with each of the η 2 -C N linkages occupying a single coordination site. In the case of 4, the transformation of the guanidinate anion into its dianionic form was concomitant with insertion of ArNC. The sources of base for the deprotonation of the guanidinate ligand are likely the amido groups of a portion of the starting material.