Mono- and dianionic guanidinate ligands. Reactivity of [(PrN)-Pr-i=C((NPr)-Pr-i)(2)]Ta(NMe2)(3) and [((PrNH)-Pr-i)C ((NPr)-Pr-i)(2)]TaCl(NMe2)(3) with Me3SiCl and ArNC (Ar=2,6-Me2C6H4)

Reactivities of the dianionic guanidinate complex of [(PrN)-Pr-i=C((NPr)-Pr -i)(2)]Ta(NMe2)(3) (1) and the monoanionic guanidinate complex Ta(NMe2)(3)C l[((PrN)-Pr-i)(2)CN(H)Pr-i] (4) have been investigated. Reaction of 1 with Me3SiCl produced compound 2, Ta(NMe2)(3)Cl[((PrN)-Pr-i)(2)CN-(SiMe3)Pr-i], which is proposed to arise from the addition of the Si-CI bond across a Ta- N(guanidinate) bond of the starting material rather than one of the Ta-NMe2 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 Complex 2 is the analogue of 4 in which H has been replaced by SiMe 3.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 PhCH2MgCl to produce T a(NMe2)(3)(CH2Ph)[((PrN)-Pr-i)(2)CN-(SiMe3)Pr-i] (3). The single-crystal X- ray-determined structural features of 3 are reported and support the spectr oscopic characterization of 3, and indirectly that of 2, by revealing a bid entate tetrasubstituted guanidinate monoanion and an yl-benzyl group. Compl exes 1 and 4 insert 2,6-dimethylphenyl isocyanide (ArNC) to yield the struc turally characterized complex Ta(NMe2)[C((NPr)-Pr-i)(3)][eta(2)-(Me2N)C=N(2 ,6-Me2C6H3)](2) (5), which possesses one dianionic bidentate guanidinate li gand and two eta(2)-iminocarbamoyl ligands derived from the insertion of Ar NC into two of the Ta-NMe2 bonds of the starting materials. The formally se ven-coordinate Ta(V) center of 5 can be viewed as pseudo trigonal bipyramid al, with each of the eta(2)-C=N linkages occupying a single coordination si te. 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.