Transition metal complexes of guanidinate dianions: Reactions between guanidines and M(NMe2)(5) (M = Ta, Nb)

The protonation of two metal-amido groups of M(NMe2)(5) with trialkylguanid ines yielded a series of novel complexes with formulas [RNC(NR)NR]M(NMe2)(3 ) (1-4) (M = Ta, Nb; R = isopropyl, cyclohexyl). These complexes contained dianionic N,N',N " trialkylguanidinate ligands which were coordinated in a chelating bidentate mode. A single-crystal X-ray study of [CyNC(NCy)NCy]Ta( NMe2)(3) (3) (C25H51N6Ta, triclinic, P (1) over bar, a = 9.4155-(2) Angstro m, b = 13.3188(2) Angstrom, c = 13.5215(2) Angstrom, alpha = 117.075(1)degr ees, beta = 101.744(1)degrees, gamma = 98.507(1)degrees, Z= 2) confirmed th e connectivity of these species. These guanidinate ligands exhibited both p lanarity of the central CN3 group and the correct orientation of the three NR substituents to allow for pi conjugation within the ligand core.