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(NMe(2))(5) with trialkylguanidines yielded a series of novel complexes with formulas [RNC(NR)NR]M(NMe(2))(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(NMe(2))(3) (3) (C(25)H(51)N(6)Ta, triclinic, P, a = 9.4155(2) Å, b = 13.3188(2) Å, c = 13.5215(2) Å, alpha = 117.075(1) degrees, beta = 101.744(1) degrees, gamma = 98.507(1) degrees, Z = 2) confirmed the connectivity of these species. These guanidinate ligands exhibited both planarity of the central CN(3) group and the correct orientation of the three NR substituents to allow for pi conjugation within the ligand core.