Theoretical study of the bonding capabilities of 1,4-diaza-1,3-butadiene and cis-1,3-butadiene ligands in cyclopentadienyl tantalum(V) complexes

A folded envelope geometry is generally encountered in structurally characterized high valent early transition metal compounds containing a substituted 1,4-diaza-1,3-butadiene ligand. In these cases the diazabutadiene ligand can be regarded as dianionic ene-diamido ligand and the folded five-membered ring can be described as a metallacyclo-2,5-diazapent-3-ene. A comparable metallacyclopent-3-ene description is attributed to η4-butadiene complexes of early transition metals, in which the ligand exhibits a σ2, π-character. For this reason, a parallel description was initially adopted by several authors for the ene-diamido ligands and the origin of the folding of the five-membered ring was attributed to the saturation of the metal center through the CC double bond. The aim of this paper is to discriminate the bonding capabilities of the 1,4-diaza-1,3-butadiene and cis-1,3-butadiene ligands on high-valent tantalum complexes, and to establish the origin of the folding of the 1,4-diaza-1,3-butadiene ligand. A Density Functional study (DFT) of the model complex [CpTaCl2(HNCHCHNH)] (1) and compound [CpTaCl2(η4-butadiene)] (2) has been carried out. The differences in the bonding description of these ligands coordinated to the identical [CpTaCl2] moiety are discussed through the examination of the HOMOs of 1 and 2 and on the basis of a FMO analysis. In complex 1, there is no evidence of any positive overlap between the tantalum center and the inner carbon atoms of the diazabutadiene ligand, while for 2 the classical σ2,π-description for the η4-butadiene ligand is well-founded. Consequently, the generally accepted bonding description of 1,4-diaza-1,3-butadiene ligands coordinated to high valent early transition complexes should be modified. The origin of the folding is the reorientation of the N hybrid orbitals with the purpose of saturating the metal center and no significant donation through the CC bond occurs.