Bonding and bending in zirconium(IV) and hafnium(IV) hydrazides

Reaction of the dichloro complexes [M(N(2) (TBS)N(py))Cl(2)] (M=Zr: 1, Hf: 2; TBS: tBuMe(2)Si; py: pyridine) with one molar equivalent of LiNHNPh(2) gave mixtures of the two diastereomeric chlorohydrazido(1-) complexes [M(N(2) (TBS)N(py))(NHNPh(2))Cl] (M=Zr: 3 a,b, Hf: 4 a,b) in which the diphenylhydrazido(1-) ligand adopts a bent kappa(1) coordination. This mixture of isomers could be cleanly converted into the deep green diphenylhydrazido(2-) complexes [Zr(N(2) (TBS)N(py))(NNPh(2))(py)] (5) and [Hf(N(2) (TBS)N(py))(NNPh(2))(py)] (6), respectively, by dehydrohalogenation with lithium hexamethyldisilazide (LiHMDS) in the presence of one molar equivalent of pyridine. Both complexes contain a linearly coordinated hydrazinediide for which a DFT-based frontier orbital analysis established bonding through one sigma and two pi orbitals. A high polarity of the M=N bond was found, in accordance with the description of hydrazinediide(2-) acting as a six-electron donor ligand. The pyridine ligand in [M(N(2) (TBS)N(py))(NNPh(2))(py)] (M=Zr: 5, Hf: 6) is substitutionally labile as established by line-shape analysis of the dynamic spectra (DeltaG(not equal)=19 kcal mol(-1)). A change in denticity of the hydrazido unit from kappa(1) to kappa(2) was studied by DFT methods. Both forms are calculated to be very close in energy and are only separated by shallow activation barriers, which supports the notion of a rapid kappa(1) to kappa(2) interconversion. This process is believed to happen early on in the N-N scission in the presence of coupling reagents. Frontier orbital and natural population analyses suggest that a primarily charge-controlled nucleophilic attack at N(alpha) is unlikely whereas interaction with an electrophile could play an important role. This hypothesis was tested by the reaction of 5 and 6 with one molar equivalent of B(C(6)F(5))(3) to give [Zr(N(2) (TBS)N(py))(NNPh(2)){B(C(6)F(5))(3)}] (7) and [Hf(N(2) (TBS)N(py))(NNPh(2)){B(C(6)F(5))(3)}] (8). In these products, B(C(6)F(5))(3) becomes attached to the N(alpha) atom of the side-on bound hydrazinediide and there is an additional interaction of an ortho-F atom of a C(6)F(5) ring with the metal centre.