RESONANCE RAMAN STUDIES OF DINUCLEAR ZIRCONIUM COMPLEXES WITH A BRIDGING DINITROGEN LIGAND - POSSIBLE N-2-COORDINATION MODELS FOR NITROGENASE

Resonance Raman spectroscopy was used to investigate the vibrational p roperties of the bridging dinitrogen ligand in two different dinuclear zirconium complexes known to contain unusually long N-N bonds. {[(Pr( 2)(i)PCH(2)SiMe(2))(2)N] Zr(eta(5)-C5H5)}(2)(mu-eta(1):eta(1)-N-2) (1) has an ''end-on'' bridging dinitrogen ligand and )PCH(2)SiMe(2))(2)N] ZrCl}(2)(mu-eta(2):eta(2)-N-2) (2) has a ''side-on'' bridging dinitrog en ligand. The nu(N-N) stretching frequency for the mu-eta(1):eta(1)-N -2 complex is observed at 1211 cm(-1) in the solid slate and confirmed by an similar to 40-cm(-1) isotope shift in the N-15(2)-substituted i sotopomer. A similar frequency and isotope shift are observed for 1 in toluene, indicating that the mu-eta(1):eta(1)-N-2 structure is preser ved in solution. Vibrational assignments for the mu-eta(2):eta(2)-N-2 complex 2 are complicated by the presence of many isotope-sensitive pe aks in the solid state. Nevertheless, we propose that the intense peak at similar to 731 cm(-1) corresponds to one of the totally symmetric A(g) modes of the Zr2N2 moiety that is predominantly nu(N-N)in charact er based on its similar to 22-cm(-1) isotope shift and its strong pola rization in THF solution. The majority of the remaining peaks in the r esonance Raman spectrum of 2 can be assigned as overtone and combinati on bands of fundamental modes at 258, 317, and 393 cm(-1) and the nu(N -N) mode at 731 cm(-1) (709 cm(-1) in the N-15 isotopomer). Comparison of solid and solution state data for 2 suggests that the structure of the side-on-bonded N-2 ligand is largely maintained in THF solution. Finally, the vibrational spectroscopic properties of 2 as the dibromo and the mixed bromochloro complex are reported. These complexes all ma intain the strong nu(N-N) mode at 709 cm(-1) (N-15 isotopomers) but ex hibit changes in the fundamental modes at similar to 300 cm(-1) and, h ence, also for the resulting overtone and combination bands. This work has shown that resonance Raman spectra reveal very marked differences and, hence,can be used tb distinguish the two modes of bonding in sid e-on versus end-on coordinated N-2 complexes of dinuclear transition m etals.