END-ON VERSUS SIDE-ON BONDING OF DINITROGEN TO DINUCLEAR EARLY TRANSITION-METAL COMPLEXES

{The preparation of a series of dinuclear complexes of zirconium and tantalum is presented in an effort to determine factors involved in the coordination mode of the bridging dinitrogen ligand. Reduction of ZrCl3[N(SiMe2-CH2PR2)2] (R = Pr(i) and B(t)) with Na/Hg under N2 generate the corresponding dinitrogen complexes {[(R2PCH2-SiMe2)2 SiMe2)2N]ZrCl}2(mu-eta2:eta2-N2); the X-ray crystal structure of the dark blue derivative having R = Pr(i) shows that the dinitrogen unit is bridging in a planar side-on bonding mode. The N-N bond distance is 1.548(7) angstrom, which is the longest such distance for any dinitrogen complex reported. Molecular orbital calculations on this molecule indicate that the N2 unit bonds in a side-on fashion due to the unavailability of one of the d orbitals on each zirconium that would normally be involved in a pi-bond with the antibonding pi*-orbitals of the N2 moiety. This particular zirconium d orbital is unavailable due to the ancillary tridentate ligand which positions the amide donor (and the chloride) to overlap effectively with this d orbital. The results of this MO study also show that the HOMO has delta-symmetry. When the related derivative Zr(eta5-C5H5)Cl2[N(SiMe2CH2PPri2)2] is reduced under N2, the corresponding dinuclear derivative {[(Pri2PCH2SiMe2)2N]Zr(eta5-C5H5)}2(mu-eta1:eta1-N2) has the bridging dinitrogen unit end-on. In this case, the MO analysis shows that the cyclopentadienyl ligand overlaps with that zirconium d orbital which could form the delta-molecular orbital of the side-on mode; as a result, the end-on form is generated. A similar argument can be used to rationalize the end-on binding mode of the dinitrogen unit in the related tantalum derivatives {[(Pri2PCH2SiMe2)2N]Ta=CHR}2(mu-eta1:eta1-N2) (R = Bu(t), Ph), since now the alkylidene unit effectively overlaps with the same d orbital Crystals of {[(Pri2PCH2-SiMe2)2N]ZrCl}(mu-eta2:eta2-N2)-CH3C6H5 are monoclinic