The dinitrogen ligand

For the other 4 fragments the NN bond length is between 1.182 and 1.236 A, where the coordinated metal atoms are Ti, Mo, Zr and W. In general it is thought that W and Ta are 

TABLE 19. Mean geometric parameters for the dinitrogen substructures MNl and MN2 

The side-on bonding mode of dinitrogen remains very rare and there appear to be no structures recorded in the April 1993 version of the CSD. The side-on bonding mode of N2 is similar to the bonding mode of ethene, v here the 7r-bond of N2 donates electrons to the metal atom, and back-donation is from metal d-type orbitals to empty N2 7r -orbitals .  

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The coordination of the dinitrogen ligand is maintained when traTO-[Re(N2)Cl(dppe)2] is exposed to NaX salts (X = NCS, NCO , Ns ) in THF/MeOH solutions. The isothiocyanato and azido complexes of type (276) have been studied by X-ray structural analysis.A Re— N2 bond length of 1.951 A is observed for the NCS compound which is somewhat shorter than that in the starting material and indicates a considerable degree of double bond character. 

The insertions of CO into Pd-Me and of various strained alkenes into Pd-COR supported by the dinitrogen ligand Ar-BIAN have been studied by Vrieze and Elsevier by means of ex situ NMR spectroscopy [22, 51], For example, the sequential insertions of CO and norbomadiene into Pd-alkyl and Pd-acyl bonds allowed the detection and isolation of several intermediates such as acyl(carbonyl) and P-chelate (Scheme 7.24). The Pd complexes isolated after alkene insertion were found to have a structure arising from cis addition of Pd-C(0)R to the exo face of the olefin. 

Figure 15.1.5(f) shows a planar, side-on bonded N2 ligand between two Zr atoms in the compound [Cp"2Zr]2(N2) [Cp" = l,3-(SiMe2)2C5H3], The N-N bond length is 147 pm. Side-on coordination of the dinitrogen ligand appears to be important for its reduction. 

The course of our reaction depends on the phosphine, the acid, and the solvent. By varying these, it has been possible to isolate products containing the dinitrogen ligand in two stages of reduction, N2H or N2H2. 

A quantitative study of the dissociation of dinitrogen from the complexes [M(N2)2(dppe)2] in THF shows that it occurs too slowly to be the initiating step (16) and it must be the protonation of one of the dinitrogen ligands that triggers the sequence of reactions leading to ammonia in the monophosphine complexes. 

The most important complexes of rhenium(I) are those containing the dinitrogen ligand, in which the rhenium atom is also complexed by phosphine ligands. 

Replacement of the dinitrogen ligand in [Os(N2)(NH3)5]2+ by pyridine under aerobic conditions90 or of the weakly bound triflate ligand in [0s(0S02CF3)(NH3)5]2+ by pyridine62 yields [Os(py)(NH3)5]3+ cyclic voltammetric studies show reversible reduction to [Os(py)(NH3)5]2 + 62,90,91 and the surface-enhanced Raman spectrum91,92 (SERS) of the complex (both normal and deuteriated) shows Os-N, ammine and internal pyridine vibrational modes. Infrared spectra were also measured.91... 

Other complexes whose crystal structures have been determined include the tetraphosphorus complex (21),191 and the dinitrogen complex (22).182 The latter complex was originally believed to contain a sideways-on dinitrogen ligand 183 however, once corrections were made for disorder and anisotropy of the dinitrogen ligand, the more usual end-on coordination was proposed.182... 

The 14-electron complexes are highly reactive and decompose even when stored in an inert atmosphere. They readily bind an additional small ligand. In particular, they react with dinitrogen under mild conditions. However, the dinitrogen ligand is easily lost and this property has been exploited in the preparation of other tricoordinate complexes (equations 3 5). 

Protons can go directly to the dinitrogen ligand if it is sufficiently basic to accept H+ from the proton donor alternatively it might temporarily form a hydride, by addition to metal, or protonate another ligand (as S or O atoms) bound to metal in the vicinity of dinitrogen. 

Some remarkable reactions of M(N2)2(dpe)2 [M = W or Mo dpe = 1,2-bis(diphenylphosphino)ethane] have been reported. Addition of an alkyl group to the dinitrogen ligand takes place during photolysis in the presence of RBr in benzene solution (reaction 67) (R = Me, Et, or Bu ).213 In THF solution compounds of type (25), in which the ligating N2 has apparently displaced the oxygen from THF, are also formed.214... 

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