Dinitrogen, complex

Numerous complexes of N2 with transition metals have been prepared, and the geometries for many of them are known. Here, we shall describe the metal- N2 bonding for CoH(N2)(PPhj)3 and [ Ru(NH3)5 2N2] .  

From the (3d) (4s) ground-state configuration of Co, we may obtain a (3d) valence-electron configuration (14) 

To form five o-bonds, the cobalt may use hybrid orbitals that are constructed from its 3d, 4s and three 4p orbitals. The cobalt is then dsp hybridized. If we assume that the cobalt and two nitrogen atoms lie along the z-axis, then the cobalt lone-pair electrons of structure (15) occupy 3d y, 3d,j and 3d j  

In structure (15), the formal charge for the cobalt atom and the adjacent nitrogen atom are negative and positive respectively, relative to their values prior to coordination. We may reduce their magnitudes by delocalizing one electron from each of the d and d orbitals into bonding Co-N and orbitals, to obtain increased-valence structure (16). 

In structure (16), the N-N vi-bonds have bond-numbers less than unity and therefore the N-N bond should be longer than that of free N. The Co-N bond of this structure has double-bond character, and therefore this bond would be expected to be shorter than a single bond. The bond lengths reported above confirm these expectations. 

Molecular nitrogen can react directly with some transition-metal compounds to form dinitrogen complexes, the structure and properties of which are of considerable interest because they may serve as models for biological nitrogen 

As a bridging ligand in dinuclear systems, dinitrogen may formally be classified 

In the mixed-metal complex [WCI(py)(PMePh)3(//3-N2)l2(AICl2)2, both WNN linkages are essentially linear, and the four metal atoms and two /2.3-N2 ligands almost lie in the same plane, as shown in Fig. 15.1.5(d). 

In [PhP(CH2SiMe2NPh)2]2Ta2(/u-H)2(N2), the dinitrogen moiety is end-on bound to one Ta atom and side-on bound to the other, as shown in Fig. 15.1.5(e). The N-N distance of 131.9 pm is consistent with a formal assignment of the bridging dinitrogen moiety as (N2)4. The shortest distance of the end-on Ta-N bond is 188.7 pm, which is consistent with its considerable double-bond character. 

In the side-on arrangement, the bonding is considered to arise from two interdependent components. In the first part, a overlap between the filled n orbital of N2 and a suitably directed vacant hybrid metal orbital forms a donor bond. In the second part, the M atom and N2 molecule are involved in two back-bonding interactions, one having it symmetry as shown in Fig. 15.1.7(a), and the other with S symmetry as shown in Fig. 15.1.7(b). These n and S-back bonds synergically reinforce the a bond. 

1 H PPI13 M63P I N = N PM63 PBUjPh MePhjP PPhjMe  

The discovery of the first dinitrogen complex, [Ru(NH3)5N2] % was made in 1965. Most complexes of this kind made since, contain phosphine groups, but these groups do not necessarily play a crucial part in determining their properties. 

Crystal-structure analyses have demonstrated that the N2 molecule is bonded end on to the metal atom as in (8.94). It is apparent that N acts as a a donor and an jt acceptor in the same way as the isoelectronic CO molecule and phosphine groups. 

Much of the interest in these compounds centres on the possibility of developing new materials for nitrogen fixation, that is, reduction of ammonia. This process is carried out in nature by bacteria (Chapter 11.5). It has been established that cM-W(N2)2(PMe2Ph)4 and a few other dinitrogen complexes 

Dinitrogen complexes can be made by direct action of gaseous nitrogen on an appropriate metal complex. In some cases direct pick-up of atmospheric nitrogen from a solution of the complex will take place by a reversible reaction (8.95). Another method of synthesis uses an azide (8.96). 

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Molybdenum(0) also forms a variety of dinitrogen complexes (41), especially when there are phosphine ligands in the molybdenum coordination sphere (see Fig. 7c). This type of complex has been extensively studied because the coordinated dinitrogen is reduced to ammonia upon acidification. 

Dinitrogen complexes are molecules that contain dinitrogen bound to a metal. The first dinitrogen complex, [Ru(N2) (NH ) ], was reported in 1965 as a product of the interaction of hydrazine and RuCl (aq) (18). There are hundreds of complexes in the 1990s with dinitrogen as a ligand (19,20). 

A similar process has been patented coveriug Cp 2ZrR [86165-24-4] Cp 2ZrR(N2) [86165-25-5] and (Cp 2ZrR)2N2 [86165-23-3] where R = CH[Si(CH3 )3 ]2. Protonolysis of the dinitrogen complexes gives hydraziae and ammonia (250). 

Analogous complexes are formed between l,2-diazaspiro[2.5]oct-l-ene (189) and the Ru(NH3)s cation in a 1 1 or 1 2 ratio. An X-ray analysis of the 1 1 complex (189a) gives evidence of a nitrogen-metal bond. Oxidation converts the 1 1 complex to the known dinitrogen complex (190) by liberation of cyclohexanone (80MI50800). 

One of the most dramatic developments in the chemistry of N2 during the past 30 years was the discovery by A. D. Allen and C. V. Senoff in 1965 that dinitrogen complexes such as [Ru(NH3)5(N2)1 could readily be prepared from aqueous RUCI3 using hydrazine hydrate in aqueous solution. Since that time virtually all transition metals have been found to give dinitrogen complexes and several hundred such compounds are now characterized.Three general preparative methods are available ... 

Frequently dinitrogen complexes have colours in the range white-yellow-orange-red-brown but other colours are known, e.g. [ Ti(t -C5H5)2 2-(N2)] is blue. 

Osmium(II) forms no hexaaquo complex and [Os(NH3)g] +, which may possibly be present in potassium/liquid NH3 solutions, is also unstable. [Os(NH3)5N2] and other dinitrogen complexes are known but only ligands with good 7r-acceptor properties, such as CN, bipy, phen, phosphines and arsines, really stabilize Os , and these form complexes similar to their Ru analogues. 

Historically, the most important ruthenium(II) ammine species is [Ru(NH3)5N2]2+, the first stable dinitrogen complex to be isolated (1965). It was initially obtained by refluxing RuC13 in hydrazine solution (but many... 

The Irans-dichloro complex can be converted into an unusual complex with bound azide and dinitrogen (Figure 1.21) with linear Ru—N=N, as in other dinitrogen complexes (z/(N=N) 2103 cm-1) [75]. 

The dinitrogen complex [Os(NH3)5N2]2+ is a useful synthetic intermediate, while the presence of the weakly nucleophilic triflate group enables it to be easily removed in the synthesis of the dihydrogen complex. 

The dinitrogen complex (Figure 1.56) has a rather short Os-N2 bond (1.842 A) indicating some multiple bond character while the trans-Os—N bond is slightly longer than the others, but not significantly different. The N2 ligand shows i/(N—N) at 2022 cm-1 in the IR spectrum [145]. 

Some bis(dinitrogen) complexes exist, generally as m-isomers (presumably this minimizes competition for the metal t2g electron density in 7r-bonding). Unlike ruthenium, osmium(III) dinitrogen complexes do exist, showing osmium(III) to be a better 7r-donor not surprisingly, they are more labile than the osmium(II) species. 

The dinitrogen complex is stable to borohydride reduction but the nitrogen is displaced by chloride... 

Figure 2.80 Synthesis of the dinitrogen complex IrCl(N2)(PPh3)2.

Potassium graphite reduction of the dichloro precursors under an atmosphere of N2 in THF according to Scheme 101 allowed access to new families of Group 4 bimetallic "side-on-bridged" dinitrogen complexes... 

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