The Diazene Ligand

Clearly, to evaluate these proposals for the intermediacy of diazene, it is important to understand the properties of this highly reactive molecule. 

In the free state diazene can occur in three isomeric forms cis- and trans-diazene (12 and 13, respectively), and iso-diazene (14). 

These molecules have been the subject of several theoretical (69) and 

Hartree-Fock treatment which predicts the triplet to be at least 21 kJ mol 1 below the singlet state (2, 345). 

Besides hydrazine and dinitrogen, dihydrogen is also observed as a product of the decomposition of diazene. Originally this was believed to 

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Upon coordination, the diazene ligand can adopt one of three potential configurations (16-18). 

Oxidation of the hydrazine complex formed by the reaction of (t] -C5H5)Mn(CO)2(THF) with hydrazine (Scheme 9.2) gives a dinitrogen complex (t -C5H5)Mn(CO)2(N2). Methylation of the N2 ligand with MeLi followed by treatment with methyl cation converts the N2 complex into a dimethyldiazene complex, from which the diazene ligand is released by applying N2 pressure (eq (3)) [10]. 

Neither 77 nor 79 could be converted to diazene complexes Cp2M=N—NH2 (M = Ti, V) by desilylation with ethereal hydrochloric acid because along with the Me3Si/H exchange, the diazene ligand is dislodged (49). The protolysis [Eq. (125)] takes place in 90% (Ti) and 75%... 

As in the case of (cis-diazene)Fe(C0)4 complexes the diazeno ligand disturbs the local symmetry (C4v) in L-M(CO)s and gives rise to four or five v(CO) bands instead of three. The low-energy absorption band of (benzo[c]cinnoline) M(CO)s (M=Cr, Mo, W) was assigned to an rt- -n transition5 ). However, in view of the assignments made for the spectra of LCr(CO)s compounds (L = amines, phosphines)52) this transition may be rather of "d-d" or CTML character. 

L = Me3CNC, P(p-MeC6H4)3, by Ibers et a/.70-72. If the diazene is regarded as a single ligand, the Ni atom has its expected trigonal coordination. Comparison with the... 

The resuits obtained from cyclic diazenes and iron carbonyls serve as an example to discuss the modes of coordination of an isolated N=N bond30 62 46 19 As predicted from the electronic structure of these ligands, a-coordination via the orbital is usually favoured over 7T-coordination. Only if the metal is a very good 7r-donor (see Table 8) and the diazene a good jr-acceptor, like azobenzene, will stable 7T-com-plexes be favoured. Thus the first reaction step usually affords the o-N-tetracarbonyl... 

We investigated (82) two iron sulfur complexes with diazene (di-imide) as the sixth ligand, [Fe(NHS4)(N2H2)] = 1 (N2H2) and [Fe(S4)PR3 (N2H2)] = 2(N2H2)(Fig.3). 

The total electronic coordination energy of frans-diazene is —73.1 kJ/mol including the estimated total hydrogen bond energy of (—)20 kJ/mol for the mononuclear complex irons-2(N2H2) (see above). Thus, the stabilization of the unstable diazene ligand by hydrogen... 

This example shows that small modifications in the second ligand sphere can be applied for a fine-tuning of the bonding between the substrate (here modeled by the diazene moiety) and the complex via hydrogen bridges. 

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