Transition-metal-coordinated dinitrogen

Since the first step of all of these reactions is dinitrogen coordination to either the surface of the catalyst or transition metal center of the complex, let us briefly discuss the nature and importance of the M-N2 interaction, and the possible coordination modes ofN2 to transition metal centers. These issues were the subjects of many discussions in the literature [10, 11] and it is commonly agreed that the interaction of the N2 molecule with transition metal centers facilitates the activation of the N=N triple bond the stronger the M-N2 interaction, the easier to break the N=N triple bond. 

The interest in these ligands has stemmed largely from their proven involvement in the chemistry of coordinated dinitrogen. However, they are of considerable intrinsic interest as their electronic flexibility permits them to bind to transition metals with a wide variety of bonding modes. 

Like electrophilic addition to diazo compounds [7] from which diazonium ions and, subsequently, carbocations are generated, transition-metal compounds that can act as Lewis acids are potentially effective catalysts for metal carbene transformations. These compounds possess an open coordination site that allows the formation of a diazo carbon-metal bond with a diazo compound and, after loss of dinitrogen, affords a metal carbene (Scheme 5.2). 

The increased basicity of a ligand when coordinated to the heavier metals in a transition metal triad has been appreciated for some time (311). This is manifest in (1) the rate of alkylation of alkyldiazenido complexes (kw/kMo = 5.4) (93), and (2) the rate of formation of hydra-zido(2-) complexes by the reaction of dinitrogen complexes with acid, in methanol ( w// Mo = 9.2 x 102) (186), and in tetrahydrofuran (kw/ kMo = 29-85, dependent upon the acid and substrate employed) (187). Clearly the electron-releasing capability of the metal has conflicting influences on the rate, but as in Section VI,El the basicity influence dominates. 

In view of the many approximations in these semi-empirical calculations their results can only be regarded as first hints for more elaborate methods. Such are DFT calculations on FeMoco-type clusters and model complexes. As it is discussed, for instance, in Ref. (31), N2 usually binds very weakly to transition metal complexes. It is thus not certain whether end-on coordination to one or two iron centers can lead to sufficiently strong coordination of dinitrogen. 

Orbital interactions of N2 and transition-metal M in end-on coordinated dinitrogen complexes. The dtt-ljt g overlap shown here also occurs in the yz plane. 

A select number of transition metal compounds are effective as catalysts for carbenoid reactions of diazo compounds (1-3). Their catalytic activity depends on coordination unsaturation at their metal center which allows them to react as electrophiles with diazo compounds. Electrophilic addition to diazo compounds, which is the rate limiting step, causes the loss of dinitrogen and production of a metal stabilized carbene. Transfer of the electrophilic carbene to an electron rich substrate (S ) in a subsequent fast step completes the catalytic cycle (Scheme I). Lewis bases (B ) such as nitriles compete with the diazo compound for the coordinatively unsaturated metal center and are effective inhibitors of catalytic activity. Although carbene complexes with catalytically active transition metal compounds have not been observed as yet, sufficient indirect evidence from reactivity and selectivity correlations with stable metal carbenes (4,5) exist to justify their involvement in catalytic transformations. 

Scheme 2 Coordination of dinitrogen in transition metal complexes 3 ATOMIC NITROGEN...

The factors that influence the mode of bonding of nitric oxide. These are important as part of the more general study of how small molecules bond to, and are activated by, transition metals. In this respect the coordination chemistry of nitric oxide exhibits greater versatility compared with that of many other simple molecules, including carbon monoxide and dinitrogen. 

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