Parent Amido —NH2 Derivatives

The thermodynamics of the oxidative addition process tends to be favored by increased electron density at the metal centre, hence the focus on later transition metal derivatives. Furthermore, as discussed above, it is believed that M—N n-bonds to the later transition metals are of significance only if the transition metal complex is unsaturated. Saturated late transition metal amides (parent or substimted) often exhibit the so-called n-conflict (see above) so that the nitrogen centre displays no n-bonding to the metal and retains its lone pair character and basicity. 

Theoretical smdies on unsaturated systems such as the d moieties IMf ri -CgMes) (CO)] and [tra -M(PH3)2X] (M = Rh or Ir) show that the thermodynamics of addition of NH3 to form MiHjNH, are more favorable for the third row element due to stronger M—H and M—NH, bonding. Experimental work available to date is in harmony with these calculations and has shown that it is possible to add NH3 to displace a weakly bound ligand and form an amido-hydride. The first such reactions under mild conditions involved an lr(l) complex, as shown in Equation (6.11).  

The Ir(III) metal centres in the products, which are bound to a terminal hydride and a bridging —NH2 group, represented the first X-ray stnictural authentication of a transition metal species with both amide and hydride bound to a metal centre. The reactivity of the complexes is low, however, and appears to be dominated by the stability of the lr(p-NH2)2lr bridging unit. More recent work has shown that olefin-iridium(l) complexes, such as the propene species [ HC(CH2CH2PBu 2)2 Ir(CH2CHMe)], react diiectly with ammonia at room temperature as shown in Equation (6.12).  

The product amido-hydride is the first structurally characterized transition metal complex that features both a terminal amide and hydride. The X-ray crystal structure shows that the nitrogen centre has planar coordination. This geometry results from 71-bonding between the nitrogen electron pair and the lowest unoccupied metal orbital which lies in the equatorial coordination plane of the atoms lr(Ci ethine)(H)(N). This is consistent with earlier calculations on related species. The possibility that the addition of 

NH3 to the iridium centre could have occurred by interaction with an Ir—H moiety derived from insertion of Ir(I) into a CH bond of the Bu groups or the methanide centre of the pincer ligand was eliminated by labelling studies All the available experimental data point to the simple insertion of the lr(l) centre into an N—H bond of ammonia. 

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