Metal-nitrogen multiple bonds

The transition metal-nitrogen multiple bond. K. Dehnicke and J. Strahle, Angew. Chem., Int. Ed. Engl., 1981, 20, 413-426 (124). 

Several reviews have dealt with the chemistry of transition metal-nitrogen derivatives,255-257 including compounds with metal-nitrogen multiple bonds,258,259 especially those encountered in high oxidation states. 

The most widely used methods for the syntheses of monohydrazido(2—) complexes have been from the parent hydrazide and metal oxo or halide complexes with formal elimination of water (equation 168), hydrogen halide (equation 169) or trimethylchlbrosilane (equation 170). The preparation of the hydrazido(2—) complexes, shown in equation (169) and (170), shows that the driving force for the formation of metal-nitrogen multiple bonds is such that the N hydrogens are lost. 

There is only one reported mononuclear hydrazido(2—) complex for the titanium group, [Ti(r/5-CjHj)2(NNSiMe3)2].498 Other hydrazido complexes are known but involve bridging hydrazido groups (see below). For reasons not yet fully understood, titanium is apparently reluctant to form four electron metal-nitrogen multiple bonds. 

Reactions of N2 Complexes Only the most basic N2 complexes, notably the bis-dinitrogen Mo and W complexes, can be protonated. According to the exact conditions, various N2H complexes are obtained, and even, in some cases, free NH3 and N2H4. As strongly reduced Mo(0) and W(0) complexes, the metal can apparently supply the six electrons required by Eq. 16.21, and so the metals are oxidized during the process. Note, too, that in Eq. 16.28, the loss of the very strong N—N triple bond is compensated by the formation of two N—H bonds and a metal nitrogen multiple bond. 

D, Bright and J. A. Ibers, Inorg. Chem., 8, 709 (1969). Studies of Metal Nitrogen Multiple Bonds. V. The Crystal Structure of Potassium Nitridopentachloroosmate(VI), K2OSNCI5. 

Nitriles. Nitriles can be prepared by a number of methods, including ( /) the reaction of alkyl haHdes with alkaH metal cyanides, (2) addition of hydrogen cyanide to a carbon—carbon, carbon—oxygen, or carbon—nitrogen multiple bond, (2) reaction of hydrogen cyanide with a carboxyHc acid over a dehydration catalyst, and (4) ammoxidation of hydrocarbons containing an activated methyl group. For reviews on the preparation of nitriles see references 14 and 15. 

The insertion of unsaturated molecules into metal-carbon bonds is a critically important step in many transition-metal catalyzed organic transformations. The difference in insertion propensity of carbon-carbon and carbon-nitrogen multiple bonds can be attributed to the coordination characteristics of the respective molecules. The difficulty in achieving a to it isomerization may be the reason for the paucity of imine insertions. The synthesis of amides by the insertion of imines into palladium(II)-acyl bonds is the first direct observation of the insertion of imines into bonds between transition metals and carbon (see Scheme 7). The alternating copolymerization of imines with carbon monoxide (in which the insertion of the imine into palladium-acyl bonds would be the key step in the chain growth sequence), if successful, should constitute a new procedure for the synthesis of polypeptides (see Scheme 7).348... 

A similar inference about the importance of the metal can be drawn from the differences in behavior with carbon-nitrogen multiple bond systems m... 

The three structurally characterized examples of this type of bonding are shown in Table 16 and all would involve a twenty electron valence count for the metal if the hydrazido(2 —) ligand functioned as a four-electron donor. The structure of [Re( 7s-CsH5)(CO)2(NNMeC6H4OMe)]460 is typical and is shown in (34). The formal localization of a lone pair on Na causes a significant increase in the metal nitrogen distance consistent with decreased multiple bonding. 

---