Nitrogen-containing ligands nitriles

Complex hydride reductions of Nj, N O, [CN] and nitriles in reactions containing complex mixtures of metal-ion species produce N—H bonded products " by reactions that may relate to those of the biological nitrogenases". Complete reaction stoichiometries are not well established. These reactions are not competitive with other methods for NH or amine synthesis. Nitrogen reacts in H O with a mixture of NaBH, S-donor ligands (e.g., NH CjM SH) and Mo and Fe salts to form NHj and NjH in low yield . In similar systems, nitriles and isonitriles are reduced to NHj and amines in low yield . 

All three structural forms contain one terminal and one bridging hydride and ten terminal carbonyl ligands. The entering ligand occupies an equatorial site (la) when L is a phosphine or a phosphite whereas it coordinates axially (Ib or Ic) when L contains a nitrogen as the donor atom (nitriles, amines). The occurrenee of transformation (1) is accompanied by a net color change from the intense violet of (//-H)20s3(CO)io to the pale yellow of the saturated adducts. 

The first part of the underlying double square [46] scheme (10) contains a single electron transfer to the 7i(C=C) bonded tetracyanoethylene (TCNE) ligand in the blue complex with W(CO)5 which leads - in a relatively slow chemical reaction on the cyclovoltammetric time scale - to a yellow isomerized anion radical product in which TCNE coordinates via one nitrile nitrogen lone pair. The high resolution EPR, UVA is and infrared data of the singly reduced complex clearly illustrate the lowered symmetry and diminished n back donation in the secondary (TiO monoanionic form [46]. 

Several synthetic Co(III) and Fe(III) complexes have been generated as models for the active-site metal center in nitrile hydratases [8, 10]. However, few have been examined in terms of water coordination and acidity. Cobalt complexes supported by Ns-type donor ligands, with two carboxamido nitrogen donors, exhibit a pfCa near 7 (Fig. 8.14a and b) [59, 60]. Introduction of two thiolate sulfur donors into the Co(III) coordination sphere increases the pK of the bound water to 8.3 (Fig. 8.14c) [61]. Interestingly, oxidation of one of the sulfur donors to a S-bound sulfmate (Fig. 8.14d) reduces the pK by around 1 unit [62]. As the active-site metal center in nitrile hydratases contain oxidatively modified cysteine residues coordinated to the metal center [8], it has been suggested that the oxidized sulfur donors play a role in modulating the acidity of the metal-bound water molecule. 

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