Nitrosyls homoleptic

The low-valent ferrate [Fe(CO)3(NO)] 76 or Hieber anion was discovered some 50 years ago by Hieber and Beutner [43, 44] in order to extend the Hieber base reaction [45,46], in which iron pentacarbonyl 78 reacts with alkaline bases to form the [Fe(CO)4] anion [47, 48]. Compared to its homoleptic analogue, the Hieber anion is more stable because the electron-withdrawing character of the nitrosyl ligand stabilizes the negative charge at the iron atom. 

It may be noted that Cr(NO)4 is the only known homoleptic metal nitrosyl complex. It is isoelectronic with Ni(CO)4 and similarly has a tetrahedral structure, with a very short Cr—N distance (1.763 A). Claims for Co(NO)3, Fe(NO)4, and Ru(NO)4 have never been confirmed. 

Neutral and cationic homoleptic (see Homoleptic Compound) tnngsten-isocyanide complexes W(CNR)6 and [W(CNR)y] + (R = alkyl, aryl) are known, and W(CO)e (CNR) (n=l-3) complexes may be prepared from W(CO)6 and excess isocyanide in the presence of catalytic amounts of C0CI2 or PdO. Iso-cyanides are isoelecfronic with CO and also insert into the W-R bonds of alkyl complexes (see Alkyl Complexes). For instance, the alkyl-nitrosyl complexes Cp W(NO)(X)(CH2Bfr) (X = NHBfr, OBfr) react with CNBu to afford -iminoacyl complexes, and the isocyanide complexes, Cp W(CO)2(Me)(CNR) (10, R = alkyl), rearrange to afford either -iminoacyl Cp W(CO)2( -MeCNR)(ll) or ij -l-azaallyl Cp W(CO)2( -CH2CHNR) (12) derivatives, depending upon the reaction conditions (equation 4). ... 

The catalytic implications of the alternative coordination modes of nitric oxide in transition metal complexes were first noted by Collman (12). He argued that the linear bent transformation, concomitant with a change in the formal oxidation state of nitrogen from (III) to (I), results in the withdrawal of electron density from the metal center and facilitates the coordination of another ligand into a vacant site. Thus, the mixed carbonyl nitrosyl complex [Co(CO)3(NO)] undergoes thermal CO substitution by an associative mechanism, whereas the iso-electronic, homoleptic carbonyl [Ni(CO)4] reacts by a dissociative pathway (13). 

Routes to homoleptic isocyanide complexes have been defined in thf (Scheme 46). Ta L6 (L = xylyl isocyanide) was reduced by ACg (A = K, Cs) to [Ta 1L6]. This anion appears to be octahedral. Its 13C NMR resonance occured at 210 ppm, the most downfield shift observed for a diamagnetic homoleptic isocyanide complex. Reaction with the NO source N-methyl-N-nitroso-/ -to 1 uenes u 1 lbnam ide provided Ta I(NO)L5 whose essentially linear NbNO fragment is consistent with the formal presence of ligand NO+. Direct nitrosylation of [M (CO)6] with 2 eq [NO][PF6] in the presence of L in CH2C12 at — 60 °C provided r/,v-[M (NO)2L4]1,662 The reaction represents a rare example of non-oxidative displacement of all CO ligands from a homoleptic carbonylme-talate. 

Lin ZS, Chiou TW et al (2012) A dinitrosylirrai complex within the homoleptic Fe(NO)4 anion NO as nitroxyl and nitrosyl ligands within a single structure. Inorg Chem 51 10092-10094... 

Many methods have been used to prepare NO complexes. The reactions of transition metal complexes with gaseous NO or solutions of the dissolved gas have been used to form NO complexes. One example of such a reaction to form, a homoleptic nitrosyl complex is shown in Equation 4.30. Alternatively, the reactions of metal complexes with nitrosonium salts form metal-nitrosyl complexes. (Note Alcohol solvents should be used with caution, because they convert NO " to alkyl nitrites. In addition, tetrahydrofuran should not be used as solvent because it can be polymerized by NO". ) Two examples of such processes are shown in Equations 4.31 and 4.32. A range of organic nitroso compounds release "NO" under appropriate conditions and have been used to introduce NO into transition metal complexes. The generation of a metal-nitrosyl complex from F3CNO is shown in Equation 4.33. ° Inorganic nitrites have also been used to prepare NO complexes, ° as shown in Equation 4.34. ... 

In one case, [Fe(/m-NO)2(5enUNO)2] , a homoleptic nitrosyl contains both linear and bent NO ligands. NS and NSe have also been recently identified as new ligands. ... 

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