Cyano and Nitrile Complexes

The vibrational spectra of cyano complexes have been studied extensively and these investigations are reviewed by Sharp, Griffith, Rigo and Turco, and Jones and Swanson,  

According to El-Sayed and Sheline, the p(CN) of cyano complexes are governed by (1) the electronegativity, (2) the oxidation state, and (3) the coordination nu.nber of the metal. The effect of electronegativity is seen in the order  

Since the electronegativity of Nil ll) is smallest, the cr-donation will be the least, and the i (CN) is expected to be the lowest. The effect of oxidation state is seen in the frequency order.  

The higher the oxidation state, the stronger the cr-bonding, and the higher the p(CN). The effect of coordination number is evident in the frequency order  

Here an increase in the coordination number results in a decrease in the positive charge on the metal, which, in turn, weakens the cr-bonding, thus decreasing the p(CN). 

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Metal-Cyano and Nitrile Complexes Ammine, Amido, Urea and Related Complexes Metal Carbonyl Compounds Metal-Acetylacetonato Compounds, Carboxylate Complexes and Complexes Involving the Carbonyl Group... 

Ruthenium(II) Treatment of [Ru(NH3)5(OH2)]2+ or [Ru(NH3)5(acetone)]2+ with L or [RuCl(NH3)5]2+ with zinc amalgam in the presence of L yields [RuL(NH3)5]2+ (L = acetonitrile, benzonitrile,358 substituted benzonitrile,196 358 359 acrylonitrile,360 hydrogen cyanide,36,37 ethyl cyano-formate,361 dicyanamide, malononitrile, substituted malononitrile, tricyanomethanide,362 4-cyano-l-methylpyridinium196). Reaction of a hundred-fold excess of RCHO (R = Ph, Me) with [Ru(NH3)6]2+ under alkaline conditions yields [Ru(NH3)sNCR]2+.363-365 The likely mechanism of this reaction is given in Scheme 12. An alternative route to nitrile complexes is by reaction of [Ru(NH3)sOH2]2+ with aldoximes, e.g. RMeC=NOH, to afford [Ru(NH3 )5 (NCMe)]2 + and... 

It should be stressed that in case of the ethynyl-acetylene reaction, a molecular hydrogen loss channel synthesizing the 1,3-butadienyl radical is open as well. Since the reactions of cyano and ethynyl radicals have no entrance barrier, are exoergic, and aU transition states involved are lower than the energy of the separated reactants, these reaction classes are extremely important to form nitriles and complex unsaturated hydrocarbons in low-temperature environments. On the other hand, the corresponding phenyl radical reactions are—due to the presence of an entrance barrier—closed in those environments. However, the elevated temperature in combustion systems helps to overcome these barriers, thus making phenyl radical reactions important pathways to form aromatic molecules in combustion flames. 

Other Nitrile Complexes. While similar additions of iron (i) and rhodium (11) hydrides to acrylonitrile to form 1-cyanoethylmetal complexes have been reported, 2-cyanoethylmetal complexes also form in certain cases (43, 51). Organotin hydrides may add to acrylonitrile in either direction, depending on the conditions of the reaction (25). Formation of the 2-cyanoalkyltin adduct apparently involves a radical mechanism, whereas a polar mechanism is operative in forming the 1-cyano-alkyl adduct. A four-center transition state was not considered probable in the latter case. 

The crystal and molecular structure79 of a carbazole/tetracyano ethylene 1 2 complex reveals the presence of a hydrogen bond between the amino group of the carbazole and a nitrile nitrogen of tetracyano ethylene in the solid state, which resembles the stronger hydrogen bond observed between amines and the cyano group. 

Metal cyanides are readily oxidised and those of some heavy metals show thermal instability. The covalent cyano group is endothermic, and hydrogen cyanide and many organic nitriles are unusually reactive under appropriate circumstances, and A-cyano derivatives are reactive or unstable. The class includes the groups 3-CYANOTRIAZENES, METAL CYANIDES (AND CYANO COMPLEXES) DIISOCYANIDE LIGANDS... 

Reactions between 5-cyanotetrazole and transition metals, when performed in boiling acetone, lead to hydrolysis of the cyano group and formation of 5-carbamyl tetrazolate complexes (68). Complexes containing 1- or 5-substituted tetrazolate anions can also be obtained by 1,3-dipolar cycloaddition of organic isonitriles (RNC) (15) or nitriles (RCN) (61), respectively, to coordinated azide ligands [Eqs. (3) and (4)]. 

In recent years there has been considerable interest in the reactions of nitriles in the coordination sphere of metal ions. Breslow et al.312 first reported that the hydrolysis of 2-cyano-l,10-phenanthro-line to the corresponding carboxamide is strongly promoted by metal ions such as copper(II), nickel(II) and zinc(II). Base hydrolysis of the 1 1 nickel complex is 107 times faster than that of the uncomplexed substrate. The entire rate acceleration arises from a more positive value of AS. Somewhat similar effects have been observed for base hydrolysis of 2-cyanopyridine to the corresponding carboxamide. In this case rate accelerations of 109 occurred with the nickel(II) complex.313... 

Like / -keto esters, /3-keto nitriles react smoothly with hydrazine the latter give aminopyrazoles.78,310-321 Under the same conditions cyanoacetic ester and its derivatives give aminopyrazolones.322-325 The reaction of malononitrile is more complex, giving 1-substituted 3-cyanomethyl-4-cyano-5-aminopyrazole and products formed by further condensation.328... 

Two sorts of metal—cyano complexes have been studied. The first contains an atom of zinc, cadmium, mercury or copper as the central atom, and the nitrogen quadrupole coupling is only a little higher than it is in nitriles (4.0—4.2 MHz against 3.8—4.0 MHz). In the second sort, the central atom is either platinum or cobalt and the observed couplings are markedly lower (3.47—3.68 MHz). 

The selective synthesis of the 2-allyltetrazoles 55 by the three-component coupling reaction of the cyano compounds 54, allyl methyl carbonate 5b, and trimethylsilyl azide 42 was accomplished in the presence of Pd2(dba)3.CHCl3 and P(2-furyl)3 (Scheme 19) [55,56]. Most probably, the formation of (r)3-allyl)( ]5-tetrazoyl)-palladium complex 56 took place through [3 + 2] dipolar cycloaddition of 7r-allylpalladium azide 44 with the nitrile 54. The complex 56 thus formed would undergo reductive elimination to form the products 55. 

Carbonylation of [CoH(CN)5]3- in strongly alkaline media results in the formation of [Co(CN)2-(CO)2]2- and the intermediacy of the short-lived tetracyanocobaltate(I) ion [Co(CN)4]3" in this process has been inferred from mechanistic studies (Scheme 9).15 Under very similar conditions [CoH(CN)s]3- catalyzes the cyanation of vinyl halides to form a,j3-unsaturated nitriles with retention of configuration (equation 6). Oxidative addition of vinyl halide to the intermediate [Co(CN)4]3- ion is stereoselective and in the a complex so produced reductive coupling of the vinyl and cyano ligands gives the stereoretentive organic product and regenerates [Co(CN)4]3-.78... 

Nitriles are excellent donors and the bonding is usually tf with the M <— N=CR moiety linear or nearly so. Bisnitriles can act as chelates cyano complexes such as bipy2 Ru(CN)2 can also act as ligands though the result is merely a cyanide bridge, M-CN-M or M-CN-M (Section 7-11). 

Cyano-derivatives can be readily obtained by a ruthenium-catalyzed addition of various hydrazines to terminal alkynes [89] in which the cyano carbon atom arises from the terminal alkyne carbon atom. The tris(pyrazolyl)borate (Tp) complex RuCl(Tp)(PPh3)2 (1 mol%) was found to be the most active catalyst, and N,N-dimethylhydrazine (5 equiv.) the best nitrogen source. The proposed mechanism involves the nucleophilic attack of the nitrogen nucleophile on the a-carbon of a vinylidene intermediate (Scheme 8.27). Proton migration in the resulting a-hydrazi-nocarbene, followed by deamination, would give the nitrile derivative and regenerate the catalytic species. 

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