Nitriles coordination numbers

Divalent and tetravalent Pt probably form as many complexes as any other metal. The platinum(II) complexes are numerous with IV. S, halogens, and C. The letranitritoplatinum complexes are soluble in basic solution. Tetranitntoplatinum(II) ion is formed when a solution of plat-inum(II) chloride is boiled, at about neutral pH, with an excess of NaNO f. The ammonium salt may explode when heated. Generally, platinum-metal nitrites should be destroyed in solution. They never should be heated in the dry form. Pladnum(II) complexes most often have a coordination number of 4. Many compounds have been prepared with olefins, cyanides, nitriles, halides, isonitnles, amines, phosphines, arsines, and nitro compounds. 

Another example is found in the various platinum (II) ammines which contain acetonitrile. One such is formed by reaction of [Pt(CH3CN)2Cl2] with ammonia. This gives a product with a formal composition corresponding to [Pt(CH3CN)2(Cl)2(NH3)2]. For many years this was considered to be an unusual complex in which platinum (II) was 6-coordinated. There are very few instances (if any) in which platinum (II) exhibits a coordination number other than 4. It has been shown by x-ray studies (67) that the product of this reaction is [Pt(acetamidine)2Cl2] and that the reaction may be written as shown at the top of the next page. Here, the coordination act accentuates the electron drift from the nitrile carbon atom and makes it more susceptible to attack by nucleophiles—in this case, ammonia. This kind of process had been essentially substantiated by chemical studies carried out previously, and the formation of... 

Nitriles. Acetonitrile adducts have been reported for several uranyl salts, including the chloride and nitrate. Although lower metal coordination numbers (e.g., five) have been suggested in the formulation of some of these compounds, it is likely that the species contain six- or seven-coordinate metal ions. As illustration of this, the molecular structure of U02Cl2(MeCN)2(H20) " possesses a seven-coordinate uranium center, with mutually traro-acetonitrile and chloride ligands. 

Well-defined 1 1 and 1 2 adducts with triphenylphosphine are formed with ease 42,45, 46). The yellow 1 1 adduct is more stable than bis(acrylo-nitrile)-nickel itself (dec. p. 185° C) and represents one of the first examples of a nickel complex with the coordination number three. The 2 1 adduct is monomeric in benzene and has a dipole moment of 6 Debye units (46). Unstable bis(pyridine) adducts were also isolated, but were found to decompose at room temperature, depositing metallic nickel (46). 

The large radius of lanthanide(lll) and the specific coordination number also attracted us, and we started to investigate the use of lanthanide triflates as Lewis-acid catalysts for the reaction of amines with nitriles under anhydrous conditions [7a]. After starting this study, we found that scandium triflate (ScfOTfls) and yttrium triflate (YfOTfla) are also excellent water-tolerant Lewis acids, and we have developed new synthetic reactions using these rare-earth triflates as catalysts. 

Cheng and Foxman 2 studied the polymerization of the square-planar nickel(II) complex, [NiBr2 P(C2H4CN)3)2]. This compound has pendant nitrile groups and the nickel ion has the potential to increase its coordination number by two. Accordingly, a thermal polymerization occurs in the solid state with formation of a blue polymeric product. Scheme 12.8. 

Only recently, nitriles were shown to increase the silicon coordination number to up to five in cationic complexes (18) [95], even though nitriles had already been shown to form adducts with sihcenium ions, thus functioning as a donor moiety in a tetrahedral Si coordination sphere [96]. Amines are scarcely encountered in silicon coordination compounds, only few examples of crystallographically evidenced silicon complexes with monodentate amines have been reported so far, which include the adduct SiF4(NH3)2 [97-100], Imines, however, are well known to add to various halosilanes. Especially IV-heterocycles with imine functionahty, such as pyridines [101-103], imidazoles [104, 105], pyrazoles [106], and related compounds, can be found as ligands in various silicon complexes (e.g., in 19 and 20). 

These rod-shaped ligands share a sterically efficient terminal N-donor and their divalent Co chemistry is well established. They will be discussed here only with selected examples. [Co (NCMe)6](TFPB)2 (TFPB- = tetrakis(3,5-bis(trifhioromethyl)phenyl)borate)) has been synthesized and characterized in the solid state along with a number of other divalent transition metal analogs.357 As a result of the extremely poor coordinating ability of the anion and facile loss of MeCN ligands from the cation, the salt is an excellent source of naked Co2+ ions. Thermolysis up to 100 °C leads to the loss of one MeCN and formation of a r -bound nitrile, whereas above 130 °C decomposition occurs with loss of MeCN and abstraction of fluoride from the anion to form CoF2. 

Non-corrin cobalt has a number of interesting applications in the chemical industry, for example in the hydroformylation (OXO) reaction between CO, H2 and olefins. A number of non-corrin Co-containing enzymes have been described, including methionine aminopep-tidase, prolidase, nitrile hydratase and glucose isomerase. We describe the best characterized of these, namely the E. coli methionine aminopeptidase, a ubiquitous enzyme, which cleaves N-terminal methionine from newly translated polypeptide chains. The active site of the enzyme (Figure 15.13) contains two Co(II) ions that are coordinated by the side-chain atoms of five amino acid residues. The distance between the two Co2+ is similar to that between the two Zn2+ atoms in leucine aminopeptidase, and indeed the catalytic mechanism of methionine aminopeptidase shares many features with other metalloproteases, in particular leucine aminopeptidases. 

A number of nucleophiles including CN , BH4, amines and alcohols react with coordinated nitriles at lO" enhanced rates compared with the free ligand. One of the most novel examples involves as a nucleophile. Acetonitrile and sodium azide form 5-methyltetrazole only after 25 hours at 150°C whereas the cobalt(III) coordinated acetonitrile reacts in 2 hours at room temperature ... 

The cobalt catalyzed cocyclization of alkynes with heterofunctional substrates is not limited to nitriles. cpCo-core complexes are capable of co-oligomerizing alkynes with a number of C,C, C,N or C,0 double bonds in a Diels-Alder-type reaction. Chen, in our laboratories, has observed that these cycloadditions are best performed with the help of stabilizers such as ketones or acetic esters that are weakly coordinated to the cobalt and prevent the alkynes from being cyclotrimerized at the metal center... 

The hydrido complex [RhH(PPr 3)3] and [Rh2(N2) P(C6H1,)3 4] have been shown to hydrogenate nitriles to primary amines at normal temperature and pressure, with turnover numbers of up to 100 being attained.124 This is a most interesting observation in view of the coordinating ability of primary amines, which might have been expected to saturate the coordination sphere of the rhodium, putting an end to catalysis. 

A number of studies have also been made of the hydrolysis of nitriles in the coordination sphere of cobalt(III). Pinnell et al.3 4 found that benzonitrile and 3- and 4-cyanophenol coordinated to pentaamminecobalt(III) are hydrolyzed in basic solution to the corresponding N-bonded carboxamide (equation 22). The reaction is first order in hydroxide ion and first order in the complex with koH= 18.8M 1s 1 at 25.6 °C for the benzonitrile derivative. As fc0H for the base hydrolysis of benzonitrile is 8.2 x 10-6 M-1 s at 25.6 °C, the rate acceleration is ca. 2.3 x 106-fold. The product of hydrolysis is converted to [(NH3)5CoNH2COPh]3+ in acidic solution and the pJC of the protonated complex is 1.65 at 25 °C. Similar effects have been observed with aliphatic nitriles.315 Thus, base hydrolysis of acetonitrile to acetamide is promoted by a factor of 2 x 106 on coordination to [Co(NH3)5]3+. 

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