Isocyanide complexes, homoleptic

Most of the substitution reactions with the homoleptic Tc(I) isocyanide complexes presented in the preceding section had to be performed at elevated temperatures and were often characterized by low yield. The reason for this behaviour is the exceptionally high kinetic and thermodynamic stability of this class of compounds. From this point of view, 4a are not very convenient or flexible starting materials, although they are prepared directly from 3a in quantitative yield. The exceptionally high kinetic and thermodynamic stability is mirrored by the fact that it was not possible to substitute more than two isocyanides under any conditions. On the other hand, oxidation to seven-coordinated Tc(III) complexes occurs very readily. Technetium compounds of this type, which are not expected to be very inert, could open up a wide variety of new compounds, but this particular field has not been investigated very thoroughly. A more convenient pathway to mixed isocyanide complexes that starts with carbonyl complexes of technetium will be described in Sects. 2.3 and 3.2. 

Compounds (RNC)AuI with R = Me, Cy, cBu are not oxidized by iodine. The products are polyiodides with the homoleptic isocyanide complex cations, for example, [(GyNG)2Au]+I5 or [(tBuNC)2Au]+[AuI2] I2. By contrast, complexes (RNC)AuBr react with bromine to give the corresponding (RNC)AuBr3 compounds (R = Me, Cy, u). Compound (tBuNC)AuBr3 has been crystallized and its structure determined (Equation (46)).200... 

Although technically organometallic, the homoleptic isocyanide complexes of Tc [Tc(CNR)6]" ... 

A recent review has highlighted the extensive and interesting chemistry of metal isocyanide complexes.1 Although synthetic procedures are varied, a vast number are based on substitution in metal carbonyl complexes by isocyanides. Such procedures are, however, not always successful. This is especially so in cases where multiple substitution of CO is required, as in the syntheses of homoleptic isocyanide complexes. Many of the inherent difficulties are illustrated by the reaction of iron pentacarbonyl with isocyanides. 

Fully Characterized Homoleptic Metal-Isocyanide Complexes... 

There are no routes yet to homoleptic metal isocyanide anions. If one considers the interesting products obtained from methyl iodide additions to molybdenum (43) and manganese (44) carbonyl isonitrile anions, negatively charged isocyanide complexes should have some interesting chemistry. Also, now that a simple route to [CpFe(CNR)2]2 complexes has been devised (45), the synthesis of the anion [CpFe(CNR)2] could provide a route to a range of products including heterometal-metal bonded systems. 

No homoleptic isocyanide complexes of niobium, tantalum, titanium, zirconium, and halfnium have yet been synthesized. The vanadium cation... 

A number of attempts have been made over the years to develop reproducible synthetic routes to six- and seven-coordinate isocyanide complexes of molybdenum and tungsten. Two of the older methods, namely, the reaction of the hexacarbonyls with halogens in the presence of an isocyanide (775,116) or reactions of the salt Ag4Mo(CN)8 with isocyanides (74), have given six- and seven-coordinate products. Recently, however, the discovery of the reductive or nonreductive cleavage of multiple metal - metal bonds in dinuclear group VIA compounds by isocyanides has provided a facile route to the synthesis of a variety of homoleptic and related isocyanide complexes of these metals in reasonable yields. 

These "one-pot" reactions, using methanol as the solvent, are an especially convenient entry into this class of homoleptic isocyanide complexes, species which had first been discovered by Lippard (U8). They have an interesting structural and reaction chemistry which has proved worth pursuing in its own right(j 7,U8, U9,52.), but these particular features will not concern us here. 

Isocyanides (RNC) are better a donors and poorer xr acceptors than CO, as indicated by the observation that typical homoleptic isonitrile complexes of many metals are in higher oxidation states than the typical carbonyl complexes of the same metal. Some metal isocyanide complexes are given in Table 7-4. It should be pointed out that there are no carbonyl analogues of those compounds in the higher oxidation... 

The interests in homoleptic (see Homoleptic Compound) isocyanide ( isonitrile, see Isocyanide Ligands) complexes of transition metals have largely been associated with their similarities to metal carbonyls. The greater versatility of isocyanide ligands in comparison to CO makes isocyanide complexes potentially valuable reagents in synthetic chemistry and catalysis. The dication [V(CNBu )6] reported in 1980 remained the sole example of a homoleptic group 5 metal isocyanide until 1999, when the first binary... 

Many of the Tc complexes (16, 17, 19) present the 0x0 Tc =0 + core and are pentacoordinate (square pyramid), but other metal oxidation states and coordination numbers are possible, for instance, in (20) (hexacoordinate with the dioxo Tc 02 " core), in (18) (heptacoordinateTc ), in (21) or (23) (hexacoordinate Tc ), or in (22) (hexacoordinate Tc ). In Tc-diphosphonate radiopharmaceuticals, used as bone imaging agents, the metal is beheved to be in the 4-4 oxidation states. The +1 oxidation state in the homoleptic isocyanide complex (22) is expected to be advantageous in terms of the kinetic inertness associated to its low-spin configuration,... 

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). ... 

Routes to homoleptic isocyanide complexes have been defined in thf (Scheme 46).616 [M L7]+ (L = xylyl isocyanide) are fluxional in solution but do not exchange ligand with free L. The cation in [ML7][BF4] is very crowded with the shortest Me- -Me contact being 4.59 A. The TaC7 core has a geometry intermediate between the capped octahedron (Cjv) and capped trigonal prism (C2v). 

Routes to homoleptic isocyanide complexes have been defined in thf (Scheme 46).616 M°L6 (L = xylyl isocyanide) are fluxional in solution but do not exchange ligand with free L. 

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. 

Many isocyanide complexes, both homoleptic and heteroleptic, may be oxidized and reduced. These reactions are often utilized for the preparation of new isocyanide metal compounds, particularly homoleptic and low valent ones ... 

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