Isocyanide complexes bonding

The subjects of structure and bonding in metal isocyanide complexes have been discussed before 90, 156) and will not be treated extensively here. A brief discussion of this subject is presented in Section II of course, special emphasis is given to the more recent information which has appeared. Several areas of current study in the field of transition metal-isocyanide complexes have become particularly important and are discussed in this review in Section III. These include the additions of protonic compounds to coordinated isocyanides, probably the subject most actively being studied at this time insertion reactions into metal-carbon bonded species nucleophilic reactions with metal isocyanide complexes and the metal-catalyzed a-addition reactions. Concurrent with these new developments, there has been a general expansion of descriptive chemistry of isocyanide-metal complexes, and further study of the physical properties of selected species. These developments are summarized in Section IV. 

A number of studies have been reported concerning azide-isocyanide condensations to give tetrazoles. Early work by Beck and co-workers 18, 19) describes the addition of various isocyanides to metal azido species [Au(N3)4]", [Au(N3)2]", Au(PPh3)N3, and M(PPh3)2(N3)2, M = Pd, Pt, Hg. The products are carbon-bonded tetrazolato-metal complexes. It is not known whether metal isocyanide complexes are intermediates in these reactions. More recently inverse reactions with azide ion addition to metal isocyanide complexes were carried out, with similar results. From... 

The hexakis(methyl isocyanide) dimers, [Pt2(CNMe)6], undergo photolytic cleavage of the Pt—Pt bond to give 15-electron radicals, Pt(CNMe)3.94 Mixtures of platinum and palladium dimers give rise to heteronuclear complexes under photolytic conditions. Mixtures of normal and deuterium-labeled methyl isocyanide complexes reveal that the metal-ligand bonds undergo thermal redistribution.94... 

Benzyne, which is generated in situ from 2-(trimethylsilyl)phenyl triflate and KF, acts as an alkyne congener in distannation in the presence of palladium-/ r/-alkyl isocyanide complex.157 A variety of substituted benzyne derivatives inserts into the Sn-Sn bond to give l,2-bis(stannyl)benzenes (Equation (59)). The reaction fails to occur in the presence of other palladium catalysts such as Pd(PPh3)4. 

Silaboration of 3-substituted 1,2-dienes takes place smoothly at the internal double bond in the presence of the catalytic Pd(acac)2-2,6-xylyl isocyanide complex and the boryl group is regioselectively introduced to the central carbon atom of an allene (Scheme 16.55) [59, 60]. The same regioselectivity is observed with the catalytic system Pd2(dba)3-P(OCH2)3CEt [59]. 

CNBu )3. Even though the isocyanides on the metal and in the complex are bonded to the same metal in the same zero oxidation state, their v(N=C) values differ by approximately 75cm . Thus, even in a case where the metal complex might be considered to be a quite good model for T -bonded isocyanide on Pt metal, the v(N=C) values are significantly different Of course, the environment around the Pt to which the isocyanide is bonded is very different in the complex and on the surface. 

Irradiation of [Re(CNC6Fl3-2,6-Me2)6] with UV light in CH2CI2 in the presence of halide ions yields complexes of the type [Re(CNCgH3-2,6-Me2)5X] (X = Cl, Br, or I). The quantum yield of this reaction is high and results in chemical yields between 30% and 40%. Products of the same composition have been obtained directly from the cleavage of the metal-metal bond in quadruply bonded rhenium complexes of various compositions in the presence of isocyanides. ... 

No isocyanide complex of Nbv or Tav appears to have been reported. The insertion of isocyanides into M—C bonds of [TaCl2Me3p40 and into the M—X bonds of MXS 345-347 has however been described. [MX4 CX(NR) (RNC)] was isolated (R = Me, Bu ). In the case of R = Me,345 further insertion occurred, yielding [MCl3 CCl(NMe) ]2-(MeNC). 

Only more recent investigations of organic isocyanide complexes of chromium are in this section, because general synthetic methods for metal isocyanide complexes, including mixed carbonyl isocyanides, and the nature of the metal-isocyanide bond are discussed in Sections... 

Comparison between the half-wave potentials (equations 2 to 4) of [Cr(CNR)6](PF6)2, e.g. for R = Bu , -1.04, -0.28 and 0.84 V (versus SCE),22 with those for [Cr(CNPh)6](PF6)2, i.e. -0.35, 0.25 and 1.00 V,20 shows that alkyl and aryl isocyanides favour respectively the higher and the lower oxidation states as expected from the greater a-donor and weaker jr-acceptor capabilities of the alkyl over the aryl isocyanides. Similarly, the phosphines in the mixed ligand complexes (Table 3), 23 relative to isocyanide ligands, stabilize the Cr111 oxidation state. The great difference in the relative stabilities of Cr—C bonds in the cyano and phenyl isocyanide complexes is indicated by the magnitude of the shift (ca. 2.0 V) between the Cr(CN) "/Cr(CN)r (-1.130 V) and the Cr(CNPh)i+/Cr(CNPh)i+ reduction potentials.28... 

Simple oxidation/reduction has also been studied for a wide range of isocyanide complexes. Again those complexes containing mainly molybdenum-carbon bonds have been reviewed in... 

Mo1 isocyanide complexes of the form [MoOX(CNR)4]Y (R = alkyl X=C1, Br Y = C1, Br, Brl2,13, PF6) have been prepared by several synthetic routes.125 These complexes contain a trans O—Mo—X arrangement, and in [MoOCl(CNMe)4](I3) the Mo—O, distance is 1.64(4),126 a length typical of the multiple terminal oxomolybdenum(IV) bond (Table 3). 

The t-butyl isocyanide complexes of nickel(O) are used as starting materials for the synthesis of a number of complexes with unsaturated groups n bonded to the nickel atom in reactions similar to those given by dioxygen or diazo complexes (Sections 50.2.7.2 and 50.2.6.3). 

Platinum(I) isocyanide complexes are formed when Na2PtCl4 reacts with methyl isocyanide, and the product isolated by addition of AgPF6 (equation 116). The structure has two square planar platinums linked by a Pt—Pt bond, with the two Pt(CNMe)3 units perpendicular. The NMR spectrum shows that intramolecular rearrangements occur involving the tetrahedral deformation of one Pt center followed by rotation about the Pt—Pt bond.353 354... 

Nuclear magnetic resonance (NMR) spectroscopic data have been reported by Takeda et al.32 for silylene-isocyanide complexes 28-30, which are stable at room temperature due to the bulky substituents. The Aic coupling constants of 38.6 Hz (28), 22.1 Hz (29), and 1.0 Hz (30) are considerably smaller than that of a Si-C single bond and indicate a weak Si-C interaction as expected for a dative bond. 

Interest in mercury(II) tetrazolates, notably Hg(5-02N-tta)2, as detonators (12, 190) has prompted thermal decomposition studies on mercury salts of 5-nitrotetrazole (34) and 5,5 -azotetrazole (169). Cycloaddition of methyl isocyanide to Hg(N3)2(PPh3)2 under mild conditions yields the C-bonded tetrazolate complex Hg(l-Me-tta)2 (15). Mercury(II) adducts HgX2(l,5-pmtta) (X = Cl, Br, or CN) and Hg(N03)2(l,5-pmtta)2 have been described (174, 236). 

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. 

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