Isocyanides stretching frequencies

Full papers have appeared on the formation and reactivity of the compounds ML(CNR)2 (M = Ni, Pd, Pt L = Oz, azobenzene, olefin, diazo-fluorene, acetylene) (231-237) (see also Sections IV,D,2 and V,D). Complexes of the type Ni(olefin)(CNBu )2 have been prepared for a large range of olefins (234, 237). The isocyanide stretching frequencies have been measured and related to the electron-withdrawing properties of the olefin. Other unsaturated molecules such as imines, diazenes, ketones, nitroso compounds, and acetylenes have been similarly studied. The effect of substituent change has been found to be cumulative and an empirical relationship has been developed to predict v(NC) (237). 

The red-brown polynuclear complex Ni4[(CH3)3CNC]7 can be recrystallized from diethyl ether in a Dry Ice-acetone bath to give a microcrystalline material which displays terminal and bridging isocyanide stretching frequencies at 2020 and 1605 cm"1, respectively. This highly air-sensitive material may be used as an intermediate in the preparation of nickel isocyanide complexes of unsaturated molecules simply by the addition of the desired molecules to a hexane or ether suspension. 

Many of the electrochemical studies of metal carbonyl derivatives, and indeed of other species, have sought to correlate redox potentials with other physical or spectroscopic properties. These studies will be described briefly where appropriate but the reader may wish to refer to a review (33) of such correlations in coordination chemistry. A more theoretical basis has also been provided for the often noted relationship between ° and infrared carbonyl, nitrosyl, or isocyanide stretching frequencies (34). 

This simple picture of bonding is convenient to use, and often completely acceptable. However, it does lack sophistication and may not be used to explain some of the subtleties of these systems. One obvious point in this regard concerns infrared spectral data. Coordination of carbon monoxide to a metal invariably leads to a lower carbonyl stretching frequency (vco). implying a lower CO bond order as predicted. However, the values for vcn may be considerably higher for metal complexes of an isocyanide than are the values for the ligand itself. The valence-bond picture cannot rationalize... 

Investigations of the adsorption of isocyanides on metal surfaces use various analytical techniques. Table 13.2 lists the techniques and their acronyms that are used throughout this chapter. Because isocyanides adsorbed on metal surfaces are often characterized by their v(N=C) stretching frequencies, these vibrational data are summarized in Table 13.3. Also given in Table 13.3 are v(N=C) values of the free (unadsorbed) isocyanides and brief descriptions of the proposed adsorption modes, which are often based on interpretations of the v(N=C) values as discussed in the following sections. 

The generally accepted valence bond and molecular orbital (MO) approach to the bonding of metal isocyanides has been well described in Treichel s review (6), and has been used to rationalize (i) variations in IR stretching frequencies between bonded and nonbonded isocyanides, and (ii) the better Tt-acceptor qualities of aryl versus alkyl isocyanide groups (53,54). In valence bond theory the canonical forms involved in metal isocyanide bonds are... 

Isocyanides generally appear to be stronger cr donors than CO, and various complexes such as [Ag(CNR)4]+, [Fe(CNR)6]2+ and [Mn(CNR)6]2 + are known where n bonding is of relatively little importance derivatives of this type are not known for CO. However, the isocyanides are capable of extensive back-acceptance of n electrons from metal atoms in low oxidation states. This is indicated qualitatively by their ability to form compounds such as Cr(CNR)6 and Ni(CNR)4, analogous to the carbonyls and more quantitatively by comparison of CO and CN stretching frequencies. As shown in Table 22-6 the extent to which CN stretching frequencies in... 

Carbon-bonded adducts include cyanide ions and isocyanides. The structure of [Rh2(02C-Me)4(MeNC)2] is reported to confirm the formation of an axial Rh—-C bond. Interest in the CO adduct has been sparked by the question of whether aorn bonding is involved in the Rh—bond. A structural study shows that in [Rh2(02 CMe)4(CO)2] the CO is linearly bonded through the carbon atom, and that the C—O bond is 0.03 A longer than in free carbon monoxide. This is difficult to understand, as several groups have reported that a small (between 39 and 60cm ) decrease in the Vc o stretching frequency occurs upon coordination to the Rh " moiety. The different experimental conditions used in the spectroscopic studies may account for the different C—O stretching frequencies, but it is difficult to rationalize the apparent contradiction in the IR and structural information. 

The IR spectra of all isocyanide complexes [MacML] and MacML2 show the intense stretching frequency of the isocyanide group at about 2080-2150 cm (see Tables 10 and 11). The shift of this absorption by changing from free to metal coordinated ligand is attributed to the or-donor and n-acceptor abilities of the metal ligand bond. The strength of the jt-acceptor bond... 

From the results of classical trajectory calculations intrinsic non-RRKM behavior has been predicted for ethane dissociation, ethyl radical dissociation,and methyl isocyanide isomerization. These predictions are supported by classical trajectory calculations for model H-C-C -> H + C=C dissociation. To generalize, classical trajectory calculations have predicted intrinsic non-RRKM behavior for molecules with isolated high frequency modes [e.g, CH3NC, clusters like Li (H20)j, and van der Waals molecules], molecules like acetylene with linear geometries for which bending and stretching motions are nearly separable, and molecules with tight activated complexes. 

Here one-photon absorption is used to prepare allyl isocyanide with 5, 6, and 7 quanta in the terminal olefinic (H2C=), nonterminal ole-finic (=CH-), or methylene (-CH2 ) CH stretches. Because of their different frequencies each of these CH stretches can be excited selectively with very little spectral impurity. For a particular overtone the methylene CH stretch contains the least energy and the terminal olefinic CH stretch the most. Unimolecular rate constants were measured by Stern-Vollmer plots, and were found not to agree with RRKM predictions. Though the terminal olefinic CH stretch is most excited for a particular overtone, excitation at this site gives a smaller unimolecular rate constant than does excitation at the nonterminal olefinic CH stretch. This result unambiguously characterizes allyl isocyanide isomerization as intrinsically non-RRKM. 

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