Mixed multiple bond

The electrostatically favored cation (Li) and anion (RE) arrangement implies the presence of two different E-, Si- and Li sorts, which has been established by solution and solid-state NMR spectroscopy. The electronic structures of the mixed-valent pnictides 10 and 11 have been simply described as electron-deficient clusters with delocalized framework electrons. Formally the latter consist of two low-valent anediyl moieties RE and eight andiides (RE)2- (E = P, As). The relatively large E-E distances of >4 A exclude the occurrence of localized E-E bonds. However, delocalization of the cluster valence electrons is achieved without Li-Li bonds via Li-mediated multiple bonding. Evidence for this has been seen in the NMR spectra (31P, 7Li, 29Si), which are in accordance with the electron delocalization model (see later discussion). 

Other solutions of (4.9a) can be found for mixed numbers of single and multiple bonds. These include the values n = 4, m = 2 for two single and two double bonds, for example, for molybdenyl fluoride (M0F2O2),... 

Infrared spectroscopy is an excellent tool in iminoborane chemistry, which readily permits, to distinguish between iminoboranes and nitrile-borane adducts and to identify monomeric and dimeric forms of iminoboranes. This event is due to the fact that the i>CN of CN multiple bonds absorbs outside the fingerprint region and can be considered to be a valuable group frequency even when mixed with other vibrational modes. In some cases other vibrations like NH, BH, B-halogen or B-S stretching modes are helpful for determining the structure of iminoboranes. 

A pivotal step in the analytical process is sample preparation. Frequently liquid-liquid extractions (LLEs) are used. Solvents, pH, and multiple back extractions are all manipulated to increase selectivity and decrease unwanted contaminants before injection on the GC system. Solid phase extraction (SPE) is more convenient than it used to be because of an increase in commercially available SPE columns. SPE columns are packed with an inert material that binds the drug of interest, allowing impurities to pass through. As with LEE, solvent choices and pH affect retention and recovery. There are three commercially available types of SPE columns, diatomaceous earth (which uses the same principles as LLE), polystyrene-divinylbenzene copolymer, and mixed mode bonded silica (Franke and de Zeeuw, 1998). 

In some cases, alkoxide displacements at a metal center proceed sequentially and at different rates so that, if redistribution processes do not intervene, mixed diketonates can be obtained by adjusting the ratios of reactants.190,191 Recently, bimetallic / -diketonates of the type M2(diketonate)2(OR)4 (M = Mo, W R = alkyl) have been prepared from diketones and M2(OR)6. X-Ray studies show that one diketonate is attached to each metal atom so that unbridged M=M units are retained although rapid rotation about this multiple bond can occur.192 Diketonate complexes with metal-metal multiple bonds are rather uncommon, one other example being C2v CK-Mo2(Me-COCHCOMe), (02 CMe)2.193... 

Pairwise interaction parameters can perhaps be interpreted in chemical bonding terms (76, 80,123, 182). Pairwise terms involving fluorine are often anomalous and this may be related to some surviving multiple bonding in the boron-fluorine bonds in the adducts (20). Pairwise interaction parameters of the Me3N mixed trihalide adduct system are reported to correlate consistently for 41, 13C, 19F, and nB chemical shifts and for nB—19F coupling constants (123). It is perhaps surprising that even the proton shifts can be correlated in this way. 

A number of elegant syntheses of mixed-metal clusters have been accomplished through the condensation of unsaturated metal complexes with metal donors. For example, condensation of the unsaturated cluster H20s3(CO)io with the alkylidyne see Alkylidyne) complex CpW(CO)2(CR) produces H20s3W(/u.3-CR)(C0)i2. Metal-metal, metal-carbon, or carbon-carbon multiple bonds may serve as the sites for metal fragment condensation. Other examples are shown in equations (80 and (9). 

Alternative high-yield preparations of many other derivatives were later developed, on the basis of the cleavage of Re-Re multiple bonded complexes, a reaction normally promoted by r-acceptors such as CO, NO, and CNR. In this way, homoleptic [Re(CNR)6]+ (153) derivatives (R = alkyl, aryl) and ReX(CNAr) are obtained in excellent yields from Re2(02CMe)4Cl2. Mixed derivatives [Re(CNR)4(PR3)2]+ (154) are likewise prepared from Re2Cl4(PR3)4 and are not... 

B. Mixed-Metal Organolithium Compounds Having Metal-Metal Multiple Bonds... 

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