Matrix isolation complexes, table

Whereas UV spectroscopic data are available for base complexes of a variety of matrix-isolated silylenes (Table IV), solution data are reported almost exclusively for complexes of dimethylsilylene (13) (Table V). Again, the complexation of this silylene is accompanied by a significant blue shift of the absorption maximum. Probably due to matrix effects of unknown nature, the Amax values of dimethylsilylene complexes in solution are shifted to wavelengths shorter than those of the corresponding matrix-isolated complexes. 

There is much evidence for silylenes reacting as Lewis bases, but complexes of silylenes acting as a Lewis acid are now well established (Fig. 14.3, Table 14.2). These complexes are also described as silaylides, R2Si —X+. Formation of silylene complexes with Lewis bases is conhrmed by a strong blue shift of the n-p transition. Matrix isolated dimesitylsilylene reacts with carbon monoxide to form the complex shown in Eq. The complex absorbs at 354 nm. 

Here, the different identified coordination modes of C02 with transition and nontransition metals are described, together with trends along the Periodic Table, and theoretical contributions to the understanding of bonding in these systems through three types of study (i) low-temperature matrix isolation spectroscopy of electron-deficient metal/C02 moieties (ii) theoretical studies of reactions of metals with C02 and (iii) the synthesis of stable complexes. 

Matrix isolation spectroscopy has proved an invaluable technique for the isolation and characterization of transition metal—noble gas complexes (see Table III). However, this technique has obvious limitations. Although photoproducts in low-temperature matrices can be made to react with added dopants, it is impossible to accurately predict their reactivity and mechanisms in solution at room temperature. Therefore, in the years following the original discovery of transition metal-noble gas interactions in matrices, new techniques have been used to probe these species in solution, gas phase, and supercritical fluids. 

Microscale (matrix isolated only) metal atom-olefin codepositions yield unstable TT-olefin-metal complexes that can be investigated by matrix isolation spectroscopy. By varying the concentrations of alkene and metal in the inert gas (at 10-50K), mono-, bisand tris-olefin-metal complexes can be observed. Table 1 lists ethylene and tetrafluoro-ethylene complexes prepared for Co, Ni and P(ji-3.io.ii.i2 nd If the... 

Table 1. Ethylene-Metal Atom and -Metal Dimer Complexes Prepared FOR Matrix Isolation Spectroscopy...

Vibrational data on the other compounds in Table XI of the stoichiometry Hg(SR)2, which have not been structurally characterized, is consistent. For linear coordination, symmetric and asymmetric vibrations may be expected in the Raman and IR, respectively, between 250 and 400 cm. If the S-n-Pr species is removed from consideration, the window is narrowed from 330 to 400 cm for the IR and 300 to 370 cm for the Raman. From the correspondence of the remaining data, one must conclude that either the S-n-Pr derivative exhibits important secondary interactions or has been misassigned, more likely the former. In any case, as we will see below, the two-coordinate window clearly distinguishes linear coordination, even in cases with strong secondary bonding interactions, from trigonal or tetrahedral coordination. There exists a need for more solution-phase and gas-phase (or matrix isolation) vibrational studies on Hg(SR)2 to obtain vibrational data when tetrahedral coordination or secondary interactions are clearly absent, and thus definitively establish the vibrational profiles for two-coordinate complexes. 

The first spectroscopic evidence for H2 coordination was obtained in matrix-isolated Cr(CO)5(H2) by Sweany172 (see Chapter 2) virtually at the same time as that for W(CO)3(PR3)2(H2). The investigations of low-temperature stable H2 complexes (Table 3.4) in solid or liquid rare-gas media have continued to constitute a subdiscipline that goes hand in hand with studies of stable complexes as shown in reviews by Sweany (1992) and Poliakoff (1998).174 In most cases the preparations involve photochemical displacement of CO either in a rare-gas matrix or in liquid Xe ... 

The first examples of alkane complexes were discovered by matrix isolation studies of photochemically generated Cr(CO)5 or Co atoms.69 The classic experiments performed by Perutz and Turner showed that CH4 binds to Cr(CO)5 in CH4 matrices and later Billups detected Co(CH4). Because IR shifts resulting from coordination of a sixth ligand are difficult to distinguish from solvent effects, UV/vis spectroscopy best demonstrates CH4 complexation in the Cr species. The absorption frequency (489 cm-1) is similar to that for Xe coordination and much higher than those for H2 and N2 binding (370 and 364 cm-1).70 Along with IR evidence (see also Table 4.3), these spectra demonstrate that CH4 is a much weaker acceptor than H2 or N2. However, the coordination mode of the alkane complex cannot be determined by such spectroscopic studies. 

There is no mention in Table 4.2 of structures, either on surfaces or in complexes, wherein two or more of the same type of hydrocarbon molecule are attached to the same metal atom or ion. Because of the wide occurrence of carbonyl complexes, where the M—CO bond is similar to that in the alkene complexes, this is somewhat surprising, but in view of their greater size it is likely that steric repulsion would inhibit their formation except perhaps at atoms of low CN. Somewhat unstable complexes of the form " E2M (M=Pt, Pd) are known, that with platinum being the more stable, and a number of complexes denoted as " E2M and " E3M (M=Co, Rh, Ni, Pd, Pt and Cu) have been formed by matrix isolation in a solid Group 0 element at very low temperature, as well as " EM species. The infrared spectrum of " EPd closely resembles that of the adsorbed structure 2, but there are additional bands that, because of the metal-surface selection rule, are not visible for the adsorbed species. Moreover the measurement of UV-visible spectra is possible. This fascinating area of -complex chemistry, which has produced several prophetic insights into catalytic mechanisms, has been sadly neglected for many years it merits renewed attention. 

In the IR spectrum of the gas at 60 to 400Torr, the rotational subbands of the three fundamental vibrations and of the overtone 2v2(see p. 150) of HOF and DOF we re observed [7]. The spectrum of matrix-isolated HOF was studied at HOF N2 ratios of -1 3000 and -1 20000. The fundamental vibrations (see p. 149) were observed as well as a small band at 1393 cm definite only in the more concentrated matrix and presumably due to the bending mode of an HOF molecule hydrogen-bonded to HF, a product of thermal decomposition of HOF [8]. The complex spectra in N2 and Ar matrices are also partly due to HF HOF [9]. IR spectra of solid films were studied at -195°C, and the intramolecular vibrations v, V2, V3, 2v2, and 2v3 of HOF, DOF, and H OF were found. Bands at 628 and 448 cm" for HOF were ascribed to a simple cyclic dimer. But infinite planar zigzag chains could not be ruled out [10], and this latter interpretation is supported by the Raman spectrum (see table p. 154). 

Coupled vPtN/vPtO modes give IR and Raman bands at 591 cm for the complex bis(l-ethylimidazol)( —)(L)malatoplatinum(ll). Skeletal mode assignments were made from IR and Raman spectra for [PtiNs) ] (Table 13), [Pt(N3)Cl5] - (Table 14) and trans-[Pt(N3)4(ECN)2] (E = S or Se Table 15). Among the matrix-isolated products of the reaction of Pt atoms with O2 were... 

Laser-ablated M (= Pd, Pt, Ag or Au) atoms and O2 give the neon matrix-isolated species M(ri -OO), M(riLOO)2 (M = Pd, Pt only), Ag(riLOO) and Au(riLoO)2 . vOO assignments for M(riLOO) are summarised in Table 11. The Raman spectra of platinum superoxo complexes gave bands due to platinum-coordinated 02 at 1020 and 1060 cm... 

If it were possible to identify or quantitatively determine any element or compound by simple measurement no matter what its concentration or the complexity of the matrix, separation techniques would be of no value to the analytical chemist. Most procedures fall short of this ideal because of interference with the required measurement by other constituents of the sample. Many techniques for separating and concentrating the species of interest have thus been devised. Such techniques are aimed at exploiting differences in physico-chemical properties between the various components of a mixture. Volatility, solubility, charge, molecular size, shape and polarity are the most useful in this respect. A change of phase, as occurs during distillation, or the formation of a new phase, as in precipitation, can provide a simple means of isolating a desired component. Usually, however, more complex separation procedures are required for multi-component samples. Most depend on the selective transfer of materials between two immiscible phases. The most widely used techniques and the phase systems associated with them are summarized in Table 4.1. 

It appears, then, that the most-often observed reactions of a metal atom with C02 in a low-temperature matrix are the formation of a metal complex, and/or the insertion into one CO bond of C02. These studies, which were conducted at low temperature on naked metal atoms, could not reproduce the reactions obtained with real metal complexes containing ligands, which can in turn influence further reactions with C02 (some examples are provided in Sections 4.3 and 4.4). However, with the assistance of theoretical calculations, the studies have allowed the identification of general trends in the Periodic Table, as well as a description of the different C02 bonding modes through the vibrational analysis of isolated M(C02) moieties. 

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