Lewis bases, complexes electronic spectra

Very recently, various DHB complexes were analyzed [39].12 The complexes of ammonia and hydronium ions were included in this analysis, in addition to the complexes with acetylene and methane, and their derivatives. Generally, in such complexes, lithium hydride and berylium hydride (and its fluorine derivative) act as the Lewis bases (proton acceptors) while hydronium ion, ammonia ion, methane, acetylene, and their simple derivatives act as the proton donors. Therefore, it was possible to investigate the wide spectrum of DHB interactions, starting from those that possess the covalent character and extending to the systems that are difficult to classify as DHBs (since they rather possess the characteristics of the van der Waals interactions). Figure 12.8 displays the relationship between H—H distance and the electron density at H—H BCP.13 One can observe the H—H distances close to 1 A, (as for the covalent bond lengths) and also the distances of about 2.2—2.5 A, typical for the van der Waals contacts. This also holds for the pc-values - of the order of 0.1 a.u. as for the covalent bonds and much smaller values as for the HBs and weaker interactions. 

Diiodine is chosen as the reference Lewis acid. The standard conditions are T = 25 °C and an alkane (e.g. n-heptane or cyclohexane) is the solvent. Diiodine provides a remarkable opportunity to study halogen-bonded complexes from three regions of the electronic spectrum in which the absorption is directly related either to the concentration of the complex (the charge-transfer band, often around 240-350 nm, and the blue-shifted band, around 400-510 nm) or to the concentration of free diiodine (the visible band at 520 nm in n-heptane). In alkanes, diiodine interacts with Lewis bases B to form stable molecular... 

Redox processes are fairly common in the formation of Z —CO— complexes of transition metals, and an example is given in Eq. (9). In this reaction, titanium is oxidized from the + 2 to the +3 state, thus becoming a better Lewis acid, and the molybdenum dimer is reductively cleaved, thus developing Z —CO— donor character (59). A characteristic low-frequency Z —CO— band is observed in the IR spectrum, and a crystal structure is available. A proposed mechanism for the redox process, based on CO mediated electron transfer, is discussed in Section IV,C. 

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