Halogen dimer

The addition of HC1, HBr, and HI to allene has been studied in some detail.145 In each case a 2-halopropene is formed, corresponding to protonation at a terminal carbon. The initial product can undergo a second addition, giving rise to 2,2-dihalopropanes. The regiochemistry reflects the donor effect of the halogen. Dimers are also formed, but we have not considered them. 

The dominant supramolecular synthon present in these compounds is the pi-halogen dimer (PHD) interaction [22], This efficient packing motif results from two inversion-related host molecules packing such that one bromine atom from each is situated in the cleft of its V-shaped partner (Figure 3), in addition to aromatic endo, endo-facial interaction. 

The performance of energy-consistent quasirelativistic 7-valence electron PPs for all halogen elements has been investigated in a study of the monohydrides and homonuclear dimers [242]. Special attention was also paid to the accuracy of valence correlation energies obtained with pseudo valence orbitals [97,98]. Some of the results for the halogen dimers is presented in Tables 13... 

Bond lengths R (A), binding energies D. (eV) and vibrational constants a>e (cm ) of the homonuclear halogen dimers from dl-electron (AE) Douglas-Kroll-HeB (DKH) and valence-only energy-consistent pseudopotential (EC-PP) Hartree-Fock self-consistent field (SCF) calculations. The effects of static and dynamic core-polarization at the valence-only level are modelled by a core-polarization potential (CPP). 

Monomeric building units of structure type D in this series (Fig. 3) are known only for ruthenium (complex 88 in Fig. 21). Method c [82b, 90] is particularly well suited for the preparation of such complexes, whereas method d [83] is limited to the few cyclophanes that can be reduced via Birch s method (Fig. 22). The halogen dimers [( -Cp )MX2]2 are known (X = Cl, I M = Co-... 

In the case of the complexes Et3P-TeX2, X = halogen, dimeric units units are formed which are held together by weak Te-X bonds (9.688a). Associated units of this kind are not uncommon, and occur with both Se and Te derivatives, for example, Ph3PSe(Ph)Br (9.688b) [11,35-42]. 

Table 6 The orientation of halogen dimers predicted from the radial density topology of the monomer...

All the other aluminium halides are covalently bonded with aluminium showing a coordination number of four towards these larger halogen atoms. The four halogen atoms arrange themselves approximately tetrahedrally around the aluminium and dimeric molecules are produced with the configuration given below ... 

Liquid Halogen Fluorides as Reaction Media. Bromine trifluoride and iodine pentafluoride are highly dimerized and behave as ionizing... 

Simple olefins do not usually add well to ketenes except to ketoketenes and halogenated ketenes. Mild Lewis acids as well as bases often increase the rate of the cyclo addition. The cycloaddition of ketenes to acetylenes yields cyclobutenones. The cycloaddition of ketenes to aldehydes and ketones yields oxetanones. The reaction can also be base-cataly2ed if the reactant contains electron-poor carbonyl bonds. Optically active bases lead to chiral lactones (41—43). The dimerization of the ketene itself is the main competing reaction. This process precludes the parent compound ketene from many [2 + 2] cyclo additions. Intramolecular cycloaddition reactions of ketenes are known and have been reviewed (7). 

Benz[e][ 1,2]azaborine, 1,2-dihydro-2-methyl-halogenation, 1, 656 Benz[6]- 1-azafulvenes dimerization, 4, 309 Benz[c]-2-azafulvenes dimerization, 4, 309 Benzazatropolone X-ray analysis, 7, 494 1 -Benzazepine, N-acetyl-6-hydroxy-bromination, 7, 528... 

Attempts to prepare the mono(cyclopenta-dienyl) derivatives are sometimes frustrated by a Schlenk-type equilibrium (see p. 132), but judicious choice of ligands, solvent etc. occasionally permits the isolation of such compounds, e.g. the centrosymmetric halogen-bridged dimer (t - -C5Me5)Ca(/i-l)(lhf)2 2] which cry.stallizes from toluene solution. The complex is isostruc-tural with the dimeric organosamarium(ll) analogue. - ... 

InPhj mp 208°, TlPhj mp 170°. For Ga and In compounds the primary M-C bonds can be cleaved by HX, X2 or MX3 to give reactive halogen-bridged dimers (R2MX)2. This contrasts with the unreactive ionic compounds of T1 mentioned above, which can be prepared by suitable Grignard reactions ... 

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