Other analogous fragments

An octahedral MLg complex, such as [Cr(CO)g], is not the only initial structure that can be used to construct inorganic fragments which are analogous to the organic fragments CH3, CH2, and CH (5-lla andb). Two other structures are often used as starting points. 

The cyclopentadienyl ligand (Cp) is very widespread in organometal-lic chemistry. When it is coordinated in the ti mode, the most common situation, it behaves as an L2X ligand (Chapter 1, 1.1.1.3). [CpMLj] complexes (L = CO, PR3,. ..) are therefore pseudo-octahedral complexes of the ML5X type, which have 18 electrons when their electronic configuration is 4 . A 4 [CpMLj] complex may therefore be used as a new initial structure, very similar to that used previously. Since the presence of the cyclopentadienyl ligand oxidizes the metal by one unit, a metal with seven valence electrons (M = Mn, Tc, Re) must be used to obtain a neutral complex. The complex [CpMn(CO)3] is therefore a 

Since the CH3 and 4 ML5 fragments have analogous electronic structures, their chemical behaviour is similar in some respects. For example. 

Another example involves the dimerization of the CH2 and butterfly d -ML4 fragments. The dimer of CH2 is just simply ethylene, a planar molecule characterized by a double bond between the two carbon atoms (a a bond and a tt bond). The isolobal analogy linking the CH2 and butterfly [Fe(CO)4] fragments (5-9) shows us that the 

The similarity in the electronic structures of the two molecules is confirmed by the interaction diagrams shown in 5-l4a for C2H4 and 

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An analogous fragmentation process occurs m the mass spectra of sulfides As with other sulfur containing compounds the presence of sulfur can be inferred by a peak at m/z of M-l-2... 

The mass spectrum of selenophene has been recorded and the fragmentation pattern compared with those of the other congener heteroaromatic compounds.27 The analogous fragments observed for the four heterocycles are given in Scheme 1. 

Analogous fragmentations to thiopyrylium ions constitute a typical model for various 2//-thiopyran derivatives155,304,345,464 along with other ions.155,299,304,345,465 A chromium complex was also measured.4542... 

Similarly to carbonyl compounds (Section 6.3.1), thiocarbonyl compounds abstract hydrogen upon irradiation however, both n,7t and n,n excited states are reactive and the hydrogen atom can be added to either the sulfur (Table 6.17, entry 1) or carbon (entry 2) atoms of the C=S bond. Aliphatic and aromatic thiocarbonyl compounds can also undergo photocycloaddition to unsaturated compounds from both singlet or triplet excited states to form thietanes (analogously to the Paterno Biichi reaction see Section 6.3.2) (entry 3) or 1,4-dithianes. On the other hand, fragmentation of the S C bond is a typical primary process observed in excited sulfones and sulfonates (entry 4), followed by efficient SO2 extrusion from the radical intermediate. 

These reactions are analogous to the McLafferty elimination of acetic acid. Other secondary fragmentations can be explained by assuming that primary fragments containing four or more carbon atoms from the original carbon-chain are degraded hy way of cyclic forms. One example is the aforementioned elimination of acetic acid another is the... 

The fragmentation chemistry of a ions appears to be more complex than for b ions. In fact, a number of other competitive fragmentation pathways have been proposed [32], which are not shown in Scheme 3, but which may be important for other a-type ions which have not yet been studied. In analogy to b ions, the sequence length is probably going to play a role in the fragmentation chemistry of a ions. 

Controversy continues over the precise nature of the transition state in the cis thermolytic ester elimination reaction. Whereas one group interpret their data as indicative of a symmetrical, non-heterolytic, non-planar transition state (12) analogous to other pericyclic fragmentations, another author proposes charge development (13), defining the order of extent of movement of electrons as 1 > 2 > 3. An unsymmetrical concerted transition state (14) has been assigned to the thermal / -elimination reaction of alkyl... 

Abstract The chapter encompasses the synthesis, chemical transformations, properties, and applications of fluorinated monocyclic 1,2,3-, 1,2,4-tiiazoles and tetrazoles as well as their fused analogs. The heterocycles directly bonded with a fluorine atom (N-F or C-F isomers) as well as trifluoromethyl, perfluoroalkyl, per-fluoroaryl, SF5, NF2 groups and some other fluorinated fragments were considered. 

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