ELECTRONIC STRUCTURE OF THE CARBONYL HALIDES

THEORETICAL DESCRIPTIONS OF THE ELECTRONIC STRUCTURES OF THE CARBONYL HALIDES... 

Formaldehyde (methanal, COH ) is an apposite starting point for a discussion of the electronic structure of the carbonyl halides, as its orbital interactions are relatively simple. Like the carbonyl halides, the ground state of COHj is planar with C2y symmetryt, and its molecular orbitals can be constructed from those of carbon monoxide and the Is atomic orbitals of two hydrogen atoms (see Fig. 17.1). Its valence electron configuration is ... 

Empirical trends in the electronic structure of the carbonyl halides... 

In the Sections above, various aspects of the electronic structure of the carbonyl halides have been discussed in some detail, and it is now appropriate to consider what insight this knowledge yields concerning their chemical reactivity. In particular, their reactivity towards nucleophilic and electrophilic substitutions will be examined. 

Acid halides. Acid halides show a strong C=0 stretching absorption band at the high frequency end of the carbonyl stretching region. This may be explained by considering the electronic structure of an acid chloride. 

A good example of the carbonylation process is the reaction of the tetracarbonyl ferrate dianion [Fe(CO)4 ] with alkyl halides. This reagent is made by dissolving metal reduction of the 18-electron Fe(0) compound Fe(CO)5. Addition of two electrons would give an unstable 20-electron species but the loss of one of the ligands with its two electrons restores the stable 18-electron structure. 

Facing these difficulties concerning the syrJanti-selectivity in carbonyl allylations, Marton et al., (1996a) extensively studied the zinc-mediated allylation of aldehydes with allyl halides in cosolvent/HaO (NH4CI) and in cosol-vent/H20 (NH4Cl)/haloorganotin media. The stereochemistry seems to be determined by the structure of the radical anions, which were presumed to be formed through an electron-transfer process. 

Just as for group 5, 6, and 7 ( -CsF MCU species, Fehlner has shown that BH3-THF or Li[BH4] react with group 8 and 9 cyclopentadienyl metal halides to result in metallaborane clusters, many of them having a metal boron ratio of 1 3 and 1 4, and much of the synthetic chemistry and reactivity shows close connections with the earlier transition metals. The main difference between the early and later transition metallaboranes that result is that the latter are generally electron precise cluster species, while as has been shown, the former often adopt condensed structures. Indeed, as has been pointed out by King, many of the later transition metallaborane clusters that result from these syntheses have structures closely related to binary boranes and, in some cases, metal carbonyl clusters such as H2Os6(CO)18.159... 

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