Electronic structure carbonyl halides

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. 

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. 

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... 

These uncertainties are increased when the existence of compounds of CO with metals, the carbonyls such as Fe(CO)5, for which no electrostatic model is conceivably possible are considered. The iron obviously is not present in the compound as an ion how then can the attraction for the CO molecules be explained Even in the straight ammoniates there is some doubt regarding the validity of the simple electrostatic representation of the structure for it is found experimentally that the magnetic properties of halides are radically altered by the taking up of molecules of ammonia. This shows that - some of the electrons of the positive ions are influenced by the ammonia molecules in a way which an electrostatic picture cannot explain. 

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