Main-group compounds

At left, the P-NMR references of phosphorus chlorides are depicted in order of decreasing coordination number of phosphorus. However, the coordination number is not expected to be the only ordering principle, as PCI/ would fall outside the range of PCl and the phosphonium salts are seen to resonate significantly upheld from PCI/. 

The chapter is divided into Lewis basic behavior and Lewis acidic behavior, respectively. The concept overlaps somewhat at the end of Sect. 6.1, as we examine examples where both the Lewis base and the Lewis acid are phosphorus containing species. 

Utilization of PF as a fluoride abstracting Lewis acid is also mentioned in Chap. 7, where the fluoride is abstracted from a fluorophosphane bonded to a transition metal. 

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Altogether, recent theoretical considerations reveal not only the background for the origin of lone-pair distortions in heavier main group compounds, but also help to understand many of the mles of thumb (see above). 

A further interesting feature of the gallium phosphides and arsenides is that the former compounds are colorless whereas the latter range from yellow to orange. Color can arise from ir- ir transitions in main group compounds for example, in the disilylenes and digermenes R2E = ER2 (E = Si, Ge) in which the it- tt transitions occur at lower energy than... 

The Reactions of Stable Nucleophilic Carbenes with Main Group Compounds... 

THE REACTIONS OF STABLE NUCLEOPHILIC CARBENES WITH MAIN GROUP COMPOUNDS... 

Compared with main-group compounds, one can therefore expect that Bent s rulelike geometrical variations in transition-metal compounds are somewhat muted (or exhibit conflicting patterns of increases and decreases, dependent on the isomer chosen), as the examples below will indicate. 

Whereas 3c/4e hypervalent interactions (4.77) tend to be relatively uncommon and fragile in main-group compounds (often leading to transition states for nucleophilic displacement reactions, rather than stable equilibrium species), the corresponding interactions in transition-metal coordination compounds are ubiquitous and robust. The far higher prevalence of hypervalent co-bonding in transition-metal chemistry may be attributed to three major factors. 

As a general conclusion, we can say that vicinal hyperconjugative interactions in transition-metal species tend to be much stronger than those in main-group compounds. Torsional degrees of freedom are therefore much more strongly hindered in metallic species, and the notion of pure torsional motion of simple rigid-rotor form lacks physical relevance in this limit. 

The method has been confined to main-group compounds presumably because of irregularities expected with unsymmetrical charge distributions in transition metal ions. The noble gas compounds remain outside the scope of the method because of the way in which electronegativity is defined (atom compactness relative to interpolated noble atom compactness). The main weakness of the method when applied to fluorides is in the somewhat arbitrary choice of fluorine bond energies. 

Table II. Some Cyclopentadlenyl and Pentamethylcyclopentadienyl Main Group Compounds with Intermediate Haptlclties.

The broader subject of the interaction of stable carbenes with main-group compounds has recently been reviewed. Accordingly, the following discussion focuses on metallic elements of the s and p blocks. Dimeric NHC-alkali adducts have been characterized for lithium, sodium, and potassium. For imidazolin-2-ylidenes, alkoxy-bridged lithium dimer 20 and a lithium-cyclopentadienyl derivative 21 have been reported. For tetrahydropyrimid-2-ylidenes, amido-bridged dimers 22 have been characterized for lithium, sodium, and potassium. Since one of the synthetic approaches to stable NHCs involves the deprotonation of imidazolium cations with alkali metal bases, the interactions of alkali metal cations with NHCs are considered to be important for understanding the solution behavior of NHCs. 

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