Nitrogen pentafluoride

A small smorgasbord of published papers will be discussed here, to show how some of the things that we have seen in previous chapters have appeared in the literature. The four topics of this section (oxirene, nitrogen pentafluoride, pyramidane and nitrogen polymers), and several others, are addressed in more detail in another book [1],... 

A more fundamental reason is that predictions for molecules very different from those outside the training set should be less reliable than those for molecules similar to the ones used for parameterization. Therefore for investigating exotic species like, say, planar carbon or nitrogen pentafluoride AMI and PM3 are considered unreliable, and even noncorrelated ab initio calculations would be considered well short of definitive nowadays [1]. 

Nitrogen pentafluoride represents an interesting contrast to oxirene. Oxirene is, on paper, a reasonable molecule there is no obvious reason why, however unstable it might be because of antiaromaticity [4] or strain, it should not be able to exist. On the other hand, NFs defies the hallowed octet rule why should it be more reasonable than, say, CHfi Yet a comprehensive computational study of this molecule left tittle doubt that it is a (relative) minimum on its potential energy surface [5]. The full armamentarium of post-HF methods, CASSDF, MRCl, CCSDT, CCSD(T), MP2 (section 5.4) and DFT (chapter 7) was employed here, and all agreed that Dan (section 2.6) NF5 is a minimum. 

Molecules that seem to violate the octet (or duplet) rule are hypercoordinate, because the central atom is bonded to an unconventionally high number of ligands . The simplest hypercoordinate, neutral, even-electron compounds of the first full row can be conceptually obtained by adding a hydrogen molecule to the normal hydrides (Fig. 4.1). Let us examine a related molecule which has substantially weathered in silico the tests that have been applied to its putative existence, namely nitrogen pentafluoride. 

The most comprehensive theoretical study of nitrogen pentafluoride so far comes from Hettinger et al., [19], They report an investigation of this molecule at several high levels CASSCF, MRCI, B3LYP, MP2, and CC with variants of these methods and of basis sets, some species were studied at 15 different levels. The ealeulated stmcture and, of prime interest, the calculated thermodynamic and kinetic stabihties, are given. In contrast to some earlier studies Synthesis, below), these workers are optimistic about the possibility of preparing NF5. 

Thermodynamic and kinetic stabilities. Further information on the challenge to our ingenuity posed by the synthesis of nitrogen pentafluoride was provided by calculated results for its thermodynamic and kinetic stabilities. The reaction energy and... 

Of the obvious routes to nitrogen pentafluoride, the NF3 + F2 -> NF5 reaction will not work, as it is very unfavorable thermodynamically. The other superficially promising approach, the reaction of the known NF4 with fluoride ion (as HF2 ) gave as the only detectable result attack by F on the F, rather than the N, of the cation. Photochemical reaction of NF3 with F2, although it has been tried, seems worth repeating. 

Bettinger et al. were optimistic that nitrogen pentafluoride can very likely exist, and their calculations indicated that it might even persist for a reasonable time at room temperature [19], and this optimism was shared by van Wazer and coworkers [25, 26, 38]. Chiiste and cowoikers, in contrast, believe that for steric reasons it is difficult if not impossible to attach five fluorines to a nitrogen atom [36, 37, 39]. 

Accurately accounting for nonbonded interactions between atoms or groups by quantum mechanical calculations requires quite high levels of theory [40], and one cannot assert unequivoeally that this was attained even in the notable paper of Bettinger et al. The possibility of the existence of nitrogen pentafluoride can neither be affirmed nor denied with confidence. 

Some molecules assault valence orthodoxy. Nitrogen pentafluoride and helium compoimds (Chapters 4 and 5) require, if they exist, that the the bonding power of the key atom exceed that shown in arty of its known compounds (for helium the known bonding power and number of its combinatorial progeny is in fact zero). 

The denouement of these considerations is that at present it is not possible to be confident that nitrogen pentafluoride can exist, beeause of the difificully of reliably taking into account the steric requirements of aceommodating the fluorines. Hehnm, however, can likely form compounds, albeit fragile ones, such as HeBeO and HHeF. Theme 3. Chapter 3 oxirene, subtly but fatally flawed ... 

The outer shells of the elements phosphorus and sulfur in period 3 of the periodic table can hold up to 18 electrons. The elements in period 2 do not form compounds with more than eight electrons in the outer shell because the second shell can only hold up to eight electrons. Hence, nitrogen trifluoride, NF3, is a stable species but nitrogen pentafluoride, NF5, is unknown. However, phosphorus trifluoride, PF3, and phosphorus pentafluoride, PF5, are both stable molecules. 

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