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“Non-Existent Compounds” and Ways to Make Them!

“Non-Existent Compounds” and Ways to Make Them!.

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“Non-Existent Compounds” and Ways to Make Them!

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  1. “Non-Existent Compounds” and Ways to Make Them! A variety of compounds were classified in a 1965 book by W. E. Dasent as being “non-existent” in that arguments could be made that such compounds were unlikely ever to be made. This was meant as more as a teaching tool (he realized that there may be ways to overcome the inherent instability of the compounds he described) and it is instructive to examine the types of arguments that he used, which include: • Electron Configuration • Thermodynamic Stability • Kinetic Stability • Orbital overlap • Electronegativity differences

  2. 1 eV = 96.49 kJ/mol 1 cal = 4.184 J NX5 vs. PX5 While the compounds PX5 are stable known compounds for X= F, Cl, Br and maybe even I, the analogous N compounds are not known. The standard explanation regarding the instability of the NX5 species is that such compounds would violate the “octet” rule and must be impossible because N does not have access to low-energy d orbitals. 2s22p3 2s12p33d1 ca. 18.9 eV 3s23p3 3s13p33d1 ca. 14.4 eV While this may seem reasonable, is the “octet rule” truly valid even for the second row elements? Are d orbitals necessary at all? Remember the alternative description of the bonding in PF5:

  3. [NF4][BF4] + [NMe4][F] [NMe4][BF4] + NF3 + F2 NF5 NF3 + F2 What about NF5 Since F is the most electronegative of the elements and the smallest of the halides, it was the best candidate to make an NX5 compound. K. O. Christie was able to make the first [NF4]+ salt in 1966 (Inorg. Nucl. Chem. Lett., 1966, 2, 83.) and Born-Haber cycle calculations revealed that [NF4][F] should have around the same energy as NF5. In 1992, Christie et al. (JACS, 1992, 114, 9934) obtained the following result: To date, all attempts to make NF5 have failed and ab initio and density functional theory calculations (H. F. Schaefer, JACS, 1998, 120, 11439) suggest that it may be metastable, at best. The energy of the disproportionation reaction is exothermic by about 42 kcal/mol.

  4. The Instability of NF5 Furthermore, radiotracer studies using 18F have not shown even the transient existence of NF5. The instability of compounds such as NF5 is probably best understood in the context of size and electronegativity. The N-F bonds in the caculated D3h structure for NF5 are 1.381Å (equatorial) and 1.608 Å (axial). This means that there would be F atom contacts of around 2.12 Å between the axial and equatorial F atoms; the van der Waals radius of F is 1.47 Å. In PF5 the distances are 1.608 Å (axial) and 1.570Å (equatorial). Perhaps more importantly, N is almost as electronegative as F so there is very little polarity to strengthen the N-F bonds, whereas the difference in electronegativity between F and P produces very polarized bonds. Questions to ponder: If size and polarity are the only important factors, could we not make NH5? What is the nature of the bonding in the molecule we usually use as an ionization source in chemical ionization mass spectrometry, CH5+? (See: G. A. Olah, Acc. Chem. Res., 1997, 30, 245) [C(AuPPh3)5]+

  5. CCl4 vs SiCl4 Some of the compounds that Dasent discussed are more surprising for their stability. For example, carbon tetrachloride is remarkbly stable in water or alcohols when one considers the bond energies involved. CCl4 should react with water to form CO2 and H-Cl in a very exothermic reaction. The heavier congener of carbon tetrachloride, SiCl4 reacts with water readily. CCl4 SiCl4 These energies are in kJ/mol The reason for the relative stability of carbon tetrachloride is the small size of carbon, which is completely surrounded by the Cl atoms. This situation does not allow for the convenient approach of the nucleophilic hydroxy group and the small size of C means that it would be impossible to form the required 5-coordinate intermediate thus carbon tetrachloride kinetically stable. Similar arguments can be used to explain the hydrolysis of SF4 versus the stability of SF6 (which should react more exothermically).

  6. AsCl5 AsCl3 + Cl2 PCl5 and SbCl5 vs. AsCl5 Perhaps one of the more surprising of Dasent’s “non-existent” compounds is AsCl5, because PCl5 and SbCl5 are both stable compounds. The relative instability of AsCl5 is related to its position in the periodic table and is explained by the relative stability of the As(III) oxidation state versus that of the As(V) oxidation state. This stability is a manifestation of the d-block contraction that also affects Ga and Ge. The disproportionation of arsenic pentachloride to the trichloride and chlorine is rendered favourable by the higher energy required to use the lone pair electrons on As (either for hybridization or planarization).

  7. AsF5 vs. AsCl5 AsX3(l) AsX3(g)DH1 X2(g) 2 X (g)DH2 AsX3(g) + 2 X(g) AsX5(g)DH3 AsX5(g) AsX5(l) DH4 The energies listed in books (kJ/mol values taken from Huheey, Keiter and Keiter, 1993) are not always helpful. E.g. As(III)-F 484; As(V)-F 406. The relative instability of AsCl5 may also seem surprising because AsF5 is a stable compound. It would seem that in this case one cannot make arguments based on the relative energies of the As(III) oxidation state versus that of the As(V) oxidation state. While this is debatable, a better answer is perhaps found in the examination of bond energies and the overall reaction energy that we might expect using equations similar to a Born-Haber cycle: Assuming that DH1 and DH4 cancel each other, this still predicts that AsCl5 should be viable, however to a much smaller extent than AsF5. One must be cautious of using such mean bond energies: experimentally the formation of PCl5 from PCl3 and Cl2 suggests that the enthalpy for the last two P-Cl bonds is only 168 kJ/mol not the 326 kJ/mol listed in books.

  8. The synthesis and structure of AsCl5 AsCl5 was eventually synthesized photochemically at low temperature by K. Seppelt (Angew. Chem., Int. Ed. Engl., 1976, 15, 377) and the structure of the molecule was recently reported (Z. Anorg. Allg. Chem., 2002, 628, 729). It is a metastable and highly reactive compound.

  9. Homonuclear Single Bond Strengths These energies are in kJ/mol and are very approximate but the trends are useful (all values taken from N. C. Norman, 1997 and Huheey, Keiter and Keiter, 1993). The general trend is for bonds to weaken down a given group because of larger internuclear distances and less effective orbital overlap. Note the anomalies, in particular, the weakness of the N-N, O-O and F-F single bonds! These anomalies are caused by “lone pair” electron repulsion.

  10. Isodesmic Equations One of the most convenient methods that one can use to assess the relative stability of valence isomers is the application of isodesmic equations. “Isodesmic” means that there are the same number of bonds on each side of the equation and the assumption is that the energies of the bonds are the only important quantity in determining the thermodynamic preference of the reactions. For example, isodesmic reactions can be used to determine the stability of different possible allotropes: 2 2 * 940 kJ/mol = 1880 kJ/mol 6 * 160 kJ/mol = 960 kJ/mol Thus the formation of N4 from dinitrogen is highly endothermic and the reaction does not proceed as drawn.

  11. 4 O2 O8DHrxn = +324 kJ/mol 4 S2 S8DHrxn = -140 kJ/mol Isodesmic Equations: 2 P2 vs. P4 In stark contrast to its lighter analogue, diphosphorus will dimerize to form P4 (white phosphorus). In doing so, all of the p-bonds are replaced with s-bonds. Such behaviour is typical of the all of the heavier elements (n > 2) in the periodic table. 2 2 * 490 kJ/mol = 980 kJ/mol 6 * 200 kJ/mol = 1200 kJ/mol For example, the relative stabilities of O2 vs. S8: These energies are in kJ/mol.

  12. Multiple-bond energy increments The results of the isodesmic comparisons are often rationalized through the use of multiple-bond energy increments. These are very rough approximations (derived empirically) of how much energy is provided by each element-element “single” bond versus that of each multiple bond. For example, the values below suggest that the double bond in O2 should have a total energy of around 495 kJ/mol because it consists of a s bond (145) and a p bond (350). Similarly the energy of the bond in N2 would be predicted to be 160 + 395 + 395 = 950 kJ/mol. (in kJ/mol; all values taken from N. C. Norman, 1997) Why do they vary in this way?

  13. D’(A-B) is the ionic resonance energy in kJ/mol (D(A-B) is in eV) A-B A+ B- 0.102 is a conversion from kJ/mol to eV Heteronuclear Single Bond Strengths These energies are in kJ/mol and are very approximate (all values taken from N. C. Norman, 1997). While the factors influencing the strength or weakness of the homonuclear bonds are still important, electronegativity differences tend to strengthen bonds (Coulombic attraction). D(A-B),theory = ½ (D(A-A) + D(B-B)) D’(A-B) = D(A-B),experimental - D(A-B),theory XA – XB = 0.102 (D’(A-B))½

  14. Heteronuclear Single Bond Strengths These energies are in kJ/mol and are very approximate (all values taken from Huheey, Keiter and Keiter, 1993). Note the relative strengthening of bonding to elements in the lower oxidation states. This (perhaps counter-intuitive) observation is readily understood in the context of the greater difference in electronegativity between the element and Cl. Remember that electronegativity is not constant and will be affected by the molecular environment or oxidation state.

  15. Just so you have a table of Pauling electronegativity values in your notes: The values of XS, XSe, and XI are all around 2.5. This observation, in conjunction with the position of I in the periodic table, suggests that S-I bonds and Se-I bonds will be relatively weak.

  16. Isodesmic Equations: Se-I bonds The prediction regarding the strength of Se-I bonds in neutral compounds is correct and such compounds were considered unlikely on the basis of isodesmic equations (energies in kJ/mol): 2 + 172 + 150 kJ/mol = 322 kJ/mol 2 * 150 kJ/mol = 300 kJ/mol Thus the reagents would be more stable than the products and the reaction is not predicted to be favourable in the indicated direction. Note that the energies of the two R-Se bonds on each side of the reaction are assumed to cancel those on the other side and can thus be ignored in the calculation.

  17. Isodesmic Equations: Se-I bonds Despite the prediction, DuMont et. al. (See: J. Organomet. Chem., 2001, 623, 14, and references therein) were able to synthesize such compounds by the direct reaction of diorganodiselenides with molecular iodine. 2 + R = Mes, Keq = 30 R = Tip, Keq = 60 R = Mes*, Keq = 300 R = Trisyl, quantitative Thus the isodesmic comparison has failed. The reason for this failure is the assumption that one can neglect all factors other than the changing bond energies. In this case, the size of the substituents has altered the thermodynamic preference of the system.

  18. Some Sterically-Demanding Substituents disyl Mesityl (Mes) Tip trisyl Supermesityl (Mes*) Terphenyl (one kind)

  19. Sterically-Demanding Substituents and the thermodynamics of Se-I bonds The bulky substituents alter the thermodynamic preference of the diselenide-iodine system primarily by destabilizing the diselenide. The destabilization is primarily enthalpic (DH) in that it raises the energy of the diselenide by introducing strain into the molecule. This strain is manifested in distortions of the metrical parameters (inter-atomic distances, angles and torsion angles) in both the ligands and the Se-Se fragments of the molecules. There is also an entropic (DS) destabilization because the presence of the two very large substituents restricts the freedom of motion in the molecule (vibrations and rotations). In the Mes*-Se fragment distortions include a non-planar benzene ring and an out-of-plane attachment of the aryl group to the Se atom. (Mes*-Se)2 Se-Se: 2.348Å (Trisyl-Se)2 Se-Se: 2.387Å Se-Se in (Ph-Se)2: 2.287Å (all data from the Cambridge Stuctural Database - CSD)

  20. Sterically-Demanding Substituents: Kinetic Shields The traditional understanding of the stabilizing role of bulky substituents is that these large groups act as “Kinetic Shields” that prevent incoming molecules from reacting with the protected functional group. Such a view implies that the energy of the product (B) is lower than that of the reagent (A) and that the role of the large substituent is to raise the activation energy (Ea) for the process. If the activation barrier is sufficiently large, the process will not occur even though the process is thermodynamically favourable. Notice that such process implies that if there were enough energy to get over the barrier, the reaction would proceed; this means that species A is “kinetically stable”. While this is a completely reasonable description for some types of reactions, it is not valid for others. Remember: Stability is relative! A situation is only stable in comparison to another possible situation. Ea Energy A DH B Reaction Coordinate

  21. Bulky Substituents: Thermodynamic and Kinetics Consider the dimerization of subsituted olefins: An isodesmic reaction predicts the cyclobutane should be favoured by around 80 kJ/mol. 2 Energy R = H, Me R = tBu, Ph R = Mes, Tip, Mes* (-85, -83) (-7, +19) (+68, +282, +395)Energies are in kJ/mol from Burford, Inorg. Chem., 1997, 36, 3204.

  22. Bulky Groups and Multiple bonding Because of the relative weakness of p-bonding between P(III) and N, as evidenced by the multiple bond energy increments (energies in kJ/mol), aminoiminophosphines R2N-P=N-R are generally susceptible to addition reactions in the presence of secondary amines. + However, the synthesis of a trisaminophosphine containing only one bulky Mes* group resulted in the spontaneous elimination of a secondary amine and the formation of an aminoiminophosphine. + Thus the thermodynamic preference of the system has clearly been reversed (Burford, Inorg. Chem., 1993, 32, 4988).

  23. Bulky Groups and Drastic Changes Sterically demanding substituents can also be used to change the thermodynamic stabilities of systems in even more extreme ways. For example, diphosphines generally have relatively strong P-P bonds (200 kJ/mol) that remain intact in all phases. For all small R groups R = Me However, the disyl-substituted derivative cleaves spontaneously when it is not in the solid state. (Hinchley, JACS, 2001, 123, 9045) 2 Melt, Solution or Gas Phase Stable Free Radicals R = CH(SiMe3)2 Some multiple bonds can also be fragmented into carbenoids in a similar way. (Lappert, JCS, Dalton., 1986, 1551 and 2387) Solution or Gas Phase

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