Computational studies of the binding energies of all possible tris(fluoroaryl)boranes B(C(6)H(x)F(5-x))(3) to NMe(3) and PMe(3) show that they (and by extension, the Lewis acidities of the boranes) can be tuned to a sizable range of values through judicious placement of fluorines. This holds despite the fact that the B-X bond distance changes little regardless of substitution, save when the aryl ring is 2,6-disubstituted. Fluorine substitution appears to affect the binding energies additively. Least-squares regression finds substitution at the 2-position to increase the binding energy by ca. 13 kcal·mol(-1), while substitution at the 3- and 5-positions increases it by ca. 3 kcal·mol(-1). Substitution at the 4-position has little to no impact, while substitution at the 6-position decreases the binding energy by ca. 3-6 kcal·mol(-1). The last observation arises from steric congestion because the 6-position can only be substituted in tandem with substitution at the 2-position. Models suggest that the pattern arises from polarization effects that decrease exponentially as the distance between boron and fluorine increases.

Estimation of ring strain energies (RSEs) of substituted cyclopropanes c-C(3)H(x)R(6-x) (R = F, Cl, Me; x = 0, 2, 4) using homodesmotic reaction methods has been plagued by implausible results. Prior work suggests that this stems from poorly canceled interactions between substituents on the acyclic reference molecules. We report a semi-homodesmotic approach that minimizes use of acyclic references, focusing instead on canceling substituent interactions. The method requires employing homodesmotic group equivalent reactions only for disubstituted cyclopropanes and relies solely on absolute energy calculations for more substituted rings. This provides RSEs consistent with chemical intuition regardless of the degree of substitution. We find that RSEs increase with substitution regardless of the electronic nature of R, although the increase is more dramatic when R is electron-withdrawing. The RSEs determined are consistent with QTAIM data, which show that progressive substitution always increases critical path angles. Overall, the semi-homodesmotic approach is simpler than hyperhomodesmotic reaction methods, and gives more trustworthy results.

Estimation of ring strain energies (RSEs) of substituted cyclohexanes c-C6H(x)R(12-x) (R = F, Cl, Me; x = 0, 2, 4, 8, 10, 12) using homodesmotic reaction methods gives implausible results for highly substituted cases, particularly, c-C6R12. Prior work suggests that this stems from poorly canceled interactions between substituents on the acyclic reference molecules. We apply here our semi-homodesmotic approach that minimizes use of acyclic references and ensures cancellation of intramolecular substituent interactions. The approach provides RSEs that are more consistent with chemical intuition, although they are higher than expected for "strain-free" cyclohexanes. The RSE for c-C6Me12 is predicted to be 11.9 kcal mol(-1). RSEs for halogenated rings rise significantly from 8-9 kcal mol(-1) for c-1,1,2,2-C6H8R4 to 44-50 kcal mol(-1) for c-C6R12 (R = F, Cl). The increase, and accompanying observation of larger RSEs for "adjacent CR2" systems, can be tied to increased bond distances in the rings upon progressive substitution. The sizable RSE for perchlorocyclohexane suggests that it may be susceptible to ring-opening reactions, a facet of its chemistry that is currently unexplored.

The kinetics and activation parameters for_NO dissociation from L(H 2 O)Co(NO) 2+ 2+ , and Cr(H 2 O) 5 NO 2+ were determined in aqueous solution in the presence of IrCl 6 2À , IrBr 6 2À , or O 2 as scavengers for _NO and/or metal(II) complexes. The rate constants k -NO at 25 C are (5.7 AE 0.1) Â 10 À3 s À1 for L 2 (H 2 O)Co(NO) 2+ , (1.0 AE 0.1) Â 10 À4 for L 1 (H 2 O) Co(NO) 2+ , (5.9 AE 2.0) Â 10 À8 for Cr(H 2 O) 5 NO 2+ and (3.5 AE 1.8) Â 10 À9 s À1 for L 2 (H 2 O)Rh(NO) 2+ . The kinetics of the reverse reaction were determined by laser flash photolysis at 25 C, k NO ¼ (1.7 AE 0.2) Â 10 7 M À1 s À1 for L 2 Co(H 2 O) 2 2+ and (1.8 AE 0.1) Â 10 8 M À1 s À1 for L 2 (H 2 O)Rh 2+ . The rate constants k NO and k -NO obtained here and those available in the literature were used to calculate the equilibrium binding constants K NO for a series of nitrosyl metal complexes. A good correlation was found between log K NO and log K O 2 , the latter corresponding to O 2 binding. The correlation includes complexes of four different transition metals as well as hydrogen atom, and covers a range of about 3 V in reduction potentials. The implications of these results are discussed.