N complexes stability

Substituents Relative Arenium Ion stability Relative n-Complex Stability Rate of Chlorination" Rate of Nitration ... 

How can we tell if 10 is present on the reaction path If it is present, there are two possibilities (1) The formation of 10 is rate determining (the conversion of 10 to 11 is much faster), or (2) the formation of 10 is rapid, and the conversion 10 to 11 is rate determining. One way to ascertain which species is formed in the rate determining step in a given reaction is to use the stability information given in Table 11.1. We measure the relative rates of reaction of a given electrophile with the series of compounds Usted in Table 11.1. If the relative rates resemble the arenium ion stabilities, we conclude that the arenium ion is formed in the slow step but if they resemble the stabilities of the Jt complexes, the latter are formed in the slow step. When such experiments are carried out, it is found in most cases that the relative rates are similar to the arenium ion and not to the n complex stabilities. For example,... 

A quantitative treatment of tt complex formation is, however, more complicated, since it is generally recognized that all three wave functions are necessary for an accurate description of the bond. For instance, it has been pointed out by Orgel (27) that n complex stability cannot solely be the result of n electron donation into empty metal d orbitals, since d and ions (Cu+, Ag+, Ni , Rh+, Pt , Pd++) form some of the strongest complexes with poor bases such as ethylene, tt Complex stability would thus appear to involve the significant back-donation of metal d electrons into vacant antibonding orbitals of the olefin. Because of the additional complication of back-donation plus the uncertainty of metal surface orbitals, it is only possible to give a qualitative treatment of this interaction at the present time. 

Table 1. Comparison of relative d and n complex stabilities with relative rates of substitution )... 

If the transition state resembles the intermediate n-complex, the structure involved is a substituted cyclohexadienyl cation. The electrophile has localized one pair of electrons to form the new a bond. The Hiickel orbitals are those shown for the pentadienyl system in Fig. 10.1. A substituent can stabilize the cation by electron donation. The LUMO is 1/13. This orbital has its highest coefficients at carbons 1, 3, and 5 of the pentadienyl system. These are the positions which are ortho and para to the position occupied by the electrophile. Electron-donor substituents at the 2- and 4-positions will stabilize the system much less because of the nodes at these carbons in the LUMO. 

A bridged intermediate exactly analogous to a bromonium ion cannot be formed as H has no electron pair available, but it may be that in some cases a n complex (21) is the intermediate. We shall, however, normally write the intermediate as a carbocation, and it is the relative stability of possible, alternative, carbocations (e.g. 23 and 24) that determines the overall orientation of addition, e.g. in the addition of HBr to propene (22) under polar conditions ... 

Table 6.1 summarizes the thermodynamic parameters relating to the macrocyclic effect for the high-spin Ni(n) complexes of four tetraaza-macrocyclic ligands and their open-chain analogues (the open-chain derivative which yields the most stable nickel complex was used in each case) (Micheloni, Paoletti Sabatini, 1983). Clearly, the enthalpy and entropy terms make substantially different contributions to complex stability along the series. Thus, the small macrocyclic effect which occurs for the first complex results from a favourable entropy term which overrides an unfavourable enthalpy term. Similar trends are apparent for the next two systems but, for these, entropy terms are larger and a more pronounced macrocyclic effect is evident. For the fourth (cyclam) system, the considerable macrocyclic effect is a reflection of both a favourable entropy term and a favourable enthalpy term. 

Figure 6.2. A comparison of the stabilities of the Cu(n) complexes of open-chain and macrocyclic ligands values are for water at 25 °C with / = 0.1 or 0.5 mol dm-3.

The capacity of cyclic ligands to stabilize less-common oxidation states of a coordinated metal ion has been well-documented. For example, both the high-spin and low-spin Ni(n) complexes of cyclam are oxidized more readily to Ni(m) species than are corresponding open-chain complexes. Chemical, electrochemical, pulse radiolysis and flash photolysis techniques have all been used to effect redox changes in particular complexes (Haines McAuley, 1982) however the major emphasis has been given to electrochemical studies. 

In the presence of a suitable heterocyclic base, the Fe(n) complex of this system is also a reversible carrier of 02 (Almog, Baldwin Huff, 1975). The stability of the dioxygen adduct depends largely upon the nature and concentration of the base present (in the absence of a base, rapid dimerization and autoxidation occurs). 

Table 5.15 compares the neutral and charged H-bonded complexes of Sections 5.2.1 and5.2.2,ordered by H-bond strengthfrom weakest (H20- H4C) to strongest (F- H- -F ). For each B- -AH complex, the table shows the total charge, the energy of the H-bond (A Hb) and the leading n->-cr stabilization (AE fr2 ). 

In parallel to the properties of H-bonded complexes (Section 5.2), the monomer properties in the CT complex of Fig. 5.42 are strongly altered by n-7t stabilization. Tables 5.23 and 5.24 summarize geometrical and NBO descriptors of the H3N- -NO+ complex that illustrate these changes. 

Stability constant for rans-[WO(X)(CN)4] n- complexes (50) as defined in Scheme 1. b Average equatorial W —CN distances from crystal structures, see Table I. 

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