Equilibrium coordinated complex

The divalent Co(salen) complex (69a) is one of the most versatile and well-studied Co coordination compounds. It has a long and well-documented history and we shall not restate this here. Recent applications of (69a) as both a synthetic oxygen carrier and as a catalyst for organic transformations are described in Sections 6.1.3.1.2 and 6.1.4.1 respectively. Isotropic shifts in the HNMR spectrum of low-spin Co(salphn) (69b) were investigated in deuterated chloroform, DMF, DMSO, and pyridine.319 Solvent-dependent isotropic shifts indicate that the single unpaired electron, delocalized over the tetradentate 7r-electron system in CHCI3, is an intrinsic property of the planar four-coordinate complex. The high-spin/low-spin equilibrium of the... 

What specific properties of these complexes have allowed isolation of five-coordinate Pt(IV), in the form of the trimethyl complex and the dihy-dridosilyl complexes These two types of complexes are significantly different, and their stability is apparently due to different factors. Comparing the trimethyl complex in Scheme 21(A) with the related but six-coordinate complexes of a similarly bulky oc-diimine ligand (98), shown in Scheme 23, is instructive. In Scheme 23A, triflate is clearly coordinated, exhibiting an O-Pt distance of 2.276(3) A (98), which is typical for Pt-coordinated triflate (108). This triflate complex A in Scheme 23 was obtained from dry tetrahydrofuran. The aqua complex cation B, also structurally characterized, was obtained from acetone containing trace water. An equilibrium between coordinated triflate and coordinated water, very likely via a common five-coordinate intermediate, was indicated by NMR spectroscopy (98). 

Under these conditions, the formation rate constant, k, can be estimated from the product of the outer sphere stability constant, Kos, and the water loss rate constant, h2o, (equation (28) Table 2). The outer sphere stability constant can be estimated from the free energy of electrostatic interaction between M(H20)q+ and L and the ionic strength of the medium [5,164,172,173]. Consequently, Kos does not depend on the chemical nature of the ligand. A similar mechanism will also apply to a coordination complex with polydentate ligands, if the rate-limiting step is the formation of the first metal-ligand bond [5]. Values for the dissociation rate constants, k, are usually estimated from the thermodynamic equilibrium constant, using calculated values of kf ... 

There are four naturally occurring isotopes of iron ( Fe 5.82%, Fe 91.66%, Fe 2.19%, Fe 0.33%), and nine others are known. The most abundant isotope ( Fe) is the most stable nuclear configuration of all the elements in terms of nuclear binding energy per nucleon. This stability, in terms of nuclear equilibrium established in the last moments of supernova events, explains the widespread occurrence of iron in the cosmos. The isotope Fe has practical applications, most notably in Mossbauer spectroscopy, which has been widely exploited to characterize iron coordination complexes. 

In aqueous solution, the equilibrium between the cis- and franj-diaqua complexes lies almost completely toward the cis isomer881 (K 0.17, pH 3-4). The sparingly soluble potassium salt of the tram isomer may, however, be prepared by the slow evaporation of a saturated solution at room temperature,878,880 and the cis isomer by cooling a hot solution or by allowing potassium dichromate and oxalic acid to react in the presence of a minimal quantity of water.878,879,882 The tris complex was resolved by Werner in 1912,883 providing the first example of an anionic optically active coordination complex. 

The reaction of vanadium salts (VC14, V202(S04)2, V(OCOMe)4, VO(acac)2) and porphyrin gives a complex which, after work-up, is isolated as a very stable oxovanadium(IV) species, VO(Por) (for VO(OEP), V—O = 1.620(2)A, V—= 2.102A, A(N4) = 0.543 A).17 -21 In the presence of a large excess of nitrogenous ligand such as pyridine and piperidine, it forms a six-coordinate complex with a small equilibrium constant (K = 10 I-10 21 M-1).18 The effect of / substituents on the association constant (Kx) is expressed by the Hammett equation (equation 4). 

The equilibrium constants K and f)2 increase as the ligand pKt increases. The increases in porphyrin basicity and solvent polarity also increase / 2, indicating the importance of the charge neutralization factor in the iron(III) porphyrin coordination chemistry (Table 6).86 For preparative purposes, five-coordinate complexes of the weak ligands are conveniently used to avoid contamination of the mixed ligand species Fe(Por)XL. 

Bis-pyridine complexes exist in equilibrium with five- and four-coofdinate species in solution.242 The four-coordinate complexes form polymers, which become predominant above a concentration of 5 x 10 3 M in chloroform. The association constants X, and K2 for 3- or 4- substituted pyridine complexes are well correlated to their pK, (BH+) values and Hammett s a values by equation (33) though deviations are rather large for 3-acetyl- and 2-methylpyridines. In general, K2 is smaller than Ki by a factor of 102-103. 7... 

Such complexes form a precursor to a full discussion of the vast and highly topical field of self-assembly (Chapter 10). We consider them here since they resemble structurally the types of compounds discussed in Section 4.7, but unlike metal-based anion receptors the simple thermodynamic equilibrium between host, anion and complex is not the only process occurring in solution. In fact multiple equilibria are occurring covering all possible combinations of interaction between anions, cations and ligands. These systems have the appeal that the formation of particular metal coordination complexes are thus subject to thermodynamic anion templating (cf. the thermodynamic template effect in macrocycle synthesis, Section 3.9.1) and vice versa. 

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