Free Ligands

A common disadvantage of many template reactions is that it is often difficult to remove the metal ion. Such syntheses are therefore in situ syntheses of metal complexes and can only occasionally be used for the synthesis of the metal-free ligands. 

Equation 14 shows that an increase in acidity of the solution increases the concentration of uncomplexed metal, which must result from the displacement of from MA2, causing a simultaneous decrease in the ratio of complexed to free ligand [MA2]/[HA]. The opposite effects result upon decreasing the acidity. This behavior occurs in the pH range where appreciable amounts of both HA and A coexist. Outside this range the ligand is present almost entirely as either HA or MA2 and A , and the system is essentially independent of pH. 

Fig. 4. pM vs pH for A Cu(II), and B Mn (IT) EDTA chelates. For each family of curves, the lowest curve represents 1% the second, 10% and the top curve, 100% of free ligand species ia excess of the amount needed to form the metal chelate. Broken lines represent soHd—solution equiUbria for... 

The concentration of the metal ion can be controlled by adjusting the ratio of the concentrations of free ligand and metal chelate. If both species are present in appreciable amounts, moderate changes in either concentration have Httie effect on the ratio. The concentration of the metal ion can thus be buffered in a manner analogous to the buffeting of pH by the presence of a weak acid and its anion... 

The IR stretching frequencies o(SN) and o(SF) occur at higher wave numbers and the S-N and S-F distances are significantly shorter in the octahedral cations [M(NSF)6] than those in the free ligand, indicating an increase in S-N and S-F bond strength upon coordination. 

In contrast to the numerous complexes of NO which have been prepared and characterized, complexes of the thionitrosyl ligand (NS) are virtually unknown, as is the free ligand itself. The first such complex [Mo(NS)(S2CNMc2)3] was obtained as orange-red air-stable crystals by treating [MoN(S2CNMe2>3] with sulfur in... 

For example, leinicyanoel%lene has a formal C=C douMe bond (l33.9[an) in the free ligand but m Ihe complex [Pl C2(CN)4l(PPh3)2) the C-C dislance (152pm) is that of a single bund and Ihe CN o[Pg.931]

The iron-free ligand, deferrioxamine E, is identical with nocard-amine,a metabolite from a Nocardia species, which was earlier erroneously formulated as 16 and then 17. ... 

The K values in Table 11 appear to be reasonably large. They correspond to the association between ligand and Zn2 + ion involving almost no interaction with the substrate in the pre-equilibrium stage so that they are the same for both L- and D-ester and for different esters (50 and 52). The enantioselectivities (kL/kD) in Table 9 and 10 involve some contributions from the free ligands in addition to those from the com-... 

If there is a means to detect (i.e., radioactivity, fluorescence) and differentiate between protein-bound and free ligand in solution, then binding can directly quantify the interaction between ligands and receptors. 

This shows that the pM value of the solution is fixed by the value of K and the ratio of complex-ion concentration to that of the free ligand. If more of M is added to the solution, more complex will be formed and the value of pM will not change appreciably. Likewise, if M is removed from the solution by some reaction, some of the complex will dissociate to restore the value of pM. This recalls the behaviour of buffer solutions encountered with acids and bases (Section 2.20), and by analogy, the complex-ligand system may be termed a metal ion buffer. 

When hemin (1) is treated with gaseous hydrogen chloride in anhydrous methanol/pyridine in the presence of iron(II) sulfate, protoporphyrin dimethyl ester (2), the metal-free ligand of hemin. is produced in almost quantitative yield. This compound is quite sensitive to oxygen... 

The cw-isobacteriochlorin 16A formed by the Claisen rearrangement has been utilized for the further transformation into porphyrin dp2311 0 the metal-free ligand of heme dr... 

Oxidation reactions were used in the synthesis of porphyrin d, the metal-free ligand system of naturally occurring heme d,. In a total synthesis of porphyrin d,12d oxo functions were introduced into isobacteriochlorin 3 by selenium dioxide oxidation to yield 4. The selenium dioxide selectively attacks the 3- and 8-positions of the partially reduced pyrrole rings of the chromophore. In another synthesis23a c of porphyrin d, an isobacteriochlorin 5, derived by... 

The dissociation constant (Kd) is the concentration of free ligand that results in occupancy of 50% of the receptors for this ligand available in the system. 

The free ligands were obtained by visible photolysis in acetonitrile and were characterized by elemental analyses and H- and 13C-NMR spectroscopies. Low-temperature reduction in ESR tubes also provided the green Fe1 19e complexes which gave the characteristic 3g values typical of the rhombic distortion of Fe1 [77],... 

Due to the difficulty in preparing the free ligand 3 complexes of the l,2,4-tri-/m-butylcyclopentadienyl ligand are virtually unexplored. l,2,4-tri-te 7-butylcyclo-... 

The palladium(ll) and heterobimetallic palladium(ll)/zinc oxalamidinate complexes depicted in Scheme 197 have been prepared analogously from the free ligands and Pd(acac)2 or ZnEt2/Pd(acac)2, respectively ... 

Reactions of the free ligands with trimethylaluminum in toluene solution afforded the bis(amidinates) as shown in Scheme 210. ... 

---