Weak-held ligands

For iron(IlI)-porphyrinato complexes, strong-held ligands lead to low-spin (5 = 2) complexes. A pair of identical weak-held ligands, such as tetrahydro-furan, leads to intermediate-spin (5 = ) species. Five-coordinate species are, with few exceptions, high-spin (5 = f), with all hve 3d electrons in separate orbitals. Spin equilibria 5 = i 5 = f and 5 = 15 = i are not unusual. Specihc examples of these spin systems are given in Table 4.4. Higher oxidation states are found in some other hemoproteins. Fe(V)-porphyrin systems actually occur as Fe(IV)-porphyrin cation radical species, and Fe(I)-porphyrin systems exist as Fe(II)-porphyrin anion radical species. 

At this point we have considered the interaction of oxygen with metal complexes in solution and have presented ways in which coordinated dioxygen is transferred to substrates which may themselves be coordinated to the metal. In several instances we have seen that the oxidized substrate leaves the coordination sphere. For catalysis to occur this is a necessary step. Thus, those substrates which on oxidation become more weakly held ligands, can be displaced by more unoxidized substrate and a catalytic cycle, equation (58), is possible. If, on the other hand, the oxidized substrate is more strongly held than the original substrate, catalysis does not occur. 

Complexes of weakly-held ligands can be favoured by increasing the size of PR3. The reaction ... 

The product is separated without the use of either heat or vacuum, conditions which might otherwise lead to loss of the weakly-bound ligand. In addition, the product is cooled somewhat by expansion of the SCF, thereby reducing the rate of decomposition. In reactions where the SCF is also a reactant (i.e. SCC2H4 in the preparation of C2H4 complexes) the product, once precipitated is held under a flow of the reactant gas, again minimizing the chances of decomposition [12]. 

RuH(COOMe)(PPh3)3] 0.089 highly distorted octahedral Ru coordination. Acetate ligand is bidentate, weakly held (Ru—O ca. 2.2 A) b... 

Phosphate, silicate, borate, arsenate, selenite, chromate, and fluoride are anions for which ligand exchange is important. Nitrate, chloride, bromide, and perchlorate are not held, while sulfate and selenate may be weakly held. As a consequence, leaching of nitrate and sulfate from soil in drainage water can be significant, but very little phosphate is lost in solution. Of the trace metals, Co, Cu, Ni, and Pb are strongly held on oxide surfaces by chemisorption, but the process is much less important for Cd and Zn. 

The meaning of the terms low or high spin as applied to transition metal complexes and the terms strong or weak held as applied to ligands... 

Most group A metal ions have the electronic configuration of the rare gases, and the interaction of the metal with a ligand is primarily electrostatic in nature. Because the charge density (oc charge radius) is small, the water molecules are weakly held and the rate of their loss is comparable with the diffusion-controlled value of around 10 —10 s. The complex formation rate-constant for an ion in this group is therefore approximately equal to the maximum possible value, unless chelation steps are important, and the stability of the complex is reflected in the dissociation rate constant. 

Many ligands of this category offer the prospect of inducing axial metal-ion coordination even for those cases where the pendant arms incorporate weak donor functions. Coordination will be enhanced simply because the donors are held near to the metal and hence their effective ... 

---