Strong-Held ligands

A strongly held ligand such as in some [MO]+ ions will only enter into addition reactions. Thus reactions of [MO]+ (M = Sc, Ti, V) with many organic compounds... 

The -CN radicals appear to dimerize rapidly to produce dicyanogen, NCCN. ) The reduction rate of [TPPFe (CN)2] depends on a variety of factors. Light and increased cyanide concentrations accelerate the reduction, while small amounts of water inhibit the reaction. Reoxidation of the dicyano Fe complex by molecular oxygen leads directly to the formation of the low-spin complex, as opposed to IJ.-OXO dimer formation, because of the presence of excess strong-held ligand (i.e. the reaction with O2 is probably outer-sphere). Autoreduction of a series of dicyanoiron(III) porphyrins studied by White-Dixon showed that... 

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. 

Since COs are small and strongly held ligands, as many will usually bind as are required to achieve coordinative saturation. This means that metal carbonyls, in common with metal hydrides, show a strong preference for the 18e configuration. 

The strong-field case gives one unpaired electron, which agrees with the experimental observation. The CN ion is a strong-held ligand toward the Fe ion. 

On the other hand, ligands that are very strongly held, (e.g., ethylenediamine) exert a blocking effect and reduce the reactivity. The order of reactivity of different copper(II) complexes was found to be acetate > sulfate > chloride > aquo > gly-dnate > ethylenediamine. 

The crystal held model can also be used to account For the stability of particular oxidation states. In aqueous solution Co(lll) is unstable with respect to reduction by water to form Co(Il)- Although there are several energy terms involved, this may be viewed as a reflection of the high third ionization energy of cobalt. If various moderate-to-strong field ligands are present in the solution, however. theCo(IIl)ion is perfectly stable. In fact, m some cases it is difficult or impossible to prevent the oxidation of Co(ll) to Coflll). 

This indicates that the stannylene ligand is strongly held. Additionally, attempts at coordinating other bases to the vacant orbital of the stannylene failed with this and related complexes of Sn[CH(SiMe3)2 (78). That is noteworthy, since analogous dialkylstannylene-chromiumpenta-carbonyl complexes with smaller alkyl groups attached to the tin are unstable in the absence of coordinated base (101). 

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. 

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