Uncharged Metal-Ligand Complexes

Metals having a zero oxidation state can still be complexed by a similarly soft bonding ligand such as carbon monoxide. Nickel carbonyl, Ni(CO)4, is a gas used in the nickel refining industry in order to volatilize and purify nickel from its ores. It is interesting to note that 

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The reactions of nucleophilic reagents with cationic and uncharged metal carbonyl complexes have received much attention in the past, and it is not surprising that these studies have now been extended to isocyanide metal complexes. Different products in these reactions can arise by three general routes these include ligand substitution, reactions involving attack at a ligand, and reduction of the metal complex. All have been observed in reactions with metal isocyanide complexes. 

For uncharged monovalent metal-ligand complexes, proton-ligand complexes (or dissolved gases), and bivalent metal-ligand complexes, some model semiempirical equations for y, are the following 11... 

The thiocyanate ion provides an interesting test of these ideas. In the SCN ion, the sulfur atom is considered to be a soft electron donor, whereas the nitrogen atom is a much harder electron donor. Accordingly, Pt2+ bonds to SCN- at the sulfur atom, whereas Cr3+ bonds to the nitrogen atom. Uncharged metal atoms are considered to be soft electron acceptors, and they form complexes with soft ligands such as CO, 11, and PR3. We will see many examples of such complexes in later chapters. On the other hand, we would not expect complexes between uncharged metal atoms and NH3 to be stable. 

Although there is a tendency for the alkene complexes to contain uncharged metals, a large number of complexes are known in which the metal ions are Pd2+, Fe2+, Cu+, Ag+, and Hg2 +. As we shall see, the formation of alkene complexes of these and other metals occurs as the metals catalyze certain reactions of the ligands. 

Exchange of transition metal loc uncharged ligand complexes... 

In Chapter 3 we described how an uncharged metal complex MA is formed from a metal ion central atom) through a stepwise reaction with the anion A (ligand) of a monobasic organic acid, HA, defining a stepwise formation constant k , and an overall formation constant (3 , where... 

Fig. 4.20 Distribution constants Kdq for uncharged metal complexes MA, vs. distribution constants Tdr for the corresponding undissociated reagent, the acid ligand HA, for various organic solvents and 1 M NaC104 open circles Cu(II), sohd circles Zn(II). Numbers refer to solvents listed in Table 4.10. (From Ref. 36.)... 

Fig. 4.21 Distribution constants, Ky)c, for uncharged metal complexes MA vs. distribution constants for the corresponding undissociated acid ligand HA solid circles Zn(II), solid triangles Co(III). Variation with ligand composition HFA hexafluoroace-tylacetone, TFA trifluoroacetylacetone, AA acetylacetone, FTA 2-furoyltrifluoroacetone, TTA 2-thenoyltrifluoroacetone, PTA pivaloyltrifluoroacetone, BFA benzoyltrifluoroace-tone, BZA benzoylacetone. (From Ref. 36.)... 

In a general way, it can be predicted that favorable bonding between a metal ion and ligands will occur when they have similar sizes and polarizabilities. Metal ions such as Co3+ and Cr3+ are hard Lewis acids and NH3 is a hard Lewis base, so it is expected that species such as [Co(NH3)6]3+ or [Cr(NH3)6]3+ would be stable. Uncharged metals are soft Lewis acids and CO is a soft Lewis base. Consequently, matching the hard-soft properties of the metal and ligands allows us to predict that Fe(CO)s will be a stable complex. Conversely, complexes such as [Co(CO)6]3+ (CO is a soft ligand) or [Fe(NH3)6] (Fe° is a soft Lewis acid) would not be expected to be stable. Both sets of predictions are in accord with experimental observations. 

Iron tricarbonyl forms exceptionally stable complexes with 1,3-dienes. The complexes are uncharged, readily soluble species, chromatographable and, for the simpler versions, distillable. They are formed by direct reaction of the 1,3-diene with Fe(CO)5, Fc2(CO)9, or Fe3(CO)i2. These iron diene complexes are known to be reactive toward electrophiles, undergoing the analogous reaction to electrophilic aromatic substitution under Friedel-Crafts conditions. However, it is clear that the metal-ligand unit increases the polarizibility of the diene unit, and, with a sufficiently reactive nucleophile, can provide a sink for electron density. How reactive does the nucleophile need to be The other important selectivity question for 1,3-dienes concerns the regioselectivity. 

Pyridine-2-aldehyde-2-pyridylhydrazone is a tridentate ligand that forms cationic complexes with dipositive transition metal ions. The protons attached to the nitrogen atoms are readily lost from the chelate rings and an uncharged metal chelate, soluble in organic solvents, is formed, accompanied by a marked color change (87, 88, 132). 

Typically for transition metal complexes, as the steric bulk is increased on the ligand, both nuclearity and the number of uncharged bound ligands (i.e., solvents) decrease because... 

Table 13.2 Number of electrons supplied by ligands to metal atoms in complexes when the metal atoms are considered to be uncharged.

Crown polyethers. Macrocyclic effects involving complexes of crown polyethers have been well-recognized. As for the all-sulfur donor systems, the study of the macrocyclic effect tends to be more straightforward for complexes of cyclic polyethers especially when simple alkali and alkaline earth cations are involved (Haymore, Lamb, Izatt Christensen, 1982). The advantages include (i) the cyclic polyethers are weak, uncharged bases and metal complexation is not pH dependent (ii) these ligands readily form complexes with the alkali and alkaline earth cations... 

Ligand—The charged or uncharged chemical species that reacts with a metal ion forming a complex ion. 

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