Of first-row metals

Much of what has been said so far in this chapter applies equally well to complexes of second- and third-row transition metals. However, there are some general differences that result from the fact that atoms and ions of the second- and third-row metals are larger in size than those of first-row metals. For example, because of their larger size (when in the same oxidation state as a first-row ion), ions of metals in the second and third rows form many more complexes in which they have a coordination number greater than 6. Whereas chromium usually has a coordination number of 6, molybdenum forms [Mo(CN)8]4 and other complexes in which the coordination number is 8. Other complexes of second- and third-row metals exhibit coordination numbers of 7 and 9. 

Because of their having larger sizes and more filled shells of electrons between the outer shell and the nucleus, the ionization energies of second- and third-row metals are lower than those of first-row metals. Consequently, it is easier for the heavier metals to achieve higher oxidation states, which also favors higher coordination numbers. In general, there is also a greater tendency of the heavier metals... 

Another significant difference between complexes of first-row metals and those of the second and third rows involves the pairing of electrons. Earlier in this chapter, it was shown that for the d4 ion Mn3 +... 

Cobalt offers many possibilities of cluster-core-geometry, but the chemistry of cobalt clusters is limited, again due to the weakness of first row metal-metal bonds and their susceptibihty to nucleophilic cleavage. Only in case of the methinyl tricobalt enneacarbonyls has a singular chemistry been developed, and therefore these compounds will be treated under a separate heading. 

These reactions show that there is no loss of configuration as substitution occurs. For these second- and third-row metals, splitting of the d orbitals produced by en and Cl- is considerably larger than it is in the case of first-row metals. As mentioned previously, the formation of a square-based pyramid transition state is accompanied by a smaller loss in LFSE than is the formation of a trigonal bipyramid transition state. Thus, attack by the... 

In keeping with the generally weaker bonds of first-row metals, the chromium complex in equation (38) has been shown to reversibly bind pyridine with cleavage of the metal-metal bond. 

The interesting antiaromatic triynes tribenzocyclyne (TB) and trithienocyclotriyne (TTC) exhibit unusual coordination chemistry. The small cavity of these compounds allows the incorporation of first-row metals including Co [69], Cu [70], and Ni[71] in novel arrangements as illustrated by structures 44-46 in Scheme 4-16. When partially reduced with alkali metals, Ni(TBC) shows a lO -fold increase in conductivity. 

Which coordination mode a complex adopts depends critically upon the size of the metal ion [162]. Anti stereochemistry, with its concomitant inversion symmetry, requires the metal ion to lie in the plane of the four donor atoms. Only the smaller metal ions can fit in this plane at normal M-S distances. Syn coordination better accommodates larger metal ions, which can (and invariably do) lie out of the S4 plane. It therefore predominates when the ionic radius of the metal exceeds the optimum for the ligand cavity [162], For example, [M(14S4)] complexes of first-row metals uniformly adopt anti coordination (as do [M(14N4)] complexes [162]). On the other hand, syn stereochemistry occurs in, e.g. [Hg(14S4)(OH2)] [35], where it arises from the attempt to circumscribe the large Hg(II) ion. 

These relative rates fit the general trend that reactions of first-row metal complexes tend to be faster than reactions of their second-row analogs, which tend to be faster than reactions of their third-row congeners, but the precise origin of this effect in associative ligand substitutions has been rationalized in more than one way. One text has attributed this trend for associative ligand substitution as reflecting the relative propensities of the metals to form five-coordinate, 18-electron complexes. In a few cases, five-coordinate... 

Reductive eliminations tend to be faster from complexes of first-row metals than from complexes of second-row metals, which in turn tend to be faster than those from complexes of third-row metals. Reductive eliminations from complexes of second-row metals are more favorable thermodynamically than reductive eliminations from third-row metals because the metal-ligand bonds in the second-row reactant are weaker. 

In an effort to move away from precious metal catalysts, various reports in recent years have focused on the use of first-row metal catalysts for direct arylations [57-60]. As a representative example of these new developments, we illustrate in Scheme 23.15 the chelate-assisted ortho-C-H arylation of arenes with Fe catalysts [61]. With iron being cheap, nontoxic, and ubiquitous, this protocol is highly attractive for pharmaceutical syntheses. Using the catalyst precursor Fe(acac)j in conjunction with bidentate pyridine ligands, Zn-aryl reagents as aryl transfer reagents and 1,2-dichloroisobutane as the oxidant, excellent yields of the arylated product were obtained. An interesting feature of this reaction is the hydrolysis of the imine moiety after work-up. The reaction conditions tolerate additional functionalities such as cyanides, chlorides, triflates, tosylates, and thiophenes. 

It should be mentioned that the work described earlier on MaMaMb systems represents an extension of the conceptual design first described by Mashima for complexes containing phosphine derivative ligands and mainly second- or third-row transition metals [66-77]. By changing from P-donor ligands to N-donors, the new EMAC complexes of first-row metals can exist in high-spin configurations and are more relevant to the types of heterometallic active sites observed in metalloproteins. 

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