First-row Metals

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. 

Alternatively, addition of two ligands to an [M(14S4)] unit can lead to cis stereochemistry, where 14S4 folds to bind to three equatorial and one apical site. Only two examples, ds-[RuCl2(14S4)] [163] and ds-[Hg(14S4) (picrate)2] [164], are known thus far. 

Both coordination mode and ring size affect the properties of the resulting complexes. For example, in Cu(II) complexes of cyclic tetrathioethers the stability constants decrease as the ring size increases from 14 to 18 [165]. Kinetic measurements reveal that the difference arises almost entirely in the dissociation rate constants. 

In 1974 Travis and Busch reported the preparation of the red-brown [Co(14S4)] cation from interaction of 14S4 with [Co(MeCN)g] in nitromethane. The Co (II) complex probably resembles [Co(dth)2(OC103)2] (dth = 2,5-dithiahexane) [92], in which two CIO4 ions bind to the axial positions of a square-planar C0S4 unit. In light of the unusual properties of other Co-crown thioether complexes the electronic structures and electrochemical behavior of the [Co(14S4)] complexes 

Air oxidation of [Co(14S4)] in the presence of LiCl affords [Co(14S4)Cl2] [50], where cis stereochemistry is established by the facile substitution of Cl by bidentateligands(e.g. oxalate). Cis-[Co(14S4)X2] (X = Br, Cl, NOj,NCS , oxalate ) and trans-[Co(14S4)X2] (X =1 ) complexes [50] can be made similarly. The cis complexes also result from substitution of X for CP in cis-[Co(14S4)Cl2]. Cobalt complexes of benzo-15S4 behave similarly. 

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The reactions of transition metals with small alkenes were also studied,45-47 94-96,98,102 103,105 and it was found that many metals from the second and third rows react with alkenes, including ethene. The measured reaction rates typically increased as the hydrocarbon was changed from ethene to propene, but levelled off for larger alkenes.94 Among the first-row metals, only Ni reacted with ethene, but several of the other metals reacted with larger alkenes.94 The observed trend in reactivity for alkene reactions was 2nd > 3rd > 1st, similar to what was observed for the M + N2O reactions (see Fig. 5). This trend was explained in both cases by the pattern of electronic states in each row, as discussed above. 

Most of the work initially was with the more volatile transition metals, i.e. the first row metals plus palladium, silver and gold, because these were easy to evaporate in reasonable quantities in simple apparatus. However, efforts to use the less volatile metals of the second and third rows gained momentum. Skell used sublimation of resistively heated wires of molybdenum and tungsten to make the remarkably stable [Mo(rj4-C4H6)3] and [W(1j4-C4H6)3] (42). Green... 

Most of the first-row transition metals and several in the second and third groups have important uses. For example, iron is the basis of the enormous range of ferrous alloys in which other first-row metals are often combined. The metallurgy of iron-based alloys is a vast and complex field. Among the many... 

Although much of the chemistry of transition metals is associated with coordination compounds, there are some important aspects of their behavior that are related to other types of compounds. In this section, a brief overview of the chemistry of transition metals will be given with emphasis on the 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... 

Table 16.5 A Summary ofTypes of Complexes Formed by First-Row Metal Ions. ...

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 +... 

Tell which of the following pairs of ligands forms more stable complexes with a first-row metal ion such as... 

Having rationalized that the transition state should be a square-based pyramid, it should be mentioned that there are numerous cases in which the transition state appears to be a trigonal bipyramid. We know that because the substitution occurs with a change in configuration. From the foregoing discussion, we would expect this to occur with first-row transition metals because if 11.48 Dq must be sacrificed, this would be more likely if Dq is smaller (which it is the case for first-row metals). If a trigonal bipyramid transition state forms, there would be more than one product possible. This can be... 

For first-row metals, the number of Dq units lost is the same as for second- and third-row metals, but the magnitude of Dq is smaller. As a result, it is possible to get some rearrangement as substitution occurs. Therefore, the product can be a mixture of both cis and trans isomers. 

Although less numerous than complexes containing heavier transition metals, some cases are known in which there is a quadruple bond between first-row metals. One very interesting complex of this type is [Cr2(C03)4(H20)2] in which the carbonate ions form bridges between the two metal atoms as shown in Figure 21.25. 

This does not apply to first-row metals where 2J(P,P) for trans-coupling may be... 

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. 

One of the characteristic features of the metal-catalysed reaction of acetylene with hydrogen is that, in addition to ethylene and ethane, hydrocarbons containing more than two carbon atoms are frequently observed in appreciable yields. The hydropolymerisation of acetylene over nickel—pumice catalysts was investigated in some detail by Sheridan [169] who found that, between 200 and 250°C, extensive polymerisation to yield predominantly C4 - and C6 -polymers occurred, although small amounts of all polymers up to Cn, where n > 31, were also observed. It was also shown that the polymeric products were aliphatic hydrocarbons, although subsequent studies with nickel—alumina [176] revealed that, whilst the main products were aliphatic hydrocarbons, small amounts of cyclohexene, cyclohexane and aromatic hydrocarbons were also formed. The extent of polymerisation appears to be greater with the first row metals, iron, cobalt, nickel and copper, where up to 60% of the acetylene may polymerise, than with the second and third row noble Group VIII metals. With alumina-supported noble metals, the polymerisation prod-... 

The most striking differences within this class of compounds (fi-methylene complexes) are encountered upon attempted protonation. First-row metal derivatives such as (jU,-CH2)[(i75-C5H5)Mn(CO)2]2 (3a) and (M-CH2)[Fe2(CO)g] (35) prove to be completely resistant to even strong protic acids (e.g., HBF4, CF3S03H), whereas second-row rhodium gives rise to some intriguing chemistry, both synthetically and mechanistically, when attacked by a proton (49, 51, 52, 61). 

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