Complexes of Second- and Third-Row Metals

In view of the number and types of hybrid orbitals that are exhibited by ions of the transition metals when complexes are formed, it is possible to arrive at a summary of the most important types of complexes that each metal ion should form. Although there are many exceptions, the summary presented in Table 16.5 is a useful starting point for considering the types of complexes that are formed by metals in the first transition series. 

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 

Number of d Electrons Most Common Ion Usual Geometry6 Hybrid Orbital Example 

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 + 

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D. R. Russell, and D. IF. Sharp Electronic spectra of some fluoride complexes of second- and third-row transition metals. J. Chem. Soc. (A) 18 (1966). 

Because of the effects described earlier, many ligands that give high-spin complexes with first-row transition metals give low-spin complexes with second- and third-row metals. For example, [NiCl4]2 is high-spin (tetrahedral), whereas [PtCl4]2- is low-spin (square planar). 

Although there are several examples of lanthanide complexes with the squarate ion 42, 55, 57), only a few squarate complexes of second-and third-row transition metals have been reported. The platinum and... 

Most polypyridine complexes of second- and third-row transition metals also display a predominantly metal-localized oxidation at positive potentials which are chemically either reversible or partly reversible. Further one-electron oxidations often occur at more positive potentials in liquid SO2 [57]. The first oxidation potential depends on the metal atom (for example, Ru > Os), the ancillary hgands in [M(W)(X)(Y)(Z)(N,N)j or [M(X)(Y)(N,N)2] and, also, on the structure of the polypyridine ligand. Empirically, oxidation potentials can be calculated using additive Lever electrochemical parameters which quantify the influence of the metal atom and individual hgands on metal-centered redox couples [9, 157, 220]. 

Spin-orbit coupling in some cases provides a mechanism of relaxing the second selection rule, with the result that transitions may be observed from a ground state of one spin multiplicity to an excited state of different spin multiphcity. Such absorption bands for first-row transition metal complexes are usually very weak, with typical molar absorptivities less than 1 L moF cm For complexes of second-and third-row transition metals, spin-orbit coupling can be more important. 

Luminescence is observed from coordination complexes of first row transition metals less often than from complexes of second or third row metals. This is a result of the presence of low-energy ligand field (LF) states into which the excitation is funneled that can rapidly deactivate through either non-radiative pathways to the ground state or photo-reaction pathways to products. Recent reports relating to the luminescence of titanium," chromium", and zinc complexes provide examples of new emissive first row coordination compounds. 

Photochemistry of Monomeric Complexes of Second- and Third-Row Transition Metals... 

PHOTOREACTIONS OF COMPLEXES OF SECOND-AND THIRD-ROW TRANSITION METALS... 

For dimeric d -d and d -d complexes of second- and third-row transition metals, the energies of the da and pa levels are likely to be sensitive to the separation between the metal centers. The intermetallic separation in Au2(dppm)2 is 2.962(1) A. By comparison, the Au-Au distance in the compound Au2(Cy2PCH2PCy2)2 is 2.935(1) A this compound being an Au2(II) dimer that does not emit at room temperature, but luminesces at 489 in the solid state at 77 For Au2(I) compounds it appears that there may be a poor correlation between the emission energy and the Au...Au separation, and it is therefore uncertain as to whether the intermetallic separation plays any significant role in the emission... 

Earlier, Ahland and coworkers56 noticed that metal and metallic salts which form complexes with Lewis bases can be divided into two classes, namely one which interacts mostly with the donors bearing a first row element as a center and a second which coordinates preferentially with those of second and third row elements. 

SUBSTITUTION STUDIES OF SECOND- AND THIRD-ROW TRANSITION METAL OXO COMPLEXES... 

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