Sn and Pb complexes

Complex ions are commonly formed by transition metals, particularly those toward die right of a transition series (MCr — 3oZn in the first transition series). Nontransition metals, including Al, Sn, and Pb, form a more limited number of stable complex ions. 

Silicon-transition metal chemistry is a relatively new area. The work of Hein and his associates (1941) on Sn—Co derivatives established the possibility of forming bonds between a Group IVB metal and a transition element 139), but it was another fifteen years before CpFe(CO)2SiMej 203), the first of many silyl derivatives, was synthesized. The interest in these compounds derives from (1) comparison with the corresponding alkyl- and Ge-, Sn-, and Pb- transition metal (M) complexes, including the role of ir-back-bonding from filled d orbitals of M into empty d orbitals on Si (or other Group IVB metal), and (2) expectation of useful catalytic properties from such heteronuclear derivatives. 

Of the group 14 elements (Si, Ge, Sn and Pb), only tin is known to form a variety of poly(pyrazoIyl)borato alkyl derivatives. For example, the reactions between K[pzTp] and the silicon derivatives Me SiCl4 (n = 1-3) have not given tractable products (115). Similarly, the reaction between K[pzTp] and Me2GeCl2 gives a complex that has been spectroscopically characterized as [pzTp]2GeMe2, but which readily decomposes. 

The term inert pair is often used for the tendency of the 6s2 electron pair to remain formally unoxidized in the compounds of Pb(n) [and also in the case of T1(I) and Bi(m) etc.]. As discussed above, this tendency can be related to relativity. Figure 59 shows the relativistic and non-relativistic valence orbital energies for Sn and Pb. The relativistic increase of the s-p gap leads to a 6s2 inert pair in the case of Pb. However, the situation is more complex if the local geometry at the heavy atom (Pb) is discussed. There are examples for both, stereochemically inactive and stereochemically active s2 lone pairs. 

The single-step electrosynthesis of O-ethylxanthato and N, A -dimethyl dithiocarbam-ato complexes of Sn(IV) and Pb(II) was carried out by using Sn and Pb anodes in acetone... 

Herrmann and coworkers183 reported a series of Cp-manganese carbonyl complexes which bind Ge, Sn and Pb as central atoms linearly coordinated in clusters, to two Mn atoms in one series and trigonal-planar coordinated to three Mn atoms in another series 8 and 9. The group 14 atoms are double-bonded to two Mn atoms in these compounds, or carry one double bond and two single bonds to three Mn atoms. Potentiometric measurements of these compounds show irreversible reductions and oxidation by CV. No products could be isolated from either reduction or oxidation. The exceptionally high oxidation potential of (/i-Pb) r/ -CsHs )Mn(CO)2]2 as compared to the apparently similar Sn compound is noteworthy (Table 15). 

A tabular listing of the Sn and Pb NMR data for the known Zintl complexes Sn4 , Sn/ , Pbg" ", and the mixed element analogs (i.e., Sng.xGox ) are given in Tables 1 and 2. Selected data for various counter cations (i.e., Na, K, etc.) are also given. Full listings can be found in [23, 28]. The spectra of the naked and Ei2 clusters have not been reported as they are only known in the gas phase or, in the case of Pbjo, only recently synthesized [29]. The transition metal derivatives of these clusters will be discussed in a later section. 

Fig. 6. The MS4 core structure in divalent Sn and Pb 1,1-dithio complexes. The heavy lines correspond with the short M-S distances (Table IV).

Some complexes which satisfy this definition are dealt with elsewhere, however. Those with X or Y as the Group V donors P, As or Sb are dealt with in Chapter 14 of this volume. Those with X or Y = Hg are described in Chapter 11 and the Group IV donors Si, Ge, Sn and Pb are included in Chapter 12.2, With very few exceptions this means that the relevant X,Y combinations are pairs derived from nitrogen, oxygen, sulfur or selenium donor groups. However, some specific examples of these combinations also occur elsewhere in this work. Amino acids (generally N—O bidentates) are covered in Chapter 20.2, Schiff base ligands (N—O or N—S) in Chapter 20.1, and several S—O... 

Half-sandwich complexes of the type MesCsEl+X- with El = Ge, Sn and Pb are stable under standard conditions and have been well characterized. However, the corresponding silicon derivatives have only been identified by mass spectrometry. An environment of very low basicity and nucleophilicity will be necessary in order to further stabilize compounds of this type. 

Only the heavier members of Group 14 exhibit the schemes (11) and (12), which require nd orbitals. A number of complex coordination geometries for Sn and Pb have been omitted from Table 6.5. 

Subvalent halides of heavier group 14 elements MX2 (M = Ge, Sn, and Pb X = F, Cl, Br, and I) and their complexes exhibit the following structural features and properties. 

In the dihalides of Ge, Sn, and Pb and their complexes, the metal atom always has one lone pair. The discrete, bent MX2 molecules are only present in the gas phase with bond angles less than 120°. Figure 14.5.1(a) shows the structure of SnCl2, which has bond angle 95° and bond distance 242 pm. 

Cyclopentadienyl complexes of Ge, Sn, and Pb exist in a wide variety of composition and structure, such as half-sandwiches CpM and CpMX, bent and parallel sandwiches Cp2M, and polymeric (Cp2M)x. 

Extensive research over the past decade has greatly increased the number of cluster complexes of Ge, Sn, and Pb. The properties of M-M bonds and structures of these compounds are similar to the C-C bonds and carbon skeletons in organic compounds. Table 14.7.1 lists some of these cluster compounds that have been synthesized, isolated, and characterized by X-ray crystallography. In... 

---