The role of the lone pair of electrons of Pb(II) in determining the coordination geometry is analyzed from crystallographic studies and ab initio molecular orbital optimizations. Of particular interest are factors that contribute to the disposition of ligands around the lead with geometries that are (1) holodirected, in which the bonds to ligand atoms are distributed throughout the surface of an encompassing globe, and (2) hemidirected, in which the bonds to ligand atoms are directed throughout only part of an encompassing globe, i.e., there is an identifiable void in the distribution of bonds to the ligands. The preferred coordination numbers for lead were found to be 4 for Pb(IV) and 4 and 6 for Pb(II). All Pb(IV) structures in the CSD have a holodirected coordination geometry. Pb(II) compounds are hemidirected for low coordination numbers (2−5) and holodirected for high coordination numbers (9, 10), but for intermediate coordination numbers (6−8), examples of either type of stereochemistry are found. A...

Lone Pair Functionality in Divalent Lead Compounds

Multifunctional ligands with podand topology provide intrinsically well-defined coordination geometries. Several subgroups of multidentate ligand systems comprising dual functionality have been developed so far. Recent advances made in this research area reflect its topicality. Work on metal complexes of ambiphilic ligands consisting of Lewis-acidic Group 13 element bridgehead atoms and additional donor functionalities is in the current focus of interest. The intrinsic topology of tetradentate ligands may introduce a fascinating reactivity and interesting electronic properties to the metal complexes. Janus-head type ligands are very promising candidates for the preparation of multimetallic complexes.

Multidentate ligand systems featuring dual functionality

New perspectives in the tailoring of hetero (bi- and tri-) metallic alkoxide derivatives

The organocalcium chemistry developed vastly during the last decade The preparation of the organocalcium compounds via direct synthesis (insertion of Ca into a C-X bond of phenyl halides, Grignard reaction) affords skilful procedures due to the inertia of the calcium metal and the extreme reactivity of the organocalcium derivatives Further suitable preparative methods include metathesis reactions of CaX2 with KR or LiR, metallation reactions of H-acidic substrates, metal-halogen exchange reactions, and transmetallation of heavy main group atoms in their compounds with calcium metal Possibilities to stabilize organocalcium compounds include steric shielding by bulky ligands at the periphery and electronic reduction of the nucleophilicity of the calcium-bound carbanions Selected applications in catalysis such as hydrophosphination are also mentioned Very recent developments and challenges in the preparation of alkaline earth metal(I) compounds are presented as well Concepts to overcome the rather large atomization energies of the metals are discussed



Die Organocalciumchemie zeigte in den letzten zehn Jahren eine sturmische Entwicklung Der Gegensatz zwischen der Reaktionstragheit des Calciummetalls und der enormen Reaktivitat der Organocalcium-Verbindungen verlangt anspruchsvolle praparative Verfahren bei der Direktsynthese der schweren Grignard-Verbindungen (Einschub von Ca in eine C-X-Bindung bei Phenylhalogeniden) Metathesereaktionen von CaX2 mit KR oder LiR, Metallierung H-acider Substrate, Metall-Halogen-Austauschreaktionen und Transmetallierungen von Verbindungen schwerer Hauptgruppenelemente mit Calciummetall stellen weitere geeignete Syntheseverfahren fur calciumorganische Verbindungen dar Moglichkeiten zur Stabilisierung von Organocalcium-Verbindungen schliesen den sterischen Schutz durch Raum fullende Reste an der Molekulperipherie ebenso ein wie die elektronische Reduzierung der Nucleophilie der Calcium-gebundenen Carbanionen Ausgewahlte katalytische Anwendungen wie beispielsweise Hydrophosphanierung werden vorgestellt Die aktuellen Entwicklungen im Bereich der einwertigen Erdalkalimetall(I)-Verbindungen werden ebenfalls diskutiert Synthesekonzepte konnen nur dann isolierbare Molekule liefern, wenn die grose Verdampfungsenthalpie der Metalle aufgebracht und das Ausfallen von Metall verhindert wird

Recent Developments in the Organic Chemistry of Calcium – An Element with Unlimited Possibilities in Organometallic Chemistry?

Recent Trends in Metal Alkoxide Chemistry

Bis(tri-tert-butoxigermanates), -stannates and -plumbates of Mg (1a), Ca (1b, 2b, 3b), Sr (1c, 2c, 3c), Ba (1d, 2d, 3d), Cd (le, 2e, 3e), Eu (1f, 2f, 3f) and Pb (1g, 2g, 3g) have been synthesized by various procedures from starting materials such as sodium tri-tert-butoxigermanate (5) and -plumbate (6) and other analogous compounds. While in 1 g and 2g the lead atom is situated in the middle of the molecule, it can also occupy two different coordination sites as in Pb(OtBu)₃Pb(OtBu)₃Pb (3g) and in Ge(OtBu)₃Pb(OtBu)₃Pb (7). X-ray structure investigations on 1a, 1b, 1e, 1f, 1g, 2b, 2e, 2g, 3c, 3d and 3g reveal that all these molecules have a common ElO₃MO₃El cage unit. This cage unit can be visualized as two ElO₃M trigonal bipyramids sharing the central metal M. Consequently in all structures the central element M is sixfold coordinated by oxygen while the outer metals (El) have three oxygen neighbours. The oxygen atoms are further bonded to tert-butyl-groups, which surround the whole ElO₃MO₃El frame. Four different structure types have been found to exist: a monoclinic type α (1a), an orthorhombic type β (1b, le, 2b, 2e), a rhombohedral type γ (1f, 1g, 3c, 3d, 3g) and a monoclinic type δ (2g). In the β-structure type the molecules are found to exist as three different isomers, depending on the arrangement of the alkoxy groups in a right- or left-handed manner within the El(OtBu)₃ groups (RR, SS and RS). Solution equilibria between these isomers are observed by NMR. In the case of 1e a more detailed picture of this intramolecular motion, attributed to the inversion of the trigonal-pyramidally coordinated oxygen atoms, can be given. CP-MAS-NMR-spectra as well as solution spectra have been obtained for le (¹³C, ¹¹³Cd), 2e (¹³C, ¹¹³Cd, ¹¹⁹Sn), 2g (¹¹⁹Sn, ²⁰⁷Pb), 3g (¹³C). While δ¹¹³Cd values of 2e in solution and in the solid are comparable (92.8 vs 73.7 ppm), the ²J(¹¹⁹Sn/¹¹³Cd) values rise from 39 Hz to 295 Hz. Detailed comparison of bond lengths and angles in the different derivatives shows that the “softness” of the El(OtBu)₃ groups (or the ability of the groups to accommodate a metal atom with respect to its size) follows the order Sn(OtBu)₃ < Ge(OtBu)₃ < Pb(OtBu)₃. From these findings it can be explained, why the lead derivative 2g shows stereochemical activity of the lone pair, while 1g and 3g do not show this effect at room temperature. All compounds except the lead derivatives 1g, 2g, 3g and 7 are colourless.

Alkoxigermanate(II), -stannate(II) und -plumbate(II) zweiwertiger Metallionen, I [1] / Alkoxigermanates(II), -stannates(II) and -plumbates(II) of Two-Valent Metal Ions, I [1]

Alkoxygermanates(II), -stannates(II), and -plumbates(II) of Divalent Metal Ions, II[1]. – Compounds of the Formula M2El2(OtBu)8



By simple salt-exchange processes the starting materials Na2El2(OtBu)6 (El = Ge, Sn, Pb) can be transformed to germanates, stannates and plumbates of divalent magnesium and divalent transition metals. Two types of compounds are formed in these reactions: MEl2(OtBu)6 [El = Ge, M = Mg (1A), Cr (1B), Mn (1C), Zn (1F); El = Pb, M = Mn (3C), M = Zn (3F)] and M2El2(OtBu)8 [El = Ge, M = Co (1d), Ni (1e); El = Sn, M = Mg (2a), Cr (2b), Mn (2c), Co (2d), Ni (2e); El = Pb, M = Co (3d)]. Single-crystal X-ray diffraction studies have been performed on 1C, 1d, 2a, 2b, 2c, 2d, and 2e, and the structures have been solved. In 1C the Mn atom occupies the center of an elongated O6 octahedron, the germanium(II) atoms displaying pyramidal coordination by three oxygen atoms. The central molecular cage can be described as two MnO3Ge trigonal bipyramids sharing the common central Mn atom and being wrapped by tert-butyl groups linked to the oxygen atoms. The other compounds of the MEl2(OtBu)6 formula seem to be isostructural with the exception of 3F, which displays a 1H-NMR spectrum which is not compatible with this structure. All X-ray structures of the compounds M2El2(OtBu)8 show the same feature: to a central M2(OtBu)2 four-membered ring are spirocyclically connected two M(OtBu)2El rings through the common metal atoms M. The structure is completed by the coordination of an exocyclic tert-butoxy group to the terminal El atoms. The metal atoms M are therefore quasi tetrahedrally coordinated while the Ge and Sn atoms are in pyramidal threefold oxygen atom environments. All molecules display an El M M El one-dimensional arrangement. From susceptibility measurements it is apparent, that in the compounds MEl2(OtBu)6 and M2El2(OtBu)8 the transition metal atoms are in high-spin configurations, which is also supported by the UV spectra. Analysis of the structural data of the series 2a–2e reveal important contributions of the electronic environments of the transition metal atoms to the M M and M Sn distances. A qualitative MO description is used to explain these features. Again it has been shown that the, „geometrical softness” of Ge(OtBu)3 and Pb(OtBu)3 is greater than of Sn(OtBu)3, as the former two can accomodate Cr2+ and Mn2+ in a sixfold coordination site by two units, while Sn(OtBu)3 coordinates Cr2+ and Mn2+ with only two alkoxy groups. When 1C and 2d are allowed to react with nonacarbonyldiiron Mn-Ge2(OtBu)6 · 2 Fe(CO)4 (4) and Co2Sn2(OtBu)8 · 2 Fe(CO)4 (5), respectively, are formed. Compound 4 displays presumably five metal atoms in a linear arrangement while 5 has six metallic elements arranged in one dimension. The latter fact has been unambigously proved by an X-ray structure determination.

Alkoxygermanate(II), -stannate(II) und -plumbate(II) zweiwertiger Metall-Ionen, II. Verbindungen des Formeltyps M2El2(OtBu)8

Bis(tri-tert-butoxigernanates), -stannates and -plumbates of Mg, Sr, Ba, Cd, Eu and Pb have been synthesized by various procedures from starting materials such as sodium tri-tert-butoxigermanate and -plumbate and other analogous compounds

Alkoxigermanate(II), -stannate(II) und -plumbate(II) zweiwertiger Metallionen. - I

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Alkoxigermanate(II), -stannate(II) und -plumbate(II) zweiwertiger Metallionen, I [1] / Alkoxigermanates(II), -stannates(II) and -plumbates(II) of Two-Valent Metal Ions, I [1]

Alkoxygermanate(II), -stannate(II) und -plumbate(II) zweiwertiger Metall-Ionen, II. Verbindungen des Formeltyps M2El2(OtBu)8

Alkoxigermanate(II), -stannate(II) und -plumbate(II) zweiwertiger Metallionen. - I