The formation of a Fischer-type transition metal complex with a W(CO) 5 fragment is evaluated for the phosphanylcarbenes, Arduengo-type carbenes, and Bertrand-type carbenes by means of quantum chemical investigations at a density functional level with effective core potential methods. Accordingly, the stabilities of the complexes depend strongly on the substitution pattern of the carbenic unit. Amino-substituted carbenes as well as the Arduengo-type carbene form stable transition metal complexes. The stabilities of the complexes decrease for the phosphanylcarbenes and are at a minimum for the hitherto unknown transition metal complexes of the push-pull-type carbene of Bertrand. The matter is analyzed in terms of distortion energies required to bring the carbene units into the geometrical standard state for complexation with the transition metal fragment. The arguments evaluated for the phosphanylcarbenes should hold equally well for other carbenes substituted with electropositive ligands. For the mono-phosphanylcarbenes, η 1 as well as η 2 structures are investigated. For the diphosphanylcarbenes, a new structural type of metal complexes is predicted in which the transition metal fragment is strongly bound to a cyclic structural valence isomer of the carbene.

The 1,4-diphosphabuta-1,3-diene (DPB) ligand as a tool for stabilizing anionic group 13 (B, Al, Ga) and neutral group 14 (C, Si, Ge) cyclic Arduengo-type carbenes is studied by quantum chemical calculations at density functional level. Accordingly, for the former group this ligand is better suited than the corresponding 1,4-diazabuta-1,3-diene (DAB) ligand. It results in larger electron affinities for the corresponding doublet states. For the latter group the DPB ligand yields essentially smaller singlet-triplet separations than the DAB ligand. An exception is the anionic boron compound with relative low singlet stability for both the DAB and DPB ligands.

The five reduced polyoxomolybdate anions [HSiMo 12 O 40 ] 4-(HI β ), [H 2 SiMo 12 O 40 ] 4-(H 2 II β ), [H 5 SiMo 12 O 40 ] 3-(H 5 IV β ), [SiMo 14 O 44 ] 4-(SiMo 14 ), and [Si 2 Mo 28 O 84 (H 2 O) 2 ] 6-(Si 2 Mo 28 ) have been synthesized as tetramethylammonium salts, by hydrothermal reactions, and characterized by IR spectroscopy, redox titrations (HI β , H 2 II β ), and X-ray crystal structure analyses. Crystal data: (TMA) 4 HI β ‚1.5H 2 O, monoclinic space group P2 1 /n, a ) 12.2487-(3) Å, b ) 39.9307(11) Å, c ) 12.4761(3) Å, β ) 116.542(1)°; (TMA) 4 H 2 II β ‚2H 2 O, monoclinic space group P2 1 /n, a ) 12.7930(2) Å, b ) 21.6171(2) Å, c ) 19.7222(2) Å, β ) 96.017(1)°; (TMA) 3 H 5 IV β ‚2H 2 O, monoclinic space group P2 1 /n, a ) 12.7763(5) Å, b ) 20.2277(8) Å, c ) 20.9784(9) Å, β ) 98.037(1)°; (TMA) 4 SiMo 14 , monoclinic space group P2 1 /n, a ) 12.0502(4) Å, b ) 11.7500(4) Å, c ) 19.8001(5) Å, β ) 100.525(1)°; (TMA) 6 Si 2 Mo 28 ‚2H 2 O, monoclinic space group P2 1 /n, a ) 19.3591(2) Å, b ) 13.1119(2) Å, c ) 21.1265(2) Å, β ) 101.124(1)°. HI β , H 2 II β , and H 5 IV β are respectively one-, two-, and four-electron-reduced {SiMo 12 O 40 } anions with the β-Keggin type structure. SiMo 14 and Si 2 Mo 28 are two novel anions containing R-Keggin cores; in SiMo 14the Keggin core is capped by two {MoO 2 } units while in Si 2 Mo 28 two bicapped anions are linked through terminal oxo groups. A comparison of the molecular structures of the five anions shows the role of the protonation, the reduction, and the aggregation of capping {MoO 2 } units on the structural features of the Keggin core. The condensation process is discussed in terms of Lewis-type interactions between reduced, highly charged basic polyanions and reactive {MoO 2 } 2+ units.

The trinuclear ferrocenophane [{Fe(eta(5)-C(5)H(4))(3)}(2)Ga(2)] (3) featuring two sp(2)-hybridized gallium atoms in bridging positions between three ferrocene-1,1'-diyl units represents a novel type of ferrocene derivative. Compound 3 is obtained by thermal treatment of 1,1'-bis(dimethylgallyl)ferrocene (1) in nondonor solvents or in diethyl ether as solvent and subsequent thermal decomplexation. The [1.1]ferrocenophane [{Fe(eta(5)-C(5)H(4))(2)}(2){GaMe}(2)] (2) is an intermediate in the formation of 3. The reaction of 3 with an excess of trimethylgallium leads back to 1 and proves the reversibility of the multistep reaction sequence. Theoretical calculations reveal a carousel-type D(3h) structure for 3. The compound can best be described as being composed of three only weakly interacting ferrocenediyl units covalently connected by gallium atoms without any pi-bond contribution in the Ga--C bonds. Owing to steric constraints 3 cannot be reduced to the dianion 3(2-), which would feature a Ga--Ga bond. Compound 3 represents a stereochemically rigid difunctional Lewis acid allowing the formation of the adducts 3 a-3 d possessing linear donor-aceptor-aceptor-donor arrangements. Crystal structure data for 3 a-3 d show a symmetry-reduced chiral ferrocenophane core (D(3h)-->D(3)). A polymeric rodlike structure is observed for 3 b and 3 d caused by pi-stacking effects (3 b) or by a difunctional donor-acceptor interaction (3 d). In solution, the chirality of the adducts is lost by rapid interconversion of the enantiomers. A cyclic voltammogram of 3 b in pyridine reveals three quasi-reversible oxidation steps at -356, -154, and 8 mV, indicating only weak electron delocalization in the cationic species. The redox potentials of the pyridine adduct 3 b are compared with those of other pyridine-stabilized gallyl-sustituted ferrocene derivatives and with ferrocene itself.

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