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The ²⁹Si chemical shifts in substituted disilynes have been analyzed by quantum-chemical studies using a detailed breakdown of paramagnetic contributions into couplings of occupied and virtual canonical molecular orbitals. The results give indications of substituent effects on shielding and confirm the importance of energy denominators in the equation for σ^p in symmetrical substituted disilynes (H3SiSiSiSiH3 and H3CSiSiCH3) on their chemical shifts. In the unsymmetrical substituted disilyne H3CSiSiSiH3 the energy denominators are identical for both sites and thus cannot explain the widely different chemical shifts. It turns out that the asymmetric charge distribution (reflected in both occupied and virtual MOs and sampled by the PSO matrix elements) allows more efficient couplings at the methyl-substituted silicon side. Furthermore, shielding values in disilynes depend strongly on the size of the trans-bent angle ϕ, as indicated clearly by analyses of the ²⁹Si chemical shifts in the model H3SiSiSiSiH3 at selected values of ϕ. Small changes in ϕ influence the ²⁹Si shielding values appreciably. At lower values of ϕ, couplings between bonding and virtual MOs exhibit stronger deshielding caused by significantly lower energy denominators.

Previous ²⁹Si NMR spectroscopic investigations of various heteroatom-substituted silyllithium compounds revealed “unexpectedly” high silicon chemical shifts. To find explanations for these observations, the ²⁹Si chemical shift tensors of various methoxy-, dimethylamino-, and methylthio-substituted chloro and lithiosilanes have been evaluated by quantum-chemical calculations. Substituent effects on shielding have been analyzed by IGLO-DFT calculations. Popular notions on the influence of atomic charge and substituent electronegativity on the central-atom shifts are not applicable. The combination of very electronegative and electropositive substituents leads to large deshielding contributions in heteroatom-substituted silyllithium compounds. As shown by analyses of localized molecular orbital (LMO) contributions to shielding, this is due to a coupling of bonding and antibonding orbitals with large silicon character and small energy differences. After a comparison of IGLO-LMO results and GIAO-based natural chemical shielding analyses, the LMO interpretations have been augmented further by an inspection of natural bond orbitals, to provide a consistent interpretation of the substituent effects on chemical shifts. The presented results should be transferable also to other heteroatom-substituted p-block main-group compounds.

The reaction of Ph2Si(CH2Li)(CH2NC5H10) (2) (CH2NC5H10 = piperidinomethyl) with trans-[PdCl2(SMe2)2] leads to dimeric [Pd{CH2SiPh2(CH2NC5H10)-κ²C,N}(μ-Cl)]2 (3), where the [CH2SiPh2(CH2NC5H10)-κ²C,N] ligand forms a palladacycle, as revealed by the crystal structure of the transoid isomer. In solution, this chloro-bridged dimer exists as a mixture of the cisoid and the transoid isomer. Monodentate ligands cleave the chloro bridges of dimer 3 to give compounds of the type [Pd{CH2SiPh2(CH2NC5H10)-κ²C,N}ClL] [L = PPh3 (4), PMe3 (5), CN^tBu (6), 4-methylpyridine (4-MePy) (7), tetrahydrothiophene (tht) (8), SMe2 (9)], where the incoming ligand and the metalated carbon center are coordinated cis. The tetrafluoroborate salt of the cationic complex [Pd{CH2SiPh2(CH2NC5H10)-κ²C,N}(4-MePy)2] (11) is obtained by treating the starting dimer 3 with TlBF4 and 4-MePy. The addition of Ph2PCH2CH2PPh2 (dppe) to the starting dimer 3 leads to [{Pd2[CH2SiPh2(CH2NC5H10)-κ²C,N]2Cl2}(μ-dppe)] (13) or to [Pd{CH2SiPh2(CH2NC5H10)-κ²C,N}(dppe)]Cl (11), depending on the stoichiometry. NMR data reveal that the Pd−N bond in 11 is transiently opened in solution. [Pd{CH2SiPh2(CH2NC5H10)-κ²C,N}(dppe)]BF4 (14) is obtained from the latter compound upon treatment with TlBF4. The chelate complexes [Pd{CH2SiPh2(CH2NC5H10)-κ²C,N}(acac)] (16) (acac = acetylacetonate) and [Pd{CH2SiPh2(CH2NC5H10)-κ²C,N}(S2CNEt2)] (15) are obtained by reaction of the starting chloro-bridged dimer 3 with Tl(acac) and NaS2CNEt2, respectively. The solid state structures of palladacycles 3, 4, 8, 9, 10, and 15 were determined by X-ray diffraction methods.

The ²⁹Si chemical shift tensors in both symmetrically and unsymmetrically substituted disilenes have been analyzed by quantum chemical calculations. A detailed breakdown of the paramagnetic contributions to nuclear shielding into individual sum-over-states terms allows new insights into the interrelations between bonding and chemical shifts in these unsaturated compounds. While the substituent effects in symmetrically substituted systems may be understood to a large extent from the energy denominators of the perturbation expression, the very different shifts for the two silicon nuclei in unsymmetrically substituted systems are mainly due to the spatial extent and orientation of the occupied and unoccupied molecular orbitals, as sampled by the matrix elements of nuclear magnetic moments at both silicon sites. The unequal shifts may thus be understood and could serve as a diagnostic tool to distinguish between symmetrically and unsymmetrically substituted disilenes. Furthermore, unequal charge transfer and an alternating charge distribution can rationalize unexpectedly larger pyramidalization of the silyl-substituted silicon center in unsymmetrically substituted systems.

The unusual trigonal prismatic structure of tris(butadiene)molybdenum, reported in 1975 by Skell, has been revisited by extensive quantum chemical calculations and by a low-temperature single-crystal X-ray diffraction study. While a trigonal prismatic coordination arrangement is confirmed by DFT and MP2 structure optimizations, the calculations provide very different bond lengths than earlier crystallographic studies:  Due to appreciable back-bonding, the terminal Mo−C1 bonds are significantly shorter than the central Mo−C2 bonds (ca. 2.29 vs ca. 2.36 Å), and the central (C2−C2A) bonds are actually shorter than the terminal ones (ca. 1.40 vs ca. 1.44 Å), as found previously for substituted complexes. Similar structures have been computed for tris(butadiene)tungsten and for related, substituted systems. A structure redetermination of tris(butadiene)molybdenum at low temperature shows that the erroneous bond lengths obtained previously are due to the presence of a disorder resulting from the superposition of two different orientations of the three butadiene ligands with different site occupation factors. Refinement of this disorder results in a physically more plausible orientation of the anisotropic displacement parameter and gives by a factor of 10 improved estimated standard deviations for the geometrical features. A much better agreement between theory and experiment is attained. It is now obvious that resonance structures involving metallacyclopentene rings contribute significantly to bonding. This conclusion has been confirmed by natural bond orbital/natural resonance theory analyses, which indicate overall larger contributions from metallacyclopentene resonance structures than from traditional resonance structures with π-bonded diolefins. Explanations are provided for the trigonal prismatic structure preferences. MO analyses differ qualitatively and quantitatively from previous work, due to the use of refined structural parameters. Computed ligand NMR chemical shifts agree well with experimental data, provided that they are calculated at the correct structures.

Tris(lithiomethyl)silanes, RSi(CH2Li)3, and tetrakis(lithiomethyl)silane, Si(CH2Li)4, were prepared by the reductive C−S bond cleavage with lithium p,p‘-di-tert-butylbiphenylide (LiDBB) and characterized by trapping with Bu3SnCl. The yields of the isolated trapping products were 42−81% (>95% NMR yield of the crude product), indicating a high-yield synthesis of the corresponding poly(lithiomethyl)silane building blocks. Tetrakis(lithiomethyl)silane is the first compound containing four lithioalkyl groups without any stabilization of the metalated carbon atoms by π-systems. Tetrakis(lithiomethyl)silane was used for the synthesis of 2,2,3,3,7,7,8,8-octamethyl-2,3,5,7,8-pentasilaspiro[4,6]nonane (10), a new spirocyclic disilane. The single-crystal X-ray diffraction study of 10 indicates two five-membered rings in envelope conformation with ecliptically arranged methyl groups connected to a spiro compound by a central silicon atom. DFT geometry optimizations [B3LYP/6-31G(d) level] and NMR calculations (GIAO method, HF/6-31+G(d,p) level) of 10 confirm the observed conformation of 10 as an energetic minimum and support the experimental results.

The heterodinuclear μ-carbonyl complexes [(OC)4M(μ-CO)(μ-dppa)Pt(PPh3)] (3a, M = Mo; 3b, M = W) are formed upon the reaction of [(OC)5M(η¹-dppa)] (1a, M = Mo; 1b, M = W) with [Pt(C2H4)(PPh3)2]. After addition of p-tosylmethyl isonitrile to 3, the CO bridge is replaced by a bridging isonitrile ligand to afford [(OC)4W(μ-C&dbd;NCH2SO2p-tolyl)(μ-dppa)Pt(PPh3)] (4a, M = Mo; 4b, M = W). If stronger electron-donating isonitriles such as benzyl or 2,6-xylyl isonitrile are added to 3, fragmentation into mononuclear complexes occurs. Protonation of 4 leads to the μ-aminocarbyne complexes [(OC)4M{μ-CN(H)CH2SO2p-tolyl}(μ-dppa)Pt(PPh3)][BF4] (5a, M = Mo; 5b, M = W), which react further with isonitriles to yield the μ-aminocarbyne complexes [(OC)3(RNC)W{μ-CN(H)CH2SO2p-tolyl}(μ-dppa)Pt(PPh3)][BF4] 6, (R = 2,6-xylyl, benzyl). Purging a solution of 4a with SO2 yields the complex [(OC)4Mo(μ-SO2)(μ-dppa)Pt(PPh3)], 7a, in which a triphenylphosphine oxide ligand is bound via a hydrogen bridge to the N−H group of the dppa backbone. X-ray diffraction studies performed on 4b and 7a reveal that the μ-CNR and the μ-SO2 ligands bridge the metal centers in an asymmetric manner, the Pt−μ-C or Pt−μ-S distances being significantly shorter than the corresponding W−μ-C or Mo−μ-S distances, respectively.