The strength of an R-agostic bond in CH 2 dTiHF has been evaluated using the generalized compliance matrix from HF (Hartree-Fock), MP2 (second-order Møller-Plesset perturbation), CCSD(T) (coupled cluster theory with single and double substitutions with noniterative triple excitations), and several formulations of DFT (density functional theory) in combination with large all-electron basis sets. While HF underestimates the agostic interaction, both MP2 and the local spin density approximation (LSDA) dramatically overestimate the agostic Ti‚‚‚H interaction strength. Hybrid DFT (B3LYP) and pure DFT (BLYP) are in line with the CCSD(T) level of theory. The DFT compliance constants and their coupling values are further used to characterize the agostic C-H‚‚‚W interaction in [W(tCCMe 3 )(dCHCMe 3 )(CH 2 -CMe 3 )(dmpe)] (1; dmpe ) (dimethylphosphino)ethane). The strength of this agostic bond is analyzed as a function of the metal atom and the molecular environment.
The relative stability of different singlet phosphinidenes (R-P) has been investigated by using isodesmic reactions. The energies of these reactions with several R groups were calculated with DFT and ab initio methods at different levels of theory. The best stabilising effect on the phosphinidene centre is exhibited by the R'2C=N-group, resulting in a singlet ground state. The analysis of the electron density in the parent H2C=N-P, indicates a considerable double bond character of the PN bond. Further tuning of the C=N pi-bond polarity is possible by variation of the R' substituents. Using trimethylsilyl substituents or incorporating the carbon atom in a pi-withdrawing pentafulvene ring the stabilization and the computed singlet triplet separation increases. The thermodynamics and the kinetics of dimerisation reactions of the most stabilised R'2C=N-P indicates that these compounds are likely synthetic targets.
Caught in the trap: Two different routes to the thermally unstable phosphinidenoid complex I are described, and chemical evidence for this novel intermediate is provided through selective reactions. For example, methyl iodide, dimethylcyanamide, or butyraldehyde furnished complexes II, III, and IV (see scheme) under very mild conditions.
P-Trityl substituted Li/Cl phosphinidenoid tungsten(0) complex (OC)5W{Ph3CP(Li/12-crown-4)Cl} (3) was prepared via chlorine/lithium exchange in complex (OC)5W{Ph3CPCl2} (2) using (t)BuLi in the presence of 12-crown-4 in tetrahydrofuran (THF) at low temperature; complex 3 possesses significantly increased thermal stability in contrast to previously reported analogue derivatives. Terminal phosphinidene-like reactivity of 3 was used in reactions with benzaldehyde and isopropyl alcohol as oxaphosphirane complex (OC)5W{Ph3CPC(Ph)O} (5) and phosphinite complex (OC)5W{Ph3CP(H)O(i)Pr} (6) were obtained selectively. Reaction of 3 with phosgene allowed to obtain the first kinetically stabilized chloroformylphosphane complex (OC)5W{Ph3CP(Cl)C(O)Cl} (4). Density functional theory (DFT) calculations revealed remarkable differences in the degree of P-Li bond dissociation 3a-d: using a continuum model 3 displays a covalent character of P-Li bond (COSMO (THF)) (a), which becomes elongated if 12-crown-4 is coordinated to lithium (b) and is cleaved if a dimethylether unit is additionally coordinated to lithium (c). A similar result was obtained for the case of 3(thf)4 in which also a solvent-separated ion pair structure is present (d). All products were unambiguously characterized by various spectroscopic means and, in the case of 2 and 4-6, by single-crystal X-ray diffraction analysis. In all structures very long P-C bonds were determined being in the range from 1.896 to 1.955 Å.
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