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Isomeric bis(phosphanyl) carbenium ions have been investigated by theoretical [MP2/6-31G(d,p) level of theory] and experimental means. On the potential energy surface (PES) of phosphorus cations of the formula [P2CH5]+, five stable isomers are found. Their structures follow the stability order (ΔErel/kcal mol−1): P-protonated diphosphiranium, [(H2P)(HP)CH2]+ 1a (0.0) > C-phosphonio phosphaalkene, [H3P–HCPH]+ 2a (4.5) ≫ bis(phosphanyl) carbenium, [CH(PH2)2]+ 3a (18.5) > C-phosphanyl methylenephosphonium, [H2P–CHPH2]+ 4a (22.6) ≫ diphosphirenium, 5a (47.1). The stability order is very different for the family of the four related stable [P2CH(NH2)4]+ ions 1b, 2b, 4b, 5b in which the phosphorus-bound hydrogens (and one carbon-bound hydrogen in the case of 1b) have been replaced by NH2 groups: 2b (−39.6) ≫ 1b (0.0) > 5b (2.2) ≥ 4b (3.7). None of these ions has a structure comparable to the classical amidinium ion [HC(NH2)2]+. While all amino carbenium ions have a positively charged carbon atom, all phosphorus analogues carry a negative charge on carbon. In some, even the p(π)-population exceeds unity suggesting that these should be considered as cationic methanides. Experimentally, a derivative of the diphosphiranium ion 1a could be prepared by methylation of 1,2-bis-(2,4,6-tri-tert-butylphenyl)diphosphirane with MeO3SCF3, and this new compound 8 was characterized by NMR methods. Although the most stable member in this family, it could only be observed under stable ion conditions (T

A synthesis of the new ligand 5-diphenylphosphanyl-10-methyl-5H-dibenzo[a,d]cycloheptene (metroppph) was developed in order to prepare highly distorted tetra-co-ordinated rhodium(+I) complexes. The ligand metroppph contains a cycloheptatriene ring in a rigid boat conformation such that a Ph2P and an olefinic binding site are perfectly arranged for transition metal complexation. Four equivalents of metroppph react with [Rh2(μ-Cl)2(cod)2] in the presence of KPF6 to yield almost quantitatively [Rh(+I)(metroppph)2]PF6, which was isolated in the form of dark red-violet crystals. A crystal structure analysis reveals that the co-ordination sphere of the rhodium centre in [Rh(+I)(metroppph)2]+ deviates strongly from a square planar arrangement (φ = 42°). One methyl group in [Rh(+I)(metroppph)2]+ can be deprotonated by KOBut to give the allyl complex [Rh(allyltroppph)(metroppph)]. This complex has a structure that may be best described as a distorted trigonal bipyramid. The boron hydride [BEt3H]− and organolithium reagents, LiR, react with [Rh(allyltroppph)(metroppph)] in allylic alkylation reactions to yield anionic rhodate complexes [Rh(RCH2troppph)(metroppph)]− (R = H, Me, n-Bu, Ph) that formally have a d10 valence electron configuration. The rhodate [Rh(metroppph)2]− can be obtained directly by reduction of cation [Rh(metroppph)2]+ with alkali metals. In a sym-proportionation reaction [Rh(metroppph)2]+ and [Rh(metroppph)2]− give the neutral d9-[Rh(metroppph)2] radical (K = 1.1 × 107), which is not stable but decomposes with loss of H2 to give the allyl complex [Rh(allyltroppph)(metroppph)]. The structures in solution of [Rh(metroppph)2]+, [Rh(allyltroppph)(metroppph)], and [Rh(nBuCH2troppph)(metroppph)]− were determined by NMR techniques, which reveal that (i) they match the solid state structures and (ii) are rather similar to each other. This fact may explain the remarkable electronic flexibility of the rhodium centre, which changes reversibly its formal oxidation state from + I to 0 to − I at rather low negative potentials (ΔE01 = − 0.882 V; ΔE02 = − 1.298 V).

The acquisition of a complementary heteronuclear 2D NMR experiment with 10-ppm carbon window allows chemists to improve by a factor 20–25 the spectral resolution and determine carbon chemical shifts with five figures from 2D spectra.