Understanding the reactivity of [WNAr(CH2tBu)2(CHtBu)] (Ar = 2,6-iPrC6H3) with silica partially dehydroxylated at low temperatures through a combined use of molecular and surface organometallic chemistry

Reaction of [WNAr(CH₂tBu)₂(CHtBu)] (Ar = 2,6-iPrC₆H₃) with silica partially dehydoxylated at 200 °C does not lead only to the expected bisgrafted [(SiO)₂WNAr(CHtBu)] species, but also surface reaction intermediates such as [(SiO)₂WNAr(CH₂tBu)₂]. All these species were characterized by infrared spectroscopy, 1D and 2D solid state NMR, elemental analysis and molecular models obtained by using silsesquioxanes. While a mixture of several surface species, the resulting material displays high activity in the olefin metathesis.     

Reaction of isocyanides with iminophosphorane-based carbene ligands: Synthesis of unprecedented ketenimine–ruthenium complexes

The RuC bond of the bis(iminophosphorano)methandiide-based ruthenium(II) carbene complexes [Ru(η⁶-p-cymene)(κ²-C,N-C[P{NP(O)(OR)₂}Ph₂]₂)] (R = Et (1), Ph (2)) undergoes a C–C coupling process with isocyanides to afford ketenimine derivatives [Ru(η⁶-p-cymene)(κ³-C,C,N-C(CNR′)[P{NP(O)(OR)₂}Ph₂]₂)] (R = Et, R′ = Bz (3a), 2,6-C₆H₃Me₂ (3b), Cy (3c); R = Ph, R′ = Bz (4a), 2,6-C₆H₃Me₂ (4b), Cy (4c)). Compounds 3–4a–c represent the first examples of ketenimine–ruthenium complexes reported to date. Protonation of 3–4a with HBF₄ · Et₂O takes place selectively at the ketenimine nitrogen atom yielding the cationic derivatives [Ru(η⁶-p-cymene)(κ³-C,C,N-C(CNHBz)[P{NP(O)(OR)₂}Ph₂]₂)][BF₄] (R = Et (5a), Ph (6a)).     

Reaction of the RuTp(PR3)Cl fragment with alkynols: Formation of carbene,vinylidene, allenylidene,and carbyne complexes

The reaction of RuTp(COD)Cl (1) with PR₃ (PR₃ = PPh₂ⁱPr, PⁱPr₃, PPh₃) and propargylic alcohols HCCCPh₂OH, HCCCFc₂OH (Fc = ferrocenyl), and HCCC(Ph)MeOH has been studied.In the case of PR₃ = PPh₂ⁱPr, PⁱPr₃ and HCCCPh₂OH, the 3-hydroxyvinylidene complexes RuTp(PPh₂ⁱPr)(CCHC(Ph)₂OH)Cl (2a) and RuTp(PⁱPr₃)(CCHC(Ph₂)OH)Cl (2b) were isolated.With PR₃ = PPh₂ⁱPr and HCCCFc₂OH as well as with PR₃ = PPh₃ and HCCCPh₂OH dehydration takes place affording the allenylidene complexes RuTp(PPh₂ⁱPr)(CCCFc₂)Cl (3b) and RuTp(PPh₃)(CCCPh₂)Cl (3c).Similarly, with PPh₂ⁱPr and HCCC(Ph)MeOH rapid elimination of water results in the formation of the vinylvinylidene complex RuTp(PPh₂ⁱPr)(CCHC(Ph)CH₂)Cl (4).In contrast to the reactions of the RuTp(PR₃)Cl fragment with propargylic alcohols, with HCC(CH₂)_nOH (n = 2, 3, 4, 5) six-, and seven-membered cyclic oxycarbene complexes RuTp(PR₃)(C₄H₆O)Cl (5), RuTp(PR₃)(C₅H₈O)Cl (6), and RuTp(PR₃)(C₆H₁₀O)Cl (7) are obtained. On the other hand, with 1-ethynylcyclohexanol the vinylvinylidene complex RuTp(PPh₂ⁱPr)(CCHC₆H₉)Cl (8) is formed. The reaction of the allenylidene complexes 3a–c with acid has been investigated. Addition of CF₃COOH to a solution of 3a–c resulted in the reversible formation of the novel RuTp vinylcarbyne complexes [RuTp(PPh₂ⁱPr)(C–CHCPh₂)Cl]⁺ (9a), [RuTp(PPh₂ⁱPr)(C–CHCFc₂)Cl]⁺ (9b), and [RuTp(PPh₃)(C–CHCPh₂)Cl]⁺ (9c). The structures of 3a, 3b, and 5b have been determined by X-ray crystallography.     

A convenient route to six-membered heterocyclic carbene complexes: Reactions of aminoallenylidene complexes with 1,3-bidentate nucleophiles

Pentacarbonyl dimethylamino(methoxy)allenylidene complexes of chromium and tungsten, [(CO)₅MCCC(NMe₂)OMe] (M = Cr (1a), W (1b)), react with 1,3-bidentate nucleophiles such as amidines and guanidine, H₂N–C(NH)R (R = Ph, C₆H₄NH₂-4, C₆H₄NO₂-3, NH₂), by displacing the methoxy substituent to give exclusively dimethylamino(imino)-allenylidene complexes, [(CO)₅MCCC{NC(NH₂)R}NMe₂] (2a–5a, 2b). Treatment of the chromium complexes 2a–5a with catalytic amounts of hydrochloric acid or HBF₄ gives rise to an intramolecular cyclization. Addition of the terminal NH₂ substituent to the C_α–C_β bond of the allenylidene chain affords pyrimidinylidene complexes 6–9 in high yield. In contrast to the chromium complexes 2a–5a, the corresponding tungsten complex 2b could not be induced to cyclize due to the lower electrophilicity of the α-carbon atom in 2b. The dimethylamino(phenyl)allenylidene complex [(CO)₅CrCCC(NMe₂)Ph] (10) reacts with benzamidine or guanidine similarly to 1a. However, the second reaction step – cyclization to give pyrimidinylidene complexes – proceeds much faster. Therefore, the formation of an imino(phenyl)allenylidene complex as an intermediate is established only by IR spectroscopy. The analogous reaction of 10 with 3-amino-5-methylpyrazole affords, via a formal [3+3]-cycloaddition, a pyrazolo[1,5a]pyrimidinylidene complex 13. Compound 13 is obtained as two isomers differing in the relative position of the N-bound proton (1H or 4H). The related reaction of 10 with thioacetamide yields a thiazinylidene complex and additionally an alkenyl(amino)carbene complex.     

Olefin metathesis for metal incorporation: Preparation of conjugated ruthenium-containing complexes and polymers

Olefin Metathesis for Metal Incorporation (OMMI) was used for the stoichiometric attachment of ruthenium to both small and large polyenes. The dinuclear complexes (PCy₃)₂C1₂RuCH(CHCH)_nCHRu(PCy₃)₂Cl₂ (n = 1, 2), were prepared by reacting 2 equiv. of the Grubbs first-generation catalyst (PCy₃)₂C1₂Ru(CHPh)) with 1 equiv. of the appropriate polyene (1,3,5-hexatriene for n = 1 and 1,3,5,7-octatetraene for n = 2). Use of excess hexatriene led to the formation of the monoruthenium complex (PCy₃)₂C1₂RuCHCH CHCHCH₂. The mono- and di-ruthenium complexes exhibited marked differences in their spectroscopic and electrochemical properties, in addition to their Z–E isomerization rates. Nucleophilic attack of PCy₃ on the end CH₂ of the mono complex was observed, leading to both isomerization and phosphonium products. Extending the OMMI strategy to the second-generation catalyst was also done, despite the reduced initiation rate. The more reactive catalyst (H₂IMes)RuCl₂(CHPh)(3-bromopyridine)₂ allowed for ruthenium incorporation into polyacetylene, leading to the formation of polymers and oligomers with high ruthenium content.     

Molecular structure of caffeine as determined by gas electron diffraction aided by theoretical calculations

The molecular structure of caffeine (3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione) was determined by means of gas electron diffraction. The nozzle temperature was 185 °C. The results of MP2 and B3LYP calculations with the 6-31G⁷ basis set were used as supporting information. These calculations predicted that caffeine has only one conformer and some of the methyl groups perform low frequency internal rotation. The electron diffraction data were analyzed on this basis. The determined structural parameters (r_g and ∠_α) of caffeine are as follows: <r(NC)_ring> = 1.382(3) Å; r(CC) = 1.382(←) Å; r(CC) = 1.446(18) Å; r(CN) = 1.297(11) Å; <r(NC_methyl)> = 1.459(13) Å; <r(CO)> = 1.206(5) Å; <r(CH)> = 1.085(11) Å; ∠N₁C₂N₃ = 116.5(11)°; ∠N₃C₄C₅ = 121. 5(13)°; ∠C₄C₅C₆ = 122.9(10)°; ∠C₄C₅N₇ = 104.7(14)°; ∠N₉–C₄=C₅ = 111.6(10)°; <∠NCH_methyl> = 108.5(28)°. Angle brackets denote average values; parenthesized values are the estimated limits of error (3σ) referring to the last significant digit; left arrow in parentheses means that this parameter is bound to the preceding one.     

Synthesis,molecular structure and reactivity of new π-conjugated diyne with ferrocenyl and phenyl units

New π-conjugated butadiynyl ligand FcC(CH₃)₂Fc′–CC–CC–Ph (L₁) has been synthesized and its reaction with Co₂(CO)₈ has been studied. New clusters [FcC(CH₃)₂Fc′–CC–CC–Ph][Co₂(CO)₆]_n [(1): n = 1; (2): n = 2] and [Fc–CC–CC–Ph][Co₂(CO)₆]_n [(3): n =  1; (4): n = 2] were obtained by the reaction of ligands FcC(CH₃)₂Fc′–CC–CC–Ph (L₁) and Fc–CC–CC–Ph (L₂) with Co₂(CO)₈ respectively and the composition and structure of the clusters and ligands have been characterized by elemental analysis, FTIR, ¹H and ¹³C NMR and MS. The crystal structures of compounds L₁, L₂, 2 and 4 have been determined by X-ray single crystal analysis.     

Nitrogen- and oxygen-bridged bidentate phosphaalkene ligands

An overview is given on synthesis and structures of new bidentate phosphaalkene ligands [(RMe₂Si)₂CP]₂E (E = O, NR, N^?) and (RMe₂Si)₂CPN(R′)PR′′₂. Exceptional properties of these ligands, extending beyond predictable properties of phosphaalkenes are: (i) the NSi bond cleavage of [(iPrMe₂Si)₂CP]₂NSiMe₃ with Au^I and Rh^I chloro complexes under mild conditions leading to binuclear complexes of the 6π-delocalised imidobisphosphaalkene anion [(iPrMe₂Si)₂CP]₂N^?, and (ii) the chlorotropic formation of molecular 1:2 Pd^II and Pt^II metallochloroylid complexes with novel ylid-type ligands [(RMe₂Si)₂CP(Cl)N(R)PR₂]^?, and the transformation of a P-platina-P-chloroylid complex into a C-platina phosphaalkene by intramolecular chlorosilane elimination. Properties of the heavier congeners [(RMe₂Si)₂CP]₂E (E = S, Se, Te, PR, P^?, As^?) and (RMe₂Si)₂CPEPR′′₂ (E = S, Se, Te) are also described.     

Nickel-catalysed asymmetric SN2′ substitution chemistry of Baylis–Hillman derived allylic electrophiles

Unsymmetrical PhCHCH(CH₂X)(CO₂Me) (X = Cl, OAc) undergoes regioselective α-substitution with AlMe₃ to afford (E)-PhCHCH(Et)(CO₂Me) under Ni(acac)₂ catalysis. On the addition of planar chiral Ferrophite ligands [(R)-CpFe(1,2-C₅H₃Ar{P(OR)₂}) (Ar = Ph, 4-CF₃Ph, 3,5-Me₂Ph, 1-naphthyl; (OR)₂ = 1,1′-binaphthylene, 1,1′-biphenylene)] regioselective methylation γ to the leaving group is possible. It is proposed that the bulky Ferrophite ligand leads to an intermediate nickel allyl species Ni^II(Me)(Ferrophite){η³-PhCHCHCH(CO₂Me)} that adopts a configuration whereby the PhCH terminus of the π-allyl and the Ni–Me are syn leading to good regio (up to 6.4:1) and stereo (up to 94% ee) selectivities.     

1,1-Bis(trifluoromethyl)butadiene-1,3—A new fluorinated building block

Although 1,1-bis(trifluoromethyl)butadiene-1,3 (1) reacts with dimethylamine with selective formation of 1,4-adduct [trans-(CF₃)₂CHCHCHCH₂N(CH₃)₂], halogenation of 1 proceeds with predominant formation (>92%) of 1,2-adducts (CF₃)₂CCHCHXCH₂X (X = Cl or Br). Electrophilic conjugated addition of “ClF” or “BrF” to 1 proceeds exclusively with the formation of 1,2-adducts (CF₃)₂CCHCHFCH₂X (major) and (CF₃)₂CCHCHXCH₂F (X = Cl or Br). Difluorocarbene adds selectively to CHCH₂ moiety of 1 forming thermally stable vinylcyclopropane. In Diels-Alder reaction with linear or cyclic dienes (butadienes, cyclopentadiene, cyclohexadiene-1,3) and quadricyclane compound 1 behaves as dienophile providing for the reaction electron-deficient CHCH₂ bond. The relative rate of cycloaddition of 1 and other fluoroolefins to quadricyclane, measured by high temperature NMR, indicates that (CF₃)₂CCH acts as very strong electron-withdrawing substituent. Synthetic utility of products based on 1 was also demonstrated.     

‘Pincer’ pyridyl- and bipyridyl-N-heterocyclic carbene analogues of the Grubbs’ metathesis catalyst

The ‘pincer’ pyridine-dicarbene and bipyridyl-carbene ruthenium benzylidene complexes, Ru(C–N–C)Cl₂(CHPh) and Ru(C–N–N)Cl₂(CHPh), (C–N–C) = 2,6-bis(DiPP-imidazol-2-ylidene)pyridine, (C–N–N) = (DiPP-imidazol-2-ylidene)bipyridine, have been prepared and characterised by spectroscopic and diffraction methods. They exhibit moderate metathesis activity. Non-symmetrical linear tridentate ether-functionalised N-heterocyclic carbene pro-ligands are also described.     

Hemilability and nonrigidity in metal complexes of bidentate P,PS donor ligands

Hemilability and nonrigidity in a series of mixed P,PS donor ligands has been studied in the complexes [Pd(P,PS)Cl₂], [Pd(η³-C₃H₅)(P,PS)][SbF₆], and [Rh(cod)(P,PS)][SbF₆] (P,PS = Ph₂P-Q-P(S)Ph₂). The effect of bite angle, the rigidity of the ligand backbone, and the role of the ancillary ligands are discussed.     

Probing the chemical reactivity of interfaces: Investigation on the interaction of dehydroindigo with Laponite by UV–vis,Raman and infrared spectroscopy

In this work, the interaction between dehydroindigo (an intermediary oxidized form of indigo) and Laponite clay was investigated. Dehydroindigo (DHI) has been detected when indigo is adsorbed by clay minerals, but it is relatively unstable and in the presence of water it turns back into indigo. It is, therefore, important to understand the factors that extend its stability and Laponite was chosen because the small aspect ratio implies in a large amount of silanol groups (SiOH) which would thus favor the DHI interaction through hydrogen bonding.A significant bathochromic shift (65 nm) of the DHI π®π* transition band in the visible region and changes in the relative intensities and position of the Raman bands at 1530, 1378 and 1167 cm⁻¹ assigned to ν(NCCN), δ(CN) and ν(CN) respectively, indicate that the interaction is stronger than expected for the van der Waals and polarization forces involved in the external surfaces interactions with the siloxane groups. It was also observed that DHI presents an enhanced photochemical stability when interacting with Laponite. These results indicate that hydrogen bonding between a DHI nitrogen atom and the −SiOH or MOH groups is mainly responsible for the behavior present in the DHI + Lap system.     

Infrared spectra and UV-induced photochemistry of methyl aziridine-2-carboxylate isolated in argon and xenon matrices

Methyl aziridine-2-carboxylate (MA2C) has been isolated in low temperature argon and xenon matrices and its structure and photochemistry were studied by FTIR spectroscopy. The reactant as well as the main photoproducts were characterized by comparison of their experimental IR spectra with spectra calculated at the DFT(B3LYP)/6-311++G(d,p) level. The theoretical calculations predicted the existence of two low energy MA2C conformers, differing by the orientation of the OCCN dihedral angle. Both conformers were identified in the studied matrices. Both narrowband tunable and broadband UV irradiations of matrix-isolated MA2C yielded isomerization photoproducts resulting from cleavage of the CC and weakest CN bonds of the aziridine ring. Irradiation with UV laser-light at λ = 235 nm resulted in the formation of the E isomer of methyl 2-(methylimino)-acetate (MMIA) and the Z isomer of methyl 3-iminopropanoate (M3IP). Subsequent irradiation at 290 nm led to observation of new bands resulting from E → Z isomerization of MMIA, while bands due to M3IP remained unchanged. The photoproduced Z isomer of MMIA could be subsequently consumed upon higher-wavelength irradiation (λ = 330 nm). The initially produced MMIA conformer was found to obey the nonequilibrium of excited rotamers (NEER) principle. No photoproducts resulting from the cleavage of the strongest CN bond of the MA2C aziridine ring were observed, nor that of methyl 3-aminoacrylate (M3AA), which could in principle be obtained also by cleavage of the weakest CN bond of the MA2C aziridine ring, but would imply a different H-atom migration simultaneous with the ring opening process. These results indicate that both the differential electronic characteristics of the CN bonds of substituted aziridine rings and the type of required H-atom migration are major factors in determining the specific photochemistries of substituted aziridines. Photofragmentation reactions of MA2C were also observed, through identification of various related products, e.g., acetonitrile, methanol, methane, CO and CO₂.     

Cyanate ester resin/graphene nanocomposite: Curing dynamics and network formation

The curing dynamics and network formation of cyanate ester resin/graphene oxide (GO) nanocomposites were studied by means of Differential Scanning Calorimetry (DSC), Fourier Transform Infrared (FT-IR) and Raman spectroscopies. The incorporation of GO into the resin showed a strong catalytic effect on the cure of the resin, especially in the initial stages. Addition of 4 wt.% GO resulted in the decrease of curing temperature significantly about 97 °C. Activation energy of the nanocomposites also maintained at a low level till the end of the cure. The most effective catalytic behavior was observed with 1 wt.% GO. Both FT-IR and Raman spectra revealed that OH group in GO reacted with cyanate group OCN in the resin to form O(CNH)O bond in the early stages of the cure. These results could provide a low temperature curing route for cyanate ester resins with improved curing efficiency.     

Reactions of α-phenylethynyl-trans-β-styryl complexes with isonitriles: hemi-labile alkyne coordination

Treatment of the complex [Ru{C(CCPh)CHPh}Cl(CO)(PPh₃)₂] (1) with one equivalent of CNR(R =^tBu, C₆H₃Me₂-2,6) gives [Ru{C(CCPh)CHPh}Cl(CNR)(CO)(PPh₃)₂]. Addition of a further equivalent of isonitrile and [NH₄]PF₆ leads to the salts [Ru{C(CCPh)CHPh}Cl(CNR)₂(CO)(PPh₃)₂]PF₆ and the mixed species [Ru{C(CCPh) CHPh}(CO)(CN^tBu)(CNC₆H₃Me₂-2,6)(PPh₃)₂]PF₆. The related [Ru{C(CCPh)CHPh}(CN^t(CO)₂     

Continous gradient temperature Raman Spectroscopy identifies flexible sites in proline and alanine peptides

Continuous gradient temperature Raman spectroscopy (GTRS) applies the temperature gradients utilized in differential scanning calorimetry (DSC) to Raman spectroscopy, providing a straightforward technique to identify molecular rearrangements that occur near phase transitions. Herein we apply GTRS and DSC to the solid dipeptides Ala-Pro, Pro-Ala, and the mixture Ala-Pro/Pro-Ala 2:1. A simple change in residue order resulted in dramatic changes in thermal stability and properties. Characteristic Pro vibrations were observed at ∼75 °C higher temperature in Pro-Ala than Ala-Pro. The appearance/disappearance of characteristic vibrational modes with increasing temperature showed that a double peak in the Ala-Pro major phase transition (174–184 °C) was due to a gauche to anti 165° rotation of H₃CC*NH₃ about C*. CH₂ rocking and wagging frequencies present in Pro-Ala were not observed in Ala-Pro. For Ala-Pro, the Ala ⁺NH_3, and Pro COO⁻ sites were flexible whereas the Pro ring moiety was not; since the OCN (C)₂ amide bond is planar the CNC moiety keeps the Pro ring rigid. For Pro-Ala, CH₂ sites in the Pro ring were flexible and the OCNH amide bond is perpendicular to the Pro ring. Since the mass of the Pro ring is significantly larger than the mass of the flexible Ala ⁺NH₃ moiety, Pro-Ala absorbs more thermal energy, corresponding to a higher phase transition temperature (240–260 °C). Ala-Pro, Pro-Ala, and Ala-Pro/Pro-Ala 2:1 exhibited α-helix, β-sheet, α-helix secondary structure conformations, respectively.     

The structure and two-dimensional correlation infrared spectroscopy study of a new 2D polyoxomolybdate complex containing β-octamolybdate linked up by potassium ions: (4,4′-Hbpy)2(K2Mo8O26)

A novel compound, (4,4′-Hbpy)₂(K₂Mo₈O₂₆) 1 (bpy = bipydine), was synthesized by hydrothermal method and characterized by X-ray single analysis, thermalgravimetric analysis, one-dimensional (1D) infrared spectroscopy and two-dimensional (2D) correlation infrared spectroscopy under thermal perturbation. In the compound 1, the [Mo₈O₂₆] units link to potassium ions to form layer structure, and the protonated 4,4′-bpy are linked to chains by hydrogen bonds. The 2D IR correlation spectroscopy study indicates that the intensity changes of MoO, NH and CC stretching vibration are sensitive to the temperature variation, and the intensity changes of asymmetry stretching vibration of the terminal MoO occur prior to that of terminal MoO linked by K atom. At the same time, the peaks of asymmetry stretching vibrations of the terminal MoO and the stretching vibrations of NH split into two peaks respectively in 2D IR correlation spectroscopy.     

Synthesis and mesomorphic investigation of calamitic liquid crystalline system ethyl-[4-(4′-decyloxy)benzoyloxy]-benzoate (4-EDBB): A temperature dependent micro-Raman study and DFT calculations

A new liquid crystalline material containing diester linking group ethyl-[4-(4′-decyloxy)benzoyloxy]-benzoate (4-EDBB) was synthesized. The phase transition temperatures were noted by differential scanning calorimetry (DSC), the texture pattern were observed by polarizing optical microscopy (POM) and temperature dependent Raman study was employed to observe the transitions as well as to understand the molecular rearrangement during phase transition. The transitions were observed with all the three techniques but the Raman signature of crystal → smectic A transition is many fold and more precise and accurate. The correlation between intermolecular interaction and phase behaviour has been discussed using temperature dependence Raman data of CH in-plane bending and CO stretching vibrations. With the help of DFT method the possible dimers of 4-EDBB were optimized and the rotational isomers were also investigated. There exists weak hydrogen bonds at room temperature, which breaks as the temperature is increased causing the CH in-plane bending to shift lower and CO stretching vibrations to shift higher. The discussion of the temperature dependent Raman data reveals that at crystal → smectic A transition as a result of intra-molecular rotation the molecules transform from trans- to cis- conformer.     

Phosphorus-substituted carbene complexes: Chelates,pincers and spirocycles

The syntheses, structural characterizations and reactivity patterns of main group and late transition metal carbene complexes of the bis(phosphoranimino)methandiide, [C(Ph₂PNSiMe₃)₂]²⁻, and the carbodiphosphorane, Ph₃PCPPh₃, are described and compared to previously reviewed early transition metal analogues. Bimetallic spirocyclic aluminum complexes of the former ligand are accessed by spontaneous double deprotonation of the central carbon atom of the parent, CH₂(Ph₂PNSiMe₃)₂, by two equivalents of AlMe₃, whereas the synthesis of platinum complexes requires the intermediacy of the tetralithium dimer, [Li₂C(Ph₂PNSiMe₃)₂]₂, and elimination of LiCl from a metal chloride precursor. In contrast to the early transition metal analogues, which are N,C,N-pincer, Schrock-type alkylidenes, the C,N-chelated platinum complexes are more akin to Fischer carbenes, and their chemistry is dominated by the nucleophilicity of free nitrogen atom and insertions into labile N–Si bonds. Chelated and pincer carbene complexes of rhodium result from single and double orthometallations, respectively, of the phenyl rings in Ph₃PCPPh₃; the latter compounds represent a wholly new class of C,C,C-pincer complexes. Electronic structure calculations show that the metal–carbon interaction in these compounds may be described as a dative, two-electron, C → M σ-bond. The free bis(phosphoranimino)methandiide and carbodiphosphorane ligands, while not having formal six valence electron resonance forms, may be thought of as having “pull–pull” Fischer carbene character, but the metal to which they become coordinated ultimately dictates their chemistry.