Weak Σ-donor Research Articles

Relativistic effects on dative carbon-coinage metal bond. Evaluation of NHC-MCl (M = Cu, Ag, Au) from relativistic DFT

The use of different N-heterocyclic carbene (NHC) as dative ligands benefits from their capabilities ranging from strong to weak σ-donor according to the Tolman electronic parameter (TEP), well spread in the formation of monometallic complexes, and recently extended to metallic nanocluster and surfaces. Here, we set to explore the role of relativistic effects in determining the stabilization of the dative carbon–metal bond on coinage metal complexes given by NHC-MCl species (M = Cu, Ag, Au), by quantifying their contribution to the different terms related to the bond formation by comparing scalar relativistic and non-relativistic DFT calculations. Our results show a strong contribution to the stabilization of the NHC-M bond, increasing along with the group, from the lighter Cu atom (7.9%) to Ag (15.3%), and most notoriously for the heavier member, Au (39.9%). This observation is similar along the different stabilizing terms related to the C-M bond, where the acceptor character of the metal center is affected, resulting in a stronger σ-donor interaction from the involved ligand. Thus, the theoretical evaluation of carbon–metal bonds requires the equal footing treatment of relativistic effects along with the group, which is beneficial for the theoretical evaluation of ligand capabilities prior to the engaging exploration of the efficient formation of related complexes involving lighter to heavy metallic centers.

Nitric Oxide Dioxygenation by O2 Adducts of Manganese Porphyrins.

We describe here nitric oxide dioxygenation (NOD) by the dioxygen manganese porphyrin adducts Mn(Por)(η2-O2) (Por2- = the meso-tetra-phenyl or meso-tetra-p-tolylporphyrinato dianions, TPP2- and TTP2-). The Mn(Por)(η2-O2) was assembled by adding O2 to sublimed layers of MnII(Por). When NO was introduced and the temperature was slowly raised from 80 to 120 K, new IR bands with correlated intensities grew concomitant with depletion of the υ(O2) band. Isotope labeling experiments with 18O2, 15NO, and N18O combined with DFT calculations provide the basis for identifying the initial intermediates as the six-coordinate peroxynitrito complexes (ON)Mn(Por)(η1-OONO). Further warming to room temperature led to formation of the nitrato complexes Mn(Por)(η1-ONO2), thereby demonstrating the ability of these metal centers to promote NOD. However, comparable quantities of the nitrito complexes Mn(Por)(η1-ONO) are also formed. In contrast, when the analogous reactions were initiated with the weak σ-donor ligand tetrahydrofuran or dimethyl sulfide present in the layers, formation of Mn(Por)(η1-ONO2) is strongly favored (∼90%). The latter are formed via a 6-coordinate intermediate (L)Mn(Por)(η1-ONO2) (L = THF or DMS) that loses L upon warming. These reaction patterns are compared to those observed previously with analogous iron and cobalt porphyrin complexes.

A SF5 Derivative of Triphenylphosphine as an Electron-Poor Ligand Precursor for Rh and Ir Complexes.

The synthesis of the triarylphosphine, P(p-C6H4SF5)3 containing a SF5 group, has been achieved. The experimental and theoretical studies showed that P(p-C6H4SF5)3 is a weaker σ-donor when compared with other substituted triarylphosphines, which is consistent with the electron-withdrawing effect of the SF5 moiety. The studies also revealed a moderate air stability of the phosphine. The σ-donor capabilities of P(p-C6H4SF5)3 were estimated from the phosphorus-selenium coupling constant in SeP(p-C6H4SF5)3 and by DFT calculations. The behavior of P(p-C6H4SF5)3 as ligand has been investigated by the synthesis of the iridium and rhodium complexes [MCl(COD){P(p-C6H4SF5)3}], [MCl(CO)2{P(p-C6H4SF5)3}2] (M = Ir, Rh), or [Rh(µ-Cl)(COE){P(p-C6H4SF5)3}]2, and the molecular structures of [IrCl(COD){P(p-C6H4SF5)3}] and [Rh(µ-Cl)(COE){P(p-C6H4SF5)3}]2 were determined by single X-ray diffraction. The structures revealed a slightly larger cone angle for P(p-C6H4SF5)3 when compared to other para-substituted triarylphosphines.

Filling the void: controlled donor-acceptor interaction facilitates the formation of an M-M single bond in the zero oxidation state of M (M = Zn, Cd, Hg).

The intriguing question of whether it is possible to form a genuine M0-M0 single bond for the M2 species (M = Zn, Cd, Hg) is addressed here. So far, all the bonds reported in the literature are exclusively MI-MI. Herein, we present viable M2(NHBMe)2 (M = Zn, Cd, Hg; NHBMe = (HCNMe)2B) complexes in which the controlled donor-acceptor interaction leads to an M0-M0 single bond. In these complexes, M2 in the 1∑g ground state with the (nσg+)2(nσu+)2 (n = 7, 10 and 14 for M = Zn, Cd and Hg, respectively) valence electron configuration forms donor-acceptor bonding with singlet 2NHBMe ligands where a combined effect of dominant (+,-) σ-backdonation from the antibonding (nσu+)2 orbital of M2 to the 2NHBMe ligands and a somewhat weaker (+,+) σ-donation from the 2NHBMe ligands to the bonding (n + 1)σg+ orbital leads to the unorthodox bonding situation of forming an M-M single bond in the zero oxidation state by eventually nullifying one effect by another. This is an unprecedented situation in the sense that the NHBMe ligand acts as a strong σ-acceptor and a weaker σ-donor. A comparison with the experimentally reported M2(PhDipp)2 complexes reveals the uniqueness of the NHBMe ligand in exhibiting such a bonding scenario. The M2(NHBMe)2 complex is thermochemically viable with respect to possible dissociation channels at room temperature, except for metal extrusion processes, M2(NHBMe)2 → M + M(NHBMe)2 and M2(NHBMe)2 → M2 + (NHBMe)2. Although the latter two processes are exergonic, they are kinetically protected by a high free energy barrier of 26.5-39.5 kcal mol-1. The experimental characterization of M2(PhDipp)2 despite similar exergonic channels reveals such kinetic stability to be enough for the viability of the M2(NHBMe)2 complexes. Furthermore, the ligand exchange reaction considering M2(PhMe)2 as the starting material also turned out to be feasible. Therefore, the M2(NHBMe)2 complexes are the first cases that feature a neutral M2 moiety with a single M0-M0 covalent bond, where M is a Group 12 metal.

Transition Metal Carbonyl Complexes of an N-Heterocyclic Carbene Stabilized Silyliumylidene Ion.

Silyliumylidene ions-Si(II) cations with a stereochemically active lone pair of electrons-can act as ligands in transition metal complexes comparable to silylenes. However, no investigations concerning their donor abilities have been carried out. Carbonyl complexes lend themselves exceptionally well to determine the donor/acceptor strength of various ligand systems. We now report the synthesis of novel group 6 and group 8 transition metal carbonyl complexes 2-5 of N-heterocyclic carbene (NHC) stabilized silyliumylidene ion 1b, including the first Cr, Mo, and Fe complexes of a Si(II) cation. The complexes were fully characterized by multinuclear NMR and IR spectroscopy and SC-XRD studies. A combination of experimentally determined IR bands together with theoretical calculations revealed weak σ-donor properties and a negligible π-acceptor ability of NHC-stabilized Si(II) cation 1b.

Triazine Annelated NHC Featuring Unprecedented Coordination Versatility

The new N-heterocyclic carbene ligand 3 is presented, which features both a weak σ-donor and a pronounced π-acceptor character. This electronic feature is realized by a side-on annelation of a bis(imidazo)triazine fragment. The coordination chemistry shows the ability of the species to act as a monodentate as well as a chelating and a bridging ligand. The last coordination mode is realized in the solid state of the complex [Rh(cod)(3)]BF4, in which three fragments are assembled around a BF4– fragment via anion−π interactions with the triazine moieties.

Synthesis, structure, and applications of pyridiniophosphines.

A new family of cationic ligands, N-alkyl/aryl pyridiniophosphines, has been synthesized through a short, scalable, and highly modular route. Evaluation of their electronic properties evidenced weak σ-donor and quite strong π-acceptor character when used as ancillary ligands. These attributes confer a substantially enhanced π-acidity to the Pt(II) and Au(I) complexes thereof derived and, as result, they depict an improved ability to activate alkynes towards nucleophilic attack. This superior performance has been demonstrated along several mechanistically diverse Pt(II)- and Au(I)-catalyzed transformations.

Preparation, Crystal Structures, and Behavior in Solution of Cobalt(III) Complexes Containing 2-Cyanoethylphosphines: trans-[Co(Me2dtc)2{P(CH2CH2CN)nPh3−n}]BF4 (n = 1–3; Me2dtc−: N,N-Dimethyldithiocarbamate)

Abstract A series of cobalt(III) complexes containing 2-cyanoethylphosphines, [Co(Me2dtc)2{P(CH2CH2CN)nPh3−n}2]BF4 {Me2dtc−: N,N-dimethyldithiocarbamate; n = 1 (1), 2 (2), and 3 (3)} have been prepared, and their molecular structures in the crystals and in solution have been characterized. X-ray analysis revealed that all complexes isolated as single-crystals were the trans-isomers. The Co–P bond lengths for the 2-cyanoethylphosphines in complexes 1–3 are almost invariant to the number of 2-cyanoethyl substituent groups. Unlike related complexes with other monodentate P-donor ligands, complexes 1–3 in solution showed dissociation and isomerization equilibriums, which were achieved immediately after dissolving the crystals. These characteristic features of the complexes would result from the weak σ-donor and π-acceptor abilities of 2-cyanoethylphosphines. Furthermore, mixing of complex 1 and NaN3 in acetonitrile gave cis-[Co(Me2dtc)2(N3){P(CH2CH2CN)Ph2}] quantitatively, and refluxing the mixture resulted in the phosphine–tetrazolate complex, [Co(Me2dtc)2(Ph2PCH2CH2CN4-κ2P,N1)] (4).

Ligand Effects in Bimetallic High Oxidation State Palladium Systems

Ligand effects in bimetallic high oxidation state systems containing a X-Pd-Pd-Y framework have been explored with density functional theory (DFT). The ligand X has a strong effect on the dissociation reaction of Y to form [X-Pd-Pd](+) + Y(-). In the model system examined where Y is a weak σ-donor ligand and a good leaving group, we find that dissociation of Y is facilitated by greater σ-donor character of X relative to Y. We find that there is a linear correlation of the Pd-Y and Pd-Pd bond lengths with Pd-Y bond dissociation energy, and with the σ-donating ability of X. These results can be explained by the observation that the Pd d(z(2)) population in the PdY fragment increases as the donor ability of X increases. In these systems, the Pd(III)-Pd(III) arrangement is favored when X is a weak σ-donor ligand, while the Pd(IV)-Pd(II) arrangement is favored when X is a strong σ-donor ligand. Finally, we demonstrate that ligand exchange to form a bimetallic cationic species in which each Pd is six-coordinate should be feasible in a high polarity solvent.

Electronic Structure of an Iron-Porphyrin−Nitrene Complex

Middle and late transition metal imido complexes (which may also be viewed as metal-nitrene adducts) are rather rare, especially for square-pyramidal and octahedral coordination geometries. However, an iron(II) porphyrin aminonitrene adduct, denoted here as Fe(Por)(NN), has been known for almost a quarter of a century. Unlike the corresponding S = 1 oxene and S = 0 carbene adducts, Fe(Por)(NN) exhibits an S = 2 ground state. DFT calculations reported herein provide a molecular orbital description of this unusual species as well as a rationale for its S = 2 ground state. The electronic configuration of Fe(Por)(NN) may be described as d(pi)(2)d(xy)(1)d(z(2))(1)d(x(2)-y(2))(1)d(pi')(1), where the z direction corresponds to the Fe-NN axis. The stability and double occupancy of one of the d(pi) orbitals may be attributed to a pi-backbonding interaction with the N-N pi* orbital. The weak sigma-donor ability of the aminonitrene ligand results in a relatively low-energy d(z(2)) orbital and an overall d orbital splitting pattern that engenders a high-spin ground state.

Paired interacting orbitals (PIO) analysis of electronic structures of copper(I) complexes with hydrotris(pyrazolyl)borate ligands

The influence of the hydrotris(pyrazolyl)borate (tpzb) ligand substituents on the electron delocalization of the copper(I) center with carbon monoxide and triphenylphosphine, i.e. the copper(I) carbonyl complexes, [Cu{HB(3,5- iPr 2pz) 3}(CO)] ( 1), [Cu{HB(3,5-Me 2pz) 3}(CO)] ( 2), [Cu{HB(pz) 3}(CO)] ( 3), [Cu{HB(3-CF 3-5-Mepz) 3}(CO)] ( 4), and the copper(I) triphenylphosphine complexes [Cu{HB(3,5- iPr 2pz) 3}(PPh 3)] ( 5), [Cu{HB(3-CF 3-5-Mepz) 3}(PPh 3)] ( 6), are studied by using paired interacting orbitals (PIO) analysis. Carbon monoxide is a weak σ-donor and strong π-acceptor ligand. The Cu(I)–CO interaction is entirely dominated by π back bonding between the two degenerate π∗ orbitals of carbon monoxide and two t 2 type d orbitals of copper(I) ion. This interaction is clearly expressed by two PIOs, PIO-2 and PIO-3, and the strength of this π-acceptability is estimated by the summation of their overlap populations (ΣOP). On the other hand, triphenylphosphine is a strong σ-donor ligand. The Cu(I)–P interaction is dominated by donation from the P atom to the copper(I) ion and expressed by PIO-1, and the strength of this donative interaction ( σ-donationability) is estimated by the overlap population of PIO-1. These results are consistent with experimental data of the C O stretching frequencies by IR spectroscopy and of 13C chemical shifts of 13C O and the broadness of the 31P signals of PPh 3 by 13C and 31P NMR spectroscopy.

Protonation of palladium(II)-allyl and palladium(0)-olefin complexes containing 2-pyridyldiphenylphosphine

The pendant nitrogen atom of the Ph 2PPy ligand in the Pd(II)-allyl complexes [PdCl(η 3-2-CH 3-C 3H 4)(Ph 2PPy)] ( 1) and [Pd(η 3-2-CH 3-C 3H 4)(Ph 2PPy) 2]BF 4 ( 3) has been protonated with methanesulfonic acid to afford the corresponding pyridinium salts [PdCl(η 3-2-CH 3-C 3H 4)(Ph 2PPyH)](CH 3SO 3) ( 1a) and [Pd(η 3-2-CH 3-C 3H 4)(Ph 2PPyH) 2](CH 3SO 3) 2(BF 4) ( 3a). Protonation strongly influences the 1H and 13C NMR spectral parameters of the allyl moieties of 1a and 3a whose signals resonate at lower fields with respect to the parent species indicating that upon protonation Ph 2PPy becomes a weaker σ-donor and a stronger Π-acceptor. The allyl moiety, which in 1 is static, becomes dynamic in 1a, the observed syn–syn and anti–anti exchange being due to deligation of the protonated phosphine from the metal centre. Treatment of complex 3 with diethylamine in the presence of fumaronitrile gives the new Pd(0)-olefin complex [Pd(η 2-fumaronitrile)(PPh 2Py) 2] ( 4) which has been characterized by elemental analysis and NMR spectroscopy. Low temperature protonation of 4 with methanesulfonic acid leads to the bis-protonated species [Pd(η 2-fumaronitrile)(Ph 2PPyH) 2](CH 3SO 3) 2 ( 4a) which is stable only at temperatures <0 °C.

Diorganoruthenium Complexes Incorporating Noninnocent [C6H2(CH2ER2)2-3,5]22- (E = N, P) Bis-Pincer Bridging Ligands: Synthesis, Spectroelectrochemistry, and DFT Studies

The dinuclear complex [(tpy)RuII(PCP-PCP)RuII(tpy)]Cl2 (bridging PCP-PCP = 3,3',5,5'-tetrakis(diphenylphosphinomethyl)biphenyl, [C6H2(CH2PPh2)2-3,5]22-) was prepared via a transcyclometalation reaction of the bis-pincer ligand [PC(H)P-PC(H)P] and the Ru(II) precursor [Ru(NCN)(tpy)]Cl (NCN = [C6H3(CH2NMe2)2-2,6]-) followed by a reaction with 2,2':6',2' '-terpyridine (tpy). Electrochemical and spectroscopic properties of [(tpy)RuII(PCP-PCP)RuII(tpy)]Cl2 are compared with those of the closely related [(tpy)RuII(NCN-NCN)RuII(tpy)](PF6)2 (NCN-NCN = [C6H2(CH2NMe2)2-3,5]22-) obtained by two-electron reduction of [(tpy)RuIII(NCN-NCN)RuIII(tpy)](PF6)4. The molecular structure of the latter complex has been determined by single-crystal X-ray structure determination. One-electron reduction of [(tpy)RuIII(NCN-NCN)RuIII(tpy)](PF6)4 and one-electron oxidation of [(tpy)RuII(PCP-PCP)RuII(tpy)]Cl2 yielded the mixed-valence species [(tpy)RuIII(NCN-NCN)RuII(tpy)]3+ and [(tpy)RuIII(PCP-PCP)RuII(tpy)]3+, respectively. The comproportionation equilibrium constants Kc (900 and 748 for [(tpy)RuIII(NCN-NCN)RuIII(tpy)]4+ and [(tpy)RuII(PCP-PCP)RuII(tpy)]2+, respectively) determined from cyclic voltammetric data reveal comparable stability of the [RuIII-RuII] state of both complexes. Spectroelectrochemical measurements and near-infrared (NIR) spectroscopy were employed to further characterize the different redox states with special focus on the mixed-valence species and their NIR bands. Analysis of these bands in the framework of Hush theory indicates that the mixed-valence complexes [(tpy)RuIII(PCP-PCP)RuII(tpy)]3+ and [(tpy)RuIII(NCN-NCN)RuII(tpy)]3+ belong to strongly coupled borderline Class II/Class III and intrinsically coupled Class III systems, respectively. Preliminary DFT calculations suggest that extensive delocalization of the spin density over the metal centers and the bridging ligand exists. TD-DFT calculations then suggested a substantial MLCT character of the NIR electronic transitions. The results obtained in this study point to a decreased metal-metal electronic interaction accommodated by the double-cyclometalated bis-pincer bridge when strong sigma-donor NMe2 groups are replaced by weak sigma-donor, pi-acceptor PPh2 groups.

Geometrical isomers of bis(benzimidazol-2-ylethyl)sulfide)cobalt(II) diperchlorates: synthesis, structure, spectra and redox behavior of pink-[Co(bbes) 2](ClO 4) 2 and blue-[Co(bbes) 2](ClO 4) 2

1:1 and 1:2 cobalt complexes of bis(benzimidazol-2-ylmethyl)amine (bbma) bis(benzimidazol-2-ylmethyl)sulfide (bbms), bis(benzimidazol-2-ylethyl)sulfide (bbes) and diethylenetriamine (dien) were prepared and their spectral and redox behavior studied. Two geometrical isomers pink-[Co(bbes) 2] 2+ and blue-[Co(bbes) 2] 2+ were obtained when the complexes were prepared by using with bbes and they were separated manually and recrystallized. The octahedral structure of pink-[Co(bbes) 2] 2+ was resolved by X-ray analysis. The electronic spectra show the presence of two geometrical isomers for Co(bbes) 2 2+ in the solid state; for example, the spectral bands of pink-[Co(bbes) 2] 2+ differs markedly with those of blue-[Co(bbes) 2] 2+. This is consistent with the results obtained from magnetic measurements (5.10 BM for pink-Co(bbes) 2 2+ and 4.72 BM for blue-[Co(bbes) 2] 2+). Further, the behavior of the ligands (bbma, bbms, bbes) at different pH conditions was determined on the basis of 13C NMR studies. The redox potentials [Co(II)/Co(I)] of the complexes follow the trend [Co(bbma) 2] 2+ < [Co(bbms) 2] 2+ ≈ [Co(bbes) 2] 2+ which demonstrates the stabilization of the Co(II) ion is more by both weak σ-donor and weak π-acceptor ligands rather than by σ-donor ligand.

Ru(ii) and Rh(iii) porphyrin complexes of primary phosphine-substituted porphyrinsDedicated to the memory of Bhaskar G. Maiya.

Primary alkynyl phosphine porphyrins were prepared by AlHCl2 reduction of the corresponding alkynyl phosphonates. Dephosphorylation of the alkyne proved to be a major side reaction. Using LiAlH4 as reducing agent, the alkyne was found to be partially reduced to give the trans-alkenyl phosphine selectively. The primary phosphines coordinate to both ruthenium(II) and rhodium(III) porphyrins and readily form bis-phosphine complexes. The 1H and 31P NMR spectra for the ruthenium complexes show a pattern characteristic of an [AX2]2 spin system with an unusually large 2JPP coupling constant of 620.6 Hz. The IR spectrum of the complex (PAPH2)Ru(CO)(porphyrin) (PAPH2 = phenylacetylenephosphine) indicates weak σ-donor properties of the ligand. In contrast to the corresponding tertiary phosphine complexes, the bis-phosphine complexes with both ruthenium(II) and rhodium(III) porphyrins are more stable than the mono-phosphine complexes, as judged by NMR spectroscopy, and they can also be detected in the gas phase by LDI-TOF MS. In all cases the complexes could not be isolated and they degrade within hours at ambient temperatures when kept in solution. These compounds may therefore not be suitable for the construction of larger multiporphyrin systems, but their accessibility makes it possible to study their coordination behaviour with other transition metals.

Redox and spin state control of Co(II) and Fe(II) N-heterocyclic complexes

Two series of complexes with the general formulas [Co(L) 2] 2+ and [Fe(L) 2] 2+, where L represents the tridentating ligands 2,2′:6′;2′′-terpyridine (L 1), 6-( N-pyrazolyl)-2,2′-bipyridine (L 2); and 2,6-bis( N-pyrazolyl)pyridine (L 3), were prepared and characterized. The electronic properties of both series of compounds were investigated as a function of stepwise variation in coordinated pyridines relative to pyrazoles. In the cobalt series, the Co(III/II) redox potential measured an average anodic shift of 0.16 V for each pyridine coordination site replaced by pyrazole, while only a slight shift was observed for the Fe(III/II) couple. Solution magnetic susceptibility measurements showed a general shift toward a high-spin ground state as pyridine was replaced by coordinated pyrazoles. The cobalt series exhibited a μ eff of 3.2 B.M. for bis-terpy (L 1) which is close to the spin crossover, while the μ eff for [Co(L 2) 2] 2+ and [Co(L 3) 2] 2+ were measured at 4.5 and 4.6 B.M., respectively. For the iron series, [Fe(L 1) 2] 2+ and Fe(L 2) 2] 2+ exhibited a stable diamagnetic low-spin state; however, [Fe(L 3) 2] 2+ measured a μ eff of 4.6 B.M. which implies a spin equilibrium that is predominantly high-spin with possible low-spin contribution. The observed redox and spin state regulation correlate with the weaker σ-donor and π-acceptor properties of coordinated pyrazole relative to pyridine.

Ligand field properties of histidine on the sharp-line excitation spectrum of trans(imidazole)-bis( L-histidinato- O, N, N′)chromium( III) nitrate

Ligand field calculations have been performd based on the data from the absorption and low temperature sharp-line excitation spectra of trans(imidazole)-[Cr( L-his) 2]NO 3. The angular overlap parameters determined for imidazole nitrogen are 6233 cm −1 for e σNim and −549 cm −1 for e πNim . This indicates that the imidazole nitrogen is a weak σ-donor and its π-interaction can be interpreted by back-donation to the electron withdrawing empty molecular orbitals on the imidazole nitrogen.

Coordination of Some Simple Molecules onto W(CO)5 in the Gas Phase

Abstract In the gas phase at room temperature, the coordination process of NH3, C2H4, CF4, SF6, and Xe onto “naked” W(CO)5, selectively prepared by pulsed 355-nm laser photolysis of W(CO)6, was investigated by laser-based time-resolved infrared absorption spectroscopy. W(CO)5 reacted with these reactant gases, except for CF 4 and SF6, to make a simple coordination complex, W(CO)5(L), where L is NH3, C2H4, or Xe. The bimolecular rate constants for these reactions were (5.6 ± 0.6) × 106 Torr−1 s−1 for W(CO)5 + NH3, (2.8 ± 0.5) × 106 Torr−1 s−1 for W(CO)5 + C2H4, and (1.0 ± 0.3) × 105 Torr−1 s−1 for W(CO)5 + Xe. W(CO)5Xe was unstable owing to its weak interaction with the coordinatively unsaturated tungsten center, the unimolecular decomposition rate of which was about 1.0 × 105 s−1, but W(CO)5(NH3) and W(CO)5(η2-C2H4) were stable under our experimental conditions. The band shifts of C–O stretching modes observed in the coordination of NH3 as a typical σ-donor, C2H4 as a typical π-acceptor, and Xe as a weak σ-donor could interpreted by MO considerations.

σ and π-interactions of the pyrrolic ligand of sandwich-like lanthanide phthalocyanines determined from magnetic susceptibility and ligand-field theory

The magnetic susceptibility data of praseodymium, neodymium, gadolinium, holmium and erbium phthalocyanines taken in the 80–300 K or 4.2–300 K ranges are reported. The σ- and π-interactions of the pyrrolic nitrogen ligand with the praseodymium, neodymium, holmium and erbium f-orbitals are obtained from angular overlap model ligand-field calculations including the full ground-state manifold. The position of the pyrrolic nitrogen ligand in the metallic two-dimensional spectrochemical series is reported. The results show that the pyrrolic nitrogen is a weak σ-donor and a moderate π-donor ligand. The cross-term AOM parameter e πσ is necessary in order to get good fits.

Factors influencing linkage isomer preference in palladium(II)-chlorinilate complexes

The factors influencing linkage isomer preference in the bonding between palladium(II) and the chloranilate dianion (CA 2−) are addressed through the synthesis and spectroscopic characterization of 16 [Pd(CA)L 2] compounds, where L 2 = 2Cl −, 2CH 3CN, 2H 2O, ox 2−, acac −, 2dmso, 2SCN −, 2F −, 2Br −, bpy, tcnq, 2CN −, CA 2− and cod. The (CCA) linkage isomer containing chloranilate ligated as dicarbanion through CCl carbon atoms is favored in the presence of weak σ-donor ligands towards the soft Pd II center (all L 2 groups except dmso, bpy and CN −), while strong σ-donor invariably give (π-CA) products in which the conventional p-quinonoid form of CA 2− is present. [Pd&(CCA)L 2] compounds are characterized by three infrared characteristics: (1) several strong CO vibrations in the 1620–1720 cm −1 interval but no 1500–1550 cm −1 CO mode; (2) 1–4 medium-intensity CC stretching bands between 1150 and 1220 but no CC feature near 1350 cm −1; and (3) a relatively high-energy CCl vibration, i.e. > 855 cm −1. Distinct C-to_Pd II LMCT transitions in the vicinity of 310 and 230–260 nm indicate that the non-equivalence of PdC bonds first demonstrated by the X-ray crystal structure of K 2[Pd(CCA)Cl 2]·4H 2O is a general feature of the (CCA) series. [Pd(CCA)L 2] complexes are surprisingly susceptible to hydrolysis, affording [Pd(CCA)(H 2O) 2] within several minutes in weakly acidic solution (pH 3.7). A kinetic trans effect linked to the strenghtening of Pd-chloranilate C bonds by the departure of competing σ donor ligands is suggested.