Ligand Bond Distances Research Articles

Modular synthesis of diphosphoramidite ligands derived from 1,8,10,9-triazaboradecalin and their complexes with Ni, Pd, and Pt

Here we report a convenient synthesis of new diphosphoramidite ligands derived from 1,8,10,9-triazaboradecalin (TBD) and describe their complexes with group 10 metals. Treating the chlorinated and structurally characterized ligand precursor ClTBDPhos (L1) with four equivalents of HOR (R = C3H7 or C3HF6) in the presence of NEt3 yielded the diphosphoramidite ligands iPrOTBDPhos (L2) and F-iPrOTBDPhos (L3) in good yields. L2 and L3 were used to prepare a variety of Ni, Pd, and Pt complexes with chloride and 1,2-benzenedithiolate ligands so their structures and spectroscopic properties could be compared to similar complexes with methoxy-substituted MeOTBDPhos such as (MeOTBDPhos)PdCl2, which is reported here for the first time. Single-crystal XRD studies on the (ROTBDPhos)PtCl2 complexes revealed that increasing the size of alkoxy substituents from MeO to iPrO to F-iPrO decreases the P-M-P bite angle from 97.47(3)° in (MeOTBDPhos)PtCl2 to 93.98(4)° in (F-iPrOTBDPhos)PtCl2. Similar changes were observed in the dithiolate complexes (iPrOTBDPhos)Pt(S2C6H4) and (F-iPrOTBDPhos)Pt(S2C6H4), and the structural studies revealed longer Pt-P bond distances compared to the dichloride complexes that correlated to ca. 1000 Hz decrease in their 195Pt-31P coupling constants. No significant changes were observed in the ligand bond distances in complexes containing the methoxy and isoproxy-substituted ligands, but complexes with fluorinated F-iPrOTBDPhos revealed subtle, but significant differences in their P-N, P-O, and B-N distances that reflect substituent-induced electronic changes in the ligand. Overall, this work establishes a more convenient synthetic entry into the chemistry of alkoxy-substituted TBDPhos ligands for ongoing studies with these and related transition metal complexes.

(Invited) X-Ray Spectroscopic View on Transition Metal-Based Electrocatalysts Structure

Significant progress has been made in replacing noble metal catalysts for the oxygen evolution and reduction reactions with materials based on transition metal (TM) compounds such as oxides1, perovskites2, molecular nitrogen-carbon catalysts, etc.3 However, the design of stable and active TM-based catalysts is jeopardized by the lack of understanding of the active sites structure, the reaction mechanism as well as the degradation pathway of these materials.4,5 In this perspective, soft and hard X-ray-based spectroscopies are used as state-of-the art methods for the analysis of the catalyst structure, namely, oxidation state of its components, their electronic structure, coordination number, metal – ligand bond distance, etc. Due to the constant instrumentation development, both soft and hard X-rays have been successfully applied for in situ and operando spectroscopic studies of electrocatalysts providing new insights on the behavior of TM-based materials.6–8 This talk will be focused on the application of X-ray Photoelectron, Absorption and Emission Spectroscopies (XPS, XAS and XES, respectively) towards the investigation of TM-based electrocatalysts. Some examples will be discussed illustrating the possibilities of each of these methods both ex and in situ to analyze TM-based electrocatalysts (where TM = Mn, Co, Fe) used in fuel cells and electrolyzers. The talk will also cover possible sources of error arising during data acquisition and subsequent treatment. Additionally, there will be a discussion of the pros and cons of the various designs of spectroelectrochemical cells currently used for in situ studies of fuel cells and electrolyzers catalysts.References Fabbri, E., Habereder, A., Waltar, K., Kötz, R. & Schmidt, T. J. Developments and perspectives of oxide-based catalysts for the oxygen evolution reaction. Catal. Sci. Technol. 4, 3800–3821 (2014).Grimaud, A. et al. Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution. Nat. Commun. 4, 2439–2446 (2013).Banham, D. & Ye, S. Current Status and Future Development of Catalyst Materials and Catalyst Layers for Proton Exchange Membrane Fuel Cells: An Industrial Perspective. ACS Energy Lett. 2, 629–638 (2017).Hu, C., Zhang, L. & Gong, J. Recent progress made in the mechanism comprehension and design of electrocatalysts for alkaline water splitting. Energy Environ. Sci. 12, 2620–2645 (2019).Hong, W. T. et al. Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis. Energy Environ. Sci. 8, 1404–1427 (2015).Saveleva, A. V. A. et al. Potential-induced spin changes in Fe/N/C electrocatalysts assessed by in situ X-ray emission spectroscopy. Angew. Chemie Int. Ed. 133, 1–7 (2021).Abbott, D. F. et al. Operando X-ray absorption investigations into the role of Fe in the electrochemical stability and oxygen evolution activity of Ni 1−x Fe x O y nanoparticles. J. Mater. Chem. A 6, 24534–24549 (2018).Zitolo, A. et al. Identification of catalytic sites in cobalt-nitrogen-carbon materials for the oxygen reduction reaction. Nat. Commun. 8, 957–968 (2017).

Coordination of a Neutral Ligand to a Metal Center of Oxohalido Anions: Fact or Fiction?

Can a neutral ligand bond to a metal center of a square pyramidal oxohalido anion at the available sixth octahedral position? Crystal structures of some compounds indeed suggest that ligands, such as THF, pyridine, H2O, NH3, and CH3CN, can interact with the central metal atom, because they are oriented with their heteroatom toward the metal center with distances being within the bonding range. However, this assumption that is based on chemical intuition is wrong. In-depth analysis of interactions between ligands and oxohalido anions (e.g., VOX4–, NbOCl4–) reveals that the bonding of a neutral ligand is almost entirely due to electrostatic interactions between the H atoms of a ligand and halido atoms of an anion. Furthermore, ab initio calculations indicate that the ligand–VOF4– interactions represent only about one-quarter of the total binding of the ligand within the crystal structure, whereas the remaining binding is due to crystal packing effects. The current study therefore shows that relying solely on the structural aspects of solved crystal structures, such as ligand orientation and bond distances, can lead to the wrong interpretation of the chemical bonding.

Origin of core-to-core x-ray emission spectroscopy sensitivity to structural dynamics.

Recently, coherent structural dynamics in the excited state of an iron photosensitizer was observed through oscillations in the intensity of Kα x-ray emission spectroscopy (XES). Understanding the origin of the unexpected sensitivity of core-to-core transitions to structural dynamics is important for further development of femtosecond time-resolved XES methods and, we believe, generally necessary for interpretation of XES signals from highly non-equilibrium structures that are ubiquitous in photophysics and photochemistry. Here, we use multiconfigurational wavefunction calculations combined with atomic theory to analyze the emission process in detail. The sensitivity of core-to-core transitions to structural dynamics is due to a shift of the minimum energy metal–ligand bond distance between 1s and 2p core-hole states. A key effect is the additional contraction of the non-bonding 3s and 3p orbitals in 1s core-hole states, which decreases electron–electron repulsion and increases overlap in the metal–ligand bonds. The effect is believed to be general and especially pronounced for systems with strong bonds. The important role of 3s and 3p orbitals is consistent with the analysis of radial charge and spin densities and can be connected to the negative chemical shift observed for many transition metal complexes. The XES sensitivity to structural dynamics can be optimized by tuning the emission energy spectrometer, with oscillations up to ±4% of the maximum intensity for the current system. The theoretical predictions can be used to design experiments that separate electronic and nuclear degrees of freedom in ultrafast excited state dynamics.

Quantifying the Interdependence of Metal–Ligand Covalency and Bond Distance Using Ligand K‐edge XAS

AbstractBond distance is a common structural metric used to assess changes in metal–ligand bonds, but it is not clear how sensitive changes in bond distances are with respect to changes in metal–ligand covalency. Here we report ligand K‐edge XAS studies on Ni and Pd complexes containing different phosphorus(III) ligands. Despite the large number of electronic and structural permutations, P K‐edge pre‐edge peak intensities reveal a remarkable correlation that spectroscopically quantifies the linear interdependence of covalent M−P σ bonding and bond distance. Cl K‐edge studies conducted on many of the same Ni and Pd compounds revealed a poor correlation between M−Cl bond distance and covalency, but a strong correlation was established by analyzing Cl K‐edge data for Ti complexes with a wider range of Ti−Cl bond distances. Together these results establish a quantitative framework to begin making more accurate assessments of metal–ligand covalency using bond distances from readily‐available crystallographic data.

Quantifying the Interdependence of Metal-Ligand Covalency and Bond Distance Using Ligand K-edge XAS.

Bond distance is a common structural metric used to assess changes in metal-ligand bonds, but it is not clear how sensitive changes in bond distances are with respect to changes in metal-ligand covalency. Here we report ligand K-edge XAS studies on Ni and Pd complexes containing different phosphorus(III) ligands. Despite the large number of electronic and structural permutations, P K-edge pre-edge peak intensities reveal a remarkable correlation that spectroscopically quantifies the linear interdependence of covalent M-P σ bonding and bond distance. Cl K-edge studies conducted on many of the same Ni and Pd compounds revealed a poor correlation between M-Cl bond distance and covalency, but a strong correlation was established by analyzing Cl K-edge data for Ti complexes with a wider range of Ti-Cl bond distances. Together these results establish a quantitative framework to begin making more accurate assessments of metal-ligand covalency using bond distances from readily-available crystallographic data.

Synthesis and Characterization of Cobalt(II) N,N'-Diphenylazodioxide Complexes.

Removal of chloride from CoCl2 with TlPF6 in acetonitrile, followed by addition of excess nitrosobenzene, yielded the eight-coordinate cobalt(II) complex salt [Co{Ph(O)NN(O)Ph}4](PF6)2, shown by single-crystal X-ray analysis to have a distorted tetragonal geometry. The analogous treatment of the bipyridyl complex Co(bpy)Cl2 yielded the mixed-ligand cobalt(II) complex salt [Co(bpy){Ph(O)NN(O)Ph}2](PF6)2, whose single-crystal X-ray structure displays a trigonal prismatic geometry, similar to that of the iron(II) cation in the previously known complex salt [Fe{Ph(O)NN(O)Ph}3](FeCl4)2. The use of TlPF6 to generate solvated metal complex cations from chloride salts or chlorido complexes, followed by the addition of nitrosobenzene, is shown to be a useful synthetic strategy for the preparation of azodioxide complex cations with the noncoordinating, diamagnetic PF6– counteranion. Coordination number appears to be more important than d electron count in determining the geometry and metal–ligand bond distances of diphenylazodioxide complexes.

Factors governing when a metal-bound water is deprotonated in proteins.

Understanding when a metal-bound water molecule in a protein is deprotonated is important as this affects the charge distribution in the metal-binding/enzyme active site and thus their interactions, the enzyme mechanism, and inhibitor design. The protonation state of the metal-bound water molecule at a given pH depends on its pKa value, which in turn depends on the properties of the cation, its ligands, and the protein environment. Here, we reveal how and the extent to which (i) the first-shell composition (type, charge, and number of ligands), (ii) the metal site's immediate surroundings (first-shellsecond-shell hydrogen-bonding interactions, metal-ligand distance constraints, and ligand-binding mode) and (iii) the protein architecture and coupled solvent interactions (long-range electrostatic interactions and solvent exposure of the site) affect the Zn2+-bound water pKa. The results, which are consistent with available experimental pKa values of Zn2+-bound water, provide guidelines to predict when Zn2+-bound water would likely be deprotonated at physiological pH.

Uncertainty quantification in theoretical spectroscopy: The structural sensitivity of X‐ray emission spectra

AbstractWe present a methodology for analyzing the dependence of molecular spectra calculated with quantum‐chemical methods on the underlying molecular structure. This analysis is applied to investigate the structural sensitivity of calculated valence‐to‐core X‐ray emission (VtC‐XES) spectra for the test case of three iron carbonyl complexes, Fe(CO)5, [FeCp(CO)2(THF)]+ (Cp = cyclopentadienyl, THF = tetrahydrofuran), and Fe(CO)3(cod) (cod = cyclooctadienyl). Based on this analysis, we discuss how the VtC‐XES spectra depend on changes of metal–ligand bond distances and bond angles as well as on the structure of the ligands. The benefits of such an analysis of the structural sensitivity are discussed. Our methodology can serve as a first step toward quantifying and accounting for uncertainties due to the underlying model structure in theoretical spectroscopy.

Coordination Modes of Hydroxamates in Neptunium (V) Complexes in Aqueous Solution

AbstractA theoretical study to understand molecular structures and coordination modes of neptunium complexes with three different substituent hydroxamates viz., acetohydroxamate (AHA), benzhydroxamate (BHA) and salicylhydroxamate (SHA) was carried out using density functional theory (DFT). The geometries for different complexes reported in literature MxLyHz (x:y:z) viz 1:1:1, 1:2:2 and 1:2:0 were optimized and interaction energies (ΔE) for complexation process were calculated to obtain the most probable structures of complexes. The ΔE value revealed, the complex stability order as NpO2+‐SHA > NpO2‐AHA > NpO2‐BHA which is in agreement with the reported experimental data. Higher negative charge on AHA compared to BHA leads to its higher interaction with NpO2 which in turn reflects in shorter Np‐Oeq (ligand) bond distances. In 1:1:1 complex of NpO2‐SHA, the binding mode involving ortho hydroxyl group and ‐NHO− oxygen atom (seven membered ring) shows most negative interaction energy.However, for binding of second ligand in 1:2:2 NpO2‐SHA complex, steric effects dominate and the mode involving five membered ring through oxygen atoms of ‐CONHO− is most stable.

DFT and TD-DFT study on structures, related energies, frontier molecular orbitals and UV–Vis spectra of [M(Tp)(PPh3)(Cl)(L)] (M = Ru and Fe; L = C3H4N2 and C13H11N)

Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) with the DZVP/DZVP2 all-electron mixed basis sets are used to study the related energies, structures, frontier molecular orbitals and UV–Vis spectra for [M(Tp)(PPh3)(Cl)(L)] (M=Ru and Fe; L=C3H4N2 and C13H11N). The related energies between the singlet state (low-spin) and nonet state (high-spin) for these complexes are reported. Because of the low related energies, these complexes are expected to be in the singlet state (low-spin). The calculated structural parameters for complexes 1 and 3 (Ru-based) are in very good agreement with the experimental values, and the geometries of complexes 2 and 4 (Fe-based) have been studied as well. The metal–ligand bond distances for the Fe-based complexes are predicted to be slightly shorter than those of the Ru-based complexes due to the small spatial extent of the 3d wave functions of the Fe atom. These complexes display a HOMO of metal d and π(Cl) orbitals in character, and the LUMO is contributed by metal d and π∗(PPh3) or π∗(C13H11N) orbitals. The HOMO–LUMO energy gap (ΔEL–H) can be reduced by a π-electron rich ligand (such as C13H11N) and a low electronegativity metal atom (such as Fe). A π-electron rich ligand (such as C13H11N) can increase the electron accepting ability, which leads to more electrons being pumped into the π∗(PPh3) and π∗(C13H11N) orbitals and results in a red-shift and intensity-enhanced absorption in the UV–Vis spectrum. The UV–Vis absorption intensity can be enhanced by solvent (such as CH3OH) as well as resulting in a blue-shift, which suggests that it is due to the polarizability and dielectric strength of the solvent. Owing to the low electronegativity of the Fe atom, a red-shift occurs in the UV–Vis spectra for complexes 2 and 4. The primary absorption features for complexes 1 and 3 are attributed to MLCT/LLCT transitions; on the other hand, a MLCT transition results in the primary absorption features for complexes 2 and 4. Our results show that Ru can be replaced by the inexpensive Fe as the photo-sensitizer. In addition, these Ru- and Fe-based complexes are good candidates for photo-sensitizers in DSSCs due to rich absorption bands and strong absorption intensities in the visible region.

Nephelauxetic Effects in the Electronic Spectra of Pr3+

The well-known nephelauxetic series of ligands describes the change in interelectronic repulsion of the central metal ion, which is reduced on going from the vapor to crystalline state. This study examines the trends and quantifies the mechanism of this series for the lanthanide ion Pr(3+), with the 4f(2) electronic configuration. A new and concise measurement by a single parameter, σee, is introduced to quantify the overall strength of interelectronic repulsion, as the alternative to the Slater parameters, F(k) (k = 2, 4, 6). Energy parameters have been derived from the literature electronic spectra of Pr(3+), in the free ion and in various crystalline hosts, with new calculations in some cases. It is found that at least the first 12 of the 13 multiplet terms of Pr(3+) must be well-determined to obtain reliable parameter values. The shifts of various energy levels for changes in the Slater parameters are not uniform in direction. For the various Pr(3+) solid-state systems, the change in σee is only up to ∼5%, with the magnitude of σee in the order F(-) > Cl(-) > O(2-) ≈ Br(-) > C, and decreasing with lower coordination number of the ligand. The decreases of the Slater parameters from the free ion values are reasonably estimated by considering the dielectric constant of the medium. In particular, the magnitude of σee (and of the spin-orbit coupling constant) is proportional to the polarizability of the ligand for F(-), Cl(-), O(2-), and Br(-). The data point scatter for oxide systems is accounted for by considering the individual ligand bond distances. A fair estimation of nephelauxetic effects can be made from some luminescence transition energies, by contrast with Eu(3+) systems where crystal field effects also play a major role. In conclusion, the nephelauxetic effect of Pr(3+) is due to the polarization of the ligand by one 4f electron, and the interaction of the other electron with the induced multipolar moments, of which the dipole moment dominates.

Investigating the Effects of Basis Set on Metal–Metal and Metal–Ligand Bond Distances in Stable Transition Metal Carbonyls: Performance of Correlation Consistent Basis Sets with 35 Density Functionals

Density functional theory (DFT) is a widely used method for predicting equilibrium geometries of organometallic compounds involving transition metals, with a wide choice of functional and basis set combinations. A study of the role of basis set size in predicting the structural parameters can be insightful with respect to the effectiveness of using small basis sets to optimize larger molecular systems. For many organometallic systems, the metal-metal and metal-carbon distances are the most important structural features. In this study, we compare the equilibrium metal-ligand and metal-metal distances of six transition metal carbonyl compounds predicted by the Hood-Pitzer double-ζ polarization (DZP) basis set, against those predicted employing the standard correlation consistent cc-pVXZ (X = D,T,Q) basis sets, for 35 different DFT methods. The effects of systematically increasing the basis set size on the structural parameters are carefully investigated. The Mn-Mn bond distance in Mn2(CO)10 shows a greater dependence on basis set size compared to the other M-M bonds. However, the DZP predictions for re(Mn-Mn) are closer to experiment than those obtained with the much larger cc-pVQZ basis set. Our results show that, in general, DZP basis sets predict structural parameters with an accuracy comparable to the triple and quadruple-ζ basis sets. This finding is very significant, because the quadruple-ζ basis set for Mn2(CO)10 includes 1308 basis functions, while the equally effective double-ζ set (DZP) includes only 366 basis functions. Overall, the DZP M06-L method predicts structures that are very consistent with experiment.

Synthesis and Characterization of a Bis(imino)-N-heterocyclic Carbene Analogue to Bis(imino)pyridine Iron Complexes

New bis(imino)-N-heterocyclic carbene ligands have been synthesized, and the corresponding iron complexes have been isolated and characterized. Whereas imidazole-derived complexes exhibited exclusively bidentate binding modes, 4,5,6-trihydropyrimidylidene-based ligands adopted a tridentate pincer conformation analogous to complexes of 2,6-bis(imino)pyridines. Bonding in the five-coordinate bis(imino)-N-heterocyclic carbene complex displayed considerably contracted iron–ligand bond distances in comparison to those in the analogous bis(imino)pyridine iron complex. Of particular note was an extraordinarily short iron–carbene carbon bond distance (1.812(2) A). In addition to demonstrating differences in bonding, the isoelectronic structures have different electrochemical properties that reflect the ability of the carbene to stabilize both oxidized and reduced forms of the parent organometallic species.

Accurate molecular structure of nickel phthalocyanine (NiN8C32H16): Gas-phase electron diffraction and quantum-chemical calculations

The molecular structure of the nickel phthalocyanine (NiPc) was determined using the combination of gas-phase electron diffraction (GED), mass spectrometry and quantum chemical calculations. The DFT calculations with employing different functionals and basis sets found the molecular structure with D4h symmetry that agrees satisfactorily to the one found in experiment at 776±5K. The important bond lengths and bond angles according to GED (total errors in parentheses) are: rh1(Ni–N)=1.913(5)Å, rh1(N–Cp)=1.385(4)Å, rh1(Cp–Nm)=1.327(5)Å, rh1(Cp–Cp)=1.462(6)Å, rh1(Cp–C)=1.399(3)Å, rh1(C–C)=1.395(3)Å, ∠NiNCp=126.6(2)°, ∠NCpNm=128.4(4)°, ∠CpNmCp=120.1(5)°, ∠NCpCp=110.0(4)°, ∠CpCpC=121.2(1)°. The nickel–nitrogen bond distance obtained is by 0.04Å longer than the one found for this compound in the literature. The structures of the crystalline and gaseous NiPc complex are compared. The effect of the metal (Ni, Cu, Zn) on structural features of the MPc complexes is discussed. Using NBO-analysis, the correlation between the metal–ligand bond distance and the total energy of donor–acceptor orbital interactions per one M–N bond was found in the MPc complexes.

Synthesis, characterization and crystal structures of new bidentate Schiff base ligand and its vanadium(IV) complex: The catalytic activity of vanadyl complex in epoxidation of alkenes

The Schiff base ligand N-salicylidin-2-bromoethylimine ( L) and its vanadium(IV) complex, VOL 2 ( 1), were synthesized and characterized by using X-ray, CHN, 1H NMR and FT-IR methods. X-ray analysis shows the Schiff base ligand L acts as a bidentate (O, N) chelating ligand and coordinates via imine nitrogen and phenolato oxygen atoms to the V(IV) center. The coordination geometry around the V(IV) center in 1 is approximately square pyramidal, as indicated by the unequal metal–ligand bond distances and angles, with the basal plane formed by the N 2O 2 donors of the two bidentate Schiff base ligands, the two phenolato O atoms and the two imine N atoms are in the trans position. The coordination sphere of the V(IV) is completed by one oxygen atom in apical position. In the Schiff base ligand, L, there are some classical intramolecular O1–H1⋯N1 and non-classical intermolecular C9–H9b⋯O1 hydrogen bonds, while in 1, there are two non-classical intermolecular C7–H7⋯O3 and C8–H8b⋯O3 hydrogen bonds. The catalytic activity of 1 in epoxidation of cyclooctene was investigated in different conditions to obtain optimum conditions. The effects of solvent, oxidant, catalyst concentration and alkene/oxidant ratio were studied and the results showed that in CCl 4 in the presence of tert-butylhydroperoxide in 1:3 alkene/oxidant ratio, high epoxide yield was obtained. The epoxidation of alkenes was also carried out in optimized conditions that high catalytic activity and selectivity were obtained.

Density Functional Analysis of Geometries and Electronic Structures of Gold-Phosphine Clusters. The Case of Au4(PR3)42+ and Au4(μ2-I)2(PR3)4.

Geometries, ligand binding energies, electronic structure, and excitation spectra are determined for Au(4)(PR(3))(4)(2+) and Au(4)(μ(2)-I)(2)(PR(3))(4) clusters (R = PH(3), PMe(3), and PPh(3)). Density functionals including SVWN5, Xα, OPBE, LC-ωPBE, TPSS, PBE0, CAM-B3LYP, and SAOP are employed with basis sets ranging from LANL2DZ to SDD to TZVP. Metal--metal and metal--ligand bond distances are calculated and compared with experiment. The effect of changing the phosphine ligands is assessed for geometries and excitation spectra. Standard DFT and hybrid ONIOM calculations are employed for geometry optimizations with PPh(3) groups. The electronic structure of the gold--phosphine clusters examined in this work is analyzed in terms of cluster ("superatom") orbitals and d-band orbitals. Transitions out of the d band are significant in the excitation spectra. The use of different basis sets and DFT functionals leads to noticeable variations in the relative intensities of strong transitions, although the overall spectral profile remains qualitatively unchanged. The replacement of PMe(3) with PPh(3) changes the nature of the electronic transitions in the cluster due to low-lying π*-orbitals. To reproduce the experimental geometries of clusters with PPh(3) ligands, computationally less expensive PH(3) or PMe(3) ligands are sufficient for geometry optimizations. However, to predict cluster excitation spectra, the full PPh(3) ligand must be considered.

Crystals in Which Some Metal Atoms are More Equal Than Others: Inequalities From Crystal Packing and Their Spectroscopic/Magnetic Consequences

Crystal structures of the heterometallic compounds CrCrFe(dpa)(4)Cl(2) (1), CrCrMn(dpa)(4)Cl(2) (2), and MoMoMn(dpa)(4)Cl(2) (3) (dpa = 2,2'-dipyridylamide) show disorder in the metal atom positions such that the linear M(A)[quadruple bond]M(A)···M(B) array for a given molecule in the crystal is oriented in one of two opposing directions. Despite the fact that the direct coordination sphere of the metals in the two crystallographically independent orientations is identical, subtle differences in some metal-ligand bond distances are observed in 1 and 3 due to differences in the orientation of a solvent molecule of crystallization. The Fe(II) and Mn(II) ions serve as sensitive local spectroscopic probes that have been interrogated by Mössbauer spectroscopy and high-field EPR spectroscopy, respectively. The subtle differences in the two independent Fe and Mn sites in 1 and 3 unexpectedly give rise to unusually large differences in the measured Fe quadrupole splitting (ΔE(Q)) in 1 and Mn zero-field splitting (D) in 3. Variable-temperature/single-crystal EPR spectroscopy has allowed us to determine that the temperature-dependent D tensors in 3 are oriented along the metal-metal axis and that they show significantly different dynamic behavior with temperature. The differences in ΔE(Q) and D are reproduced by density functional calculations on truncated models for 1 and 3 that lack the quadruply bonded M(A)[quadruple bond]M(A) groups, though the magnitude of the calculated effect is not as large as that observed experimentally. We suggest that the large observed differences in ΔE(Q) and D for the individual sites could be due to the influence of the strong diamagnetic anisotropy of the quadruply bonded M[quadruple bond]M unit.

ChemInform Abstract: Binding Energies and Bond Distances of Ni(CO)x, x = 1-4: An Application of Coupled-Cluster Theory.

The accuracy of the single and double excitation coupled‐cluster (CCSD) method that includes a perturbational estimate of connected triple excitations, denoted CCSD(T), has been tested for some representative transition metal complexes. For both the binding energy and metal to ligand bond distance of NiCO and Ni(CO)2, the CCSD(T) method yields results in very good agreement with multireference averaged coupled‐pair functional (ACPF) calculations. The results are much better than those obtained using either the coupled‐pair functional (CPF) or modified CPF (MCPF) methods. The contribution of connected triples to the binding energy is significant for all Ni(CO)x, x=1–4 ranging from 15 kcal/mol for NiCO to 30 kcal/mol for Ni(CO)4. In contrast, for the geometries the connected triples are only of minor importance. In this case, the correct treatment of disconnected quadruple excitations appears to be more important. For Ni(CO)4, the calculated binding energy is 125 kcal/mol (expt. 140 kcal/mol) and the bond d...

Pt(II) complexes with diimine and chelating 5-ring iminocarbene ligands: synthesis, characterization, and structural and spectroscopic trends

A series of new L2PtMe2 and L2PtPh2 complexes have been prepared, where L2 = chelating diimine or NHC iminocarbene ligands with similar substituent patterns. These have been investigated by 1H- and 13C-NMR spectroscopy and single-crystal X-ray structure determinations to assess differences between the two ligand systems. All the three approaches underscore the strong trans influence of the N-heterocyclic carbene moiety relative to the imine group. This is supported by trends in J(195Pt–H) and J(195Pt–C) coupling constants in the hydrocarbyl NMR resonances as well as in differences in Pt–ligand bond distances trans to the imine and carbene moieties. This is paralleled by a clear trans effect of the carbene ligand in protonolysis; treatment of the Pt(II) dimethyl or diphenyl iminocarbene complexes with acid causes the elimination of methane and benzene, respectively, with selective loss of the methyl or phenyl groups from the trans position relative to the carbene part of the chelating ligand.