Cationic Complexes Research Articles

In-silico antibacterial activity of piperazinyl-based copper(II) metallo-organic pharmacophores

The piperazinyl-based Cu(II)-Schiff base complex {[Cu(HL1′)(N3)]2∙(ClO4)2·4H2O} (1) [HL1′ is the Zwitter ionic form of (E)-2-((2-piperazin-1-ylethylimino)methyl)phenol] was synthesized and characterized by spectroscopic and single crystal X-ray diffraction studies. The X-ray single crystal structure analysis shows that the asymmetric unit of 1 contains a four-coordinate discrete complex cation, [Cu(HL1′)(N3)]+ (1a), with a perchlorate anion and a lattice water molecule. Two square planar units of 1a are dimerized by weak Cu···N intermolecular interaction, forming [Cu(HL1′)(N3)]2 2+ (1′a). DFT and TD-DFT calculations of the monomeric cationic unit, [Cu(HL1′)(N3)]+ (1a) were performed. In-silico antibacterial activities of HL1 as well as monomeric and dimeric cationic units 1a and 1'a, respectively, were performed and study shows that 1a (EC50 0.30 μM) and 1′a (EC50 0.06 μM) may be treated as antibacterial drugs after in-vivo analysis and clinical testing.

Synthesis and Characterization of Copper(II) and Nickel(II) Complexes with 3-(Morpholin-4-yl)propane-2,3-dione 4-Allylthiosemicarbazone Exploring the Antibacterial, Antifungal and Antiradical Properties.

The eleven new copper(II) and nickel(II) coordination compounds [Cu(L)Br]2 (1), [Cu(L)Cl] (2), [Cu(L)NO3] (3), [Ni(L)Cl] (4), [Ni(HL)2](NO3)2 (5), and [Cu(A)(L)]NO3, where A is 1,10-phenanthroline (6), 2,2'-bipyridine (7), 3,4-dimethylpyridine (8), 3-methylpyridine (9), pyridine (10) and imidazole (11) were synthesized with 3-(morpholin-4-yl)propane-2,3-dione 4-allylthiosemicarbazone (HL). The new thiosemicarbazone was characterized by NMR and FTIR spectroscopy. All the coordination compounds were characterized by elemental analysis and FTIR spectroscopy. Also, the crystal structures of HL and complexes 1, 6, 7, and 11 were determined using single-crystal X-ray diffraction analysis. Complex 1 has a dimeric molecular structure with two bromide bridging ligands, while 6, 7, and 11 are ionic compounds and comprise monomeric complex cations. The studied complexes manifest antibacterial and antifungal activities and also have an antiradical activity that, in many cases, surpasses the activity of trolox, which is used as a standard antioxidant in medicine. Copper complexes 1-3 have very weak antiradical properties (IC50 > 100 µM), but nickel complexes 4-5 are strong antiradicals with IC50 values lower than that of trolox. The mixed ligand copper complexes with additional ligand of N-heteroaromatic base are superior to complexes without these additional ligands. They are 1.4-5 times more active than trolox.

Benzoquinone Ligand-Enabled Ruthenium-Catalyzed Deaminative Coupling of 2-Aminoaryl Aldehydes and Ketones with Branched Amines for Regioselective Synthesis of Quinoline Derivatives.

The catalytic system generated in situ from the cationic Ru-H complex [(C6H6)(PCy3)(CO)RuH]+BF4- (1) with 2,3,4,5-tetrachloro-1,2-benzoquinone (L1) was found to be highly effective for promoting the deaminative coupling reaction of 2-aminoaryl aldehydes with branched amines to form 2-substituted quinoline products. The analogous deaminative coupling reaction of 2-aminoaryl ketones with branched amines led to the regioselective formation of 2,4-disubstituted quinoline products. A number of biologically active quinoline derivatives including graveolinine and a triplex DNA intercalator have been synthesized by using the catalytic method.

Structures and unimolecular chemistry of alkali metal cation complexes with glutathione in the gas phase

This study investigates the unimolecular reactions of glutathione complexes with alkali metal cations in the gas phase through sustained off-resonance irradiation collision-induced dissociation and examines their structures using a combination of infrared multiphoton dissociation spectroscopy and computational techniques. Under soft CID conditions, glutathione complexes with charge-dense cations such as Li⁺, Na⁺, and K⁺ show significant fragmentation of glutathione, while complexes with heavier cations, Rb⁺ and Cs⁺, primarily undergo loss of glutathione. This behavior is attributed to the stronger non-covalent binding between smaller metal cations and glutathione, which competes with the dissociation thresholds of covalent interactions within the peptide complex. Using CREST, a tool for determining trial structures which were submitted to density functional theory calculations, a thorough investigation of the conformational space revealed many possible structures, including pentacoordinated structures for the Na⁺ and K⁺ complexes, as well as tetra-tri-, bi-, and monocoordinated structures along with zwitterionic structures for all metal cation/GSH complexes. For all alkali metal cation complexes, the thermodynamically most stable structures were found to be tetracoordinated A-type structures where the metal cation is bound to the amine nitrogen and three of the carbonyl oxygens—all except O2, the amide between glycine and cysteine. These computed infrared spectra for these lowest energy complexes were also consistent with the experimental vibrational spectra in the fingerprint region. Based on relative energies and the comparison of computed and experimental infrared spectra in the fingerprint region, the tetracoordinate A-type structures are concluded to be the dominant forms of the [M(GSH)]+ complexes in the gas phase.

Synthesis and Luminescent Properties of Zwitterionic Platinum Bis‐Phosphine Complexes Based on a Pendant Closo‐Monocarborane Cluster

ABSTRACTThe synthesis, structures and photophysical properties of a series of zwitterionic platinum complexes are reported. These cyclometalated complexes have the general structure Pt(C^N)(P^P), where C^N is a cyclometalating ligand with a pendant monocarborane cluster and P^P is a bis‐phosphine ligand (dppe or dppb). Their solid‐state structures have been characterized by x‐ray diffraction, and no Pt–Pt interactions are observed. All complexes are emissive at room temperature in solid states with quantum yields of 0.06–0.58 in PMMA films. The emission energy of these complexes is governed by the nature of the C^N cyclometalated ligands, allowing the emission to be tuned from blue to green. In particular, the introduction of an anionic carborane unit provokes a more than twofold improvement in the photoluminescence quantum yield of [Pt(CB11‐ppy)(dppe)] (3b) compared to the parent cationic complex [Pt(ppy)(dppe)]+. In addition, density functional theory (DFT) calculations were performed on the ground states and first triplet excited states of 1a, 2a, and 3a for interpretation of the observed photophysical properties.

Coordination Chemistry of Calix[3]Pyrrole-Related Macrocycles

Calix[3]pyrrole is a ring-contracted porphyrinogen-like macrocycle in which three pyrrole units are linked by three sp3-carbon atoms at the a-positions. While synthetic routes to calix[3]pyrrole and its furan and thiophene derivatives have been established using oligoketone precursors,[1–3] their coordination chemistry remains fully unexplored. Here, the author reports the direct macrocyclization synthesis and metal complexation properties of novel thiazole-containing calix[3]pyrrole analogues that exhibit better metal chelation than that of calix[3]pyrrole.Treatment of calix[3]pyrrole (1) with n-butyl lithium (4 eq.) resulted in the deprotonation of all the NH protons to generate a tridentate trianionic ligand. Successive reaction with BBr3 furnished a boron(III)-complex 2 in 25% isolated yield. However, attempts to synthesize stable complexes with other metal ions were unsuccessful. The obtained boron complex 2 were unusually stable towards acids; even in pure trifluoroacetic acid (TFA) or methanesulfonic acid, no deboronation was observed, instead, protonation at a pyrrole carbon atom was detected by NMR spectroscopy.In order to generate isolable metal complexes with calix[3]pyrrole-related macrocycles, we have designed calix[1]pyrrole[2]thiazole (4). Calix[1]furan[2]thiazole (3), a precursor of 4, was synthesized by direct macrocyclization between bis(a-bromoketone) and 2,2-dimethylpropanebis(thioamide) in 60% yield. While calix[3]pyrrole (1)undergoes a strain-induced ring expansion reaction to give calix[6]pyrrole under acidic conditions, less strained 3 was stable in TFA, thus its furan moiety was hydrolyzed to a diketone moiety. Further Paal–Knorr pyrrole formation reaction led to the formation of 4 in 78% yield.When macrocycle 4 was mixed with Et2Zn, monoethyl zinc complex 5 was formed in 72% yield. Single crystal X-ray analysis revealed that 4 adopted a C s-symmetric cone conformation to behave as a monoanionic tridentate ligand. Due to the steric protection of the ethyl zinc center by meso-methyl groups, organozinc complex 5 was stable in the presence of excess water or alcohol at room temperature.Complexation of 4 with Pd(MeCN)4(BF4)2 afforded cationic palladium complex 6 in which macrocycle 4 adopted a partial cone conformation to chelate a Pd2+ ion as a neutral bidentate ligand. Similar complexation behavior was also observed with AgBF4, demonstrating conformational flexibility of macrocycle 4 upon metal complexation. Furthermore, coordination-driven self-assembly was confirmed for Ag+ complex of 4.In this presentation, the detailed synthesis and electronic properties of the metal complexes of calix[3]pyrrole-related macrocycles will be discussed.

First Isolation of Solvated MgCl+ species as the Sole Cations in Electrolyte Solutions for Rechargeable Mg Batteries

Synthesis of complex magnesium cations in ethereal solutions, is receiving a lot of attention due to their potential utilization in rechargeable magnesium batteries (RMB). The simplest complex cation, namely, solvated MgCl+, was hypothesized and reported as the most important cation in nonaqueous magnesium electrolyte solutions chemistry. However, such ions have never isolated as the only cationic species in ethereal solutions developed for RMB. In this study, we report on a successful isolation of the pure electrolyte MgCl(THF)5 + - PhAlCl3 - , and on the electrochemical behaviour of its ethereal solutions. The structure of this compound was proved by single-crystal X-ray diffraction, Raman, and, NMR spectroscopies. The novel MgCl(THF)5PhAlCl3/THF electrolyte solutions exhibit reversible Mg deposition/dissolution processes with anodic stability up to 2.7 V. The application of electrochemical cleaning pre-treatment for these solutions enabled to profoundly increase the coulombic efficiency of their Mg deposition/dissolution processes. Examining Mg prototype batteries with Mg foil anodes, composite MgxMo6S8Chevrel phase (CP) cathodes containing these solutions exhibited a viable battery performance, indicating that upon a further optimization, electrolyte solutions based on the complex MgCl(THF)5 + - PhAlCl3 - electrolyte may be suitable for practical RMB. Figure 1

(1,2,5,6-η)-Cyclo-octa-1,5-diene](1-ethyl-4-isobutyl-1,2,4-triazol-5-yl-idene)(tri-phenyl-phosphane)rhodium(I) tetra-fluorido-borate.

A new, cationic N-heterocyclic carbene RhI complex with a tetra-fluorido-borate counter-anion, [Rh(C8H12)(C8H15N3)(C18H15P)]BF4, has been synthesized and structurally characterized. There are two independent ion pairs in the asymmetric unit. Each complex cation exhibits a distorted square-planar conformation around the RhI atom. Bond lengths and bond angles are as expected for an Rh-NHC complex. There are several close, non-standard C-H⋯F hydrogen-bonding inter-actions between the ions. One of the tetra-fluorido-borate anions shows statistical disorder of the F atoms.

Synergy of {Co(H2O)6}2+ with a Polyoxometalate Leads to Aqueous Homogeneous Hydrogen Evolution: Experiments and Computations.

In this work, we have described a polyoxometalate (POM)-based inexpensive and easily synthesizable compound [Co(H2O)6]2[{K(H2O)}2V10O28]·2H2O (1), which exhibits electrocatalytic hydrogen evolution in its aqueous solution without its decomposition (or electrodeposition), acting as a rare homogeneous electrocatalyst. Even though the compound [Co(H2O)6]2[{K(H2O)}2V10O28]·2H2O (1) (soluble in water) shows electrocatalytic hydrogen evolution reaction (HER) activity because of the Coulombic attraction, including H-bonding interactions, between the [Co(H2O)6]2+ cationic species and [{K(H2O)}2V10O28]4-anionic species, the individual homogeneous solutions of [V10O28]6- (source: Na6[V10O28]·18H2O) and [Co(H2O)6]2+ (source: CoCl2·6H2O) do not show any electrocatalytic HER activity. We have thus established that the synergy of [V10O28]6- with [Co(H2O)6]2+ in crystal matrix as well as in the aqueous solution of 1 makes the compound 1 a stable and highly active electrocatalyst for homogeneous HER in an aqueous solution. In order to corroborate these homogeneous HER studies, we performed density functional theory (DFT) calculations to show that decavanadate cluster anion [V10O28]6- interacts with hexa-aqua complex cation [Co(H2O)6]2+ via strong H-bonding interactions, leading to a synergy effect that enables the cobalt center of [Co(H2O)6]2+ to be an active site of HER in the present work.

From Coordination to Noncoordination: Syntheses and Substitution Lability Studies of Titanium Triflato Complexes.

A new concept for obtaining cationic complexes with triflate counteranions from coordinating triflato ligands was developed. Various routes are leading to titanium(IV) and titanium(III) triflato complexes efficiently. The reactions of pentafulvene titanium complexes with either triflic acid or silver triflate give the corresponding titanium(IV) triflato complexes in excellent yields. Hydrolysis of the titanium(IV) bistriflato complexes leads to cationic aqua complexes via displacement of the triflato ligand, which consequently acts as a noncoordinating anion. A functionalized titanium(IV) monotriflato complex was synthesized by insertion of a nitrile into the Ti-C bond and the triflato ligand was displaced by an NHC. While the titanium(IV) complexes are mostly inert toward substrates, the donor-free titanium(III) triflato complex is a strong Lewis acid and forms various adducts with monodentate Lewis bases. The titanium(III) complex was oxidized by reaction with TEMPO, resulting in a diamagnetic titanium(IV) complex. The reaction with bidentate ligands results in cationic titanium(III) complexes due to displacement of the triflato ligand by the bidentate ligands. Treatment with acetone leads to an aldol reaction of two acetone molecules and the formation of a cationic diacetone alcohol complex.

Counterion Effects in [Ru(bpy)3](X)2-Photocatalyzed Energy Transfer Reactions.

Photocatalysis that uses the energy of light to promote chemical transformations by exploiting the reactivity of excited-state molecules is at the heart of a virtuous dynamic within the chemical community. Visible-light metal-based photosensitizers are most prominent in organic synthesis, thanks to their versatile ligand structure tunability allowing to adjust photocatalytic properties toward specific applications. Nevertheless, a large majority of these photocatalysts are cationic species whose counterion effects remain underestimated and overlooked. In this report, we show that modification of the X counterions constitutive of [Ru(bpy)3](X)2 photocatalysts modulates their catalytic activities in intermolecular [2 + 2] cycloaddition reactions operating through triplet-triplet energy transfer (TTEnT). Particularly noteworthy is the dramatic impact observed in low-dielectric constant solvent over the excited-state quenching coefficient, which varies by two orders of magnitude depending on whether X is a large weakly bound (BArF 4 -) or a tightly bound (TsO-) anion. In addition, the counterion identity also greatly affects the photophysical properties of the cationic ruthenium complex, with [Ru(bpy)3](BArF 4)2 exhibiting the shortest 3MLCT excited-state lifetime, highest excited state energy, and highest photostability, enabling remarkably enhanced performance (up to >1000 TON at a low 500 ppm catalyst loading) in TTEnT photocatalysis. These findings supported by density functional theory-based calculations demonstrate that counterions have a critical role in modulating cationic transition metal-based photocatalyst potency, a parameter that should be taken into consideration also when developing energy transfer-triggered processes.

Rare-Earth Metallocenes for Polymerization of Olefins and Conjugated Dienes: From Fundamental Studies to Olefin Block Copolymers.

The various steps in the mechanism of olefin polymerizations mediated by neutral rare-earth metallocene complexes are discussed. The complexes are either trivalent hydride and alkyl rare-earth compounds or divalent metallocenes that are activated by the monomer via an oxidation step. The stereospecific polymerizations of conjugated dienes based on the association of a cationic metallocene complex and an alkylaluminum and the polymerization mechanism based on monomer insertion into an aluminum-carbon bond are also discussed. The exploitation of metallocene complexes for the copolymerization of olefins with conjugated dienes is the subject of a third part of this review. The synthesis of new elastomers called ethylene butadiene rubber (EBR) is highlighted. Finally, the use of rare-earth metallocenes in macromolecular engineering is detailed. This includes the synthesis of functional polyolefins and block copolymers including thermoplastic elastomers.

Optical Activity of Nonactin and Its Cation Complexes.

Nonactin is a non-enantiomorphous (S4 symmetric), optically active natural product with a specific rotation of zero in solutions at all frequencies and temperatures. All optically active, non-enantiomorphous natural products have specific rotations of zero as a consequence of the spatial average of bisignate chiroptical (magnetoelectric or gyration) tensors with equal and opposite eigenvalues. Zeros that arise in the spatial average are distinct in principle, though not necessarily in practice, from zeros that arise in optical inactivity-chiroptical tensors with zero values for all elements as in centric molecules. Nonactin would be measurably optically active when oriented. The anisotropy of the optical activity of nonactin and its cation complexes, likewise S4 symmetric, are studied here by computation to emphasize the infelicitous linkage between optical activity and chirality. Computations show that changes in the conformation of the nonactin macrocycle upon complexation principally are responsible for diminishing the computed optical activity; the metals are incidental.

New Catalyst Systems for the Polymerization of Norbornene and Its Derivatives Based on Cationic Palladium Cyclopentadienyl Complexes

The catalytic properties of systems based on [Pd(Cp)(L)n]m[BF₄]m (where Cp = η5-C5H5; n = 2, m = 1: L = tris(orthomethoxyphenyl)phosphine, triphenylphosphine, tris(2-furyl)phosphine (TFP), n = 1, m = 1: L = 1,1'-bis(diphenylphosphino)ferrocene, 1,3-bis(diphenylphosphino)propane, 1,4-bis(diphenylphosphino)butane, 1,5-bis(diphenylphosphino)pentane; n = 1, m = 2 or 3: L = 1,6-bis(diphenylphosphino)hexane) in the addition homoand copolymerization of norbornene (NB) and its derivatives have been studied. These complexes can be activated with the Lewis-acids (BF₃ ∙ OEt2 or AlCl3). The productivity of the [Pd(Cp)(PPh3)2][BF₄]/BF₃ ∙ OEt2 catalyst system in the polymerization of norbornene is up to 188800 molNB molPd⁻¹. The homopolymerization of 5-methoxycarbonylnorbornene and copolymerization of NB with 5-methoxycarbonylnorbornene or 5-phenylnorbornene were studied in the presence of BF₃ ∙ OEt2 and [Pd(Cp)(L)2][BF₄] (L = PPh3 or TFP). A hypothesis for the catalyst’s formation via the intramolecular rearrangement of the η5-Cp ligand into the η1-Cp form upon interaction with a Lewis acid is suggested. The structure of the [Pd(Cp)(TFP)2]BF₄ (I) was determined by X-ray diffraction. In the crystal structure of I, the palladium coordination sphere is characterized by a slight distortion of the square planar geometry of the central atom, and the cyclopentadiyl fragment is in an eclipsed conformation. Based on the XRD data, the steric hindrance of the TFP ligand was estimated (the cone angle is 149°).

Synthesis and Characterization of Bridging-Diazene Diiron Half-Sandwich Complexes: The Role of Sulfur Hydrogen Bonding.

We report two bridging-diazene diiron complexes [Cp*Fe(8-quinolinethiolate)]2(μ-N2H2) (1-N2H2) and [Cp*Fe(1,2-Cy2PC6H4S)]2(μ-N2H2) (2-N2H2), synthesized by the reaction of hydrazine with the corresponding thiolate-based iron half-sandwich complex, [Cp*Fe(8-quinolinethiolate)]2 (1) and Cp*Fe(1,2-Cy2PC6H4S) (2). Crystallographic analysis reveals that the thiolate sites in 1-N2H2 and 2-N2H2 can engage in N-H···S hydrogen bonding with the diazene protons. 1-N2H2 is thermally stable in both solid and solution states, allowing for one-electron oxidation to afford a cationic diazene radical complex [1-N2H2]+ at room temperature. In contrast, 2-N2H2 tends to undergo N2H2/N2 transformation, leading to the formation of a Fe(III)-H species by the loss of N2. In addition to stabilizing HN=NH species through the hydrogen bonding, the thiolate-based ligands also seem to facilitate proton-coupled electron transfer, thereby promoting N-H cleavage.

Bifunctional Ligands: Evaluating the Role of Acidic Protons in the Secondary Coordination Sphere.

To evaluate bifunctional ligand reactivity involving NH acidic sites in the secondary coordination sphere, complexes where the proton has been substituted with a methyl group (NMe) are often investigated. An alternative strategy involves substitution of the NH group for an O. This contribution considers and compares the merits of these approaches; the synthesis and characterization of cationic square-planar Rh carbonyl complexes bearing diprotic bispyrazole pyridine ligand L1, and the bis-methylated pyrazole pyridine ligand L1Me are described. The syntheses and characterization of the novel monoprotic pyrazole isoxazole pyridine ligand L2 and aprotic bisisoxazole pyridine ligand L3, and their corresponding Rh carbonyl complexes are also described. Comparison of the CO stretching frequencies of the four Rh complexes suggest that substitutions of NH with NMe, as well as with O, lead to significant electronic differences. These electronic differences result in different reactivities with respect to ligand addition/substitution of the Rh carbonyl complexes. Overall, the data suggest that electronic differences arising due to the NH substitutions can be significant and should be considered when the NH group is substituted in investigations of the participation of the NH proton in a reaction.

(1R,3R,5S,Z)-2-Ethylidene-6,6-dimethyl-3-vinylbicyclo[3.1.1]-heptane

(1R,3R,5S,Z)-2-ethylidene-6,6-dimethyl-3-vinylbicyclo[3.1.1]heptane was prepared by hydrovinylation of nopadiene catalyzed by a cationic Ru complex. The structure was fully characterized by 1H- and 13C-NMR spectroscopy, including 2D-COSY and 2D-NOESY spectra, optical rotation, and combustion analysis. In contrast to the previously reported 1,2-hydrovinylation of 1-vinylcycloalkenes by this catalyst, the reaction with nopadiene proceeds by 1,4-addition of ethylene.

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This paper presents a novel macrocycle based on conformation‐adaptive and electron‐rich dihydrophenazine. The macrocycle exhibits host‐guest interaction with tetracyanoquinodimethane (TCNQ) driving by charge transfer interaction between them. The host‐guest complex forms stable radical cation species upon oxidation, resulting in the release of TCNQ from the macrocyclic cavity, thereby successfully achieving the reversible binding of guest molecules. More details are discussed in the article by Yang et al. on page 1895—1900.image

1-(Anthracen-9-ylmeth-yl)-1,4,7,10-tetra-aza-cyclododeca-ne]chlorido-zinc(II) nitrate.

In the title salt, [ZnCl(C23H30N4)]NO3, the central ZnII atom of the complex cation is coordinated in a square-pyramidal arrangement by four nitro-gen atoms from cyclen (1,4,7,10-tetra-aza-cyclo-dodeca-ne) in the basal plane and one chlorido ligand in the apical position. The anthracene group attached to cyclen contributes to the crystal packing through inter-molecular T-shaped π inter-actions. Additionally, the nitrate anion participates in inter-molecular N-H⋯O hydrogen bonds with cyclen.

Origin of Reactivity Trends of an Elusive Metathesis Intermediate from NMR Chemical Shift Analysis of Surrogate Analogues.

Olefin metathesis has become an efficient tool in synthetic organic chemistry to build carbon-carbon bonds, thanks to the development of Grubbs- and Schrock-type catalysts. Olefin coordination, a key and often rate-determining elementary step for d0 Schrock-type catalysts, has been rarely explored due to the lack of accessible relevant molecular analogues. Herein, we present a fully characterized surrogate of this key olefin-coordination intermediate, namely, a cationic d0 tungsten oxo-methylidene complex bearing two N-heterocyclic carbene ligands─[WO(CH2)Cl(IMes)2](OTf) (1) (IMes = 1,3-dimesitylimidazole-2-ylidene, OTf-triflate counteranion), resulting in a trigonal bipyramidal (TBP) geometry, along with its neutral octahedral analogue [WO(CH2)Cl2(IMes)2] (2)─and an isostructural oxo-methylidyne derivative [WO(CH)Cl(IMes)2] (3). The analysis of their solid-state 13C and 183W MAS NMR signatures, along with computed 17O NMR parameters, helps to correlate their electronic structures with NMR patterns and evidences the importance of the competition among the three equatorial ligands in the TBP complexes. Anchored on experimentally obtained NMR parameters for 1, computational analysis of a series of olefin coordination intermediates highlights the interplay between σ- and π-donating ligands in modulating their stability and further paralleling their reactivity. NMR spectroscopy descriptors reveal the origin for the advantage of the dissymmetry in σ-donating abilities of ancillary ligands in Schrock-type catalysts: weak σ-donors avoid the orbital-competition with the oxo ligand upon formation of a TBP olefin-coordination intermediate, while stronger σ-donors compromise M≡O triple bonding and thus render olefin coordination step energy demanding.