The complexes with end-to-end dicyanamide bridges: syntheses, characterization and crystal structures of [Cu(μ1,5-dca)2(phen)]n and [Cd(μ1,5-dca)2(py)2]n (phen=phenanthroline; py=pyridine; dca=dicyanamide, N(CN)2 )

Two new dicyanamide (dicyanamide=[N(CN)₂]⁻, dca) bridged complexes, [Cu(μ_1,5-dca)₂(phen)]_n (1) and [Cd(μ_1,5-dca)₂(py)₂]_n (2) have been synthesized and their structures have been determined by X-ray crystallography diffraction. Complex 1 crystallizes in the monoclinic space group P2(1)/c with a=10.1502(3), b=10.9815(4), c=14.5839(4) Å and Z=4. The adjacent copper atoms are connected by single end-to-end dca bridges to form a chain structure along the b axis. The chains are linked via Cu?N weak interactions to give rise to a 2D layer structure, which furthermore into a 3D structure by the π-π interaction between aromatic rings of adjacent layers. Complex 2 crystallizes in the monoclinic space group C2/m with a=6.6849(10), b=17.476(2), c=13.231(2) Å and Z=4. The cadmium(II) center is six-coordinated with a distorted octahedral geometry, bounded to four N atoms of four dca ligands and two N atoms of two-chelated py ligands. Neighbor Cd(II) atoms are linked by the double end-to-end dca bridges to generate a chain structure, which result in a 2D layer structure through the π-π interactions between the adjacent chains with distances 3.641 Å. EPR and magnetic results of 1 suggest that the complex exhibits a weak ferromagnetic interaction through CuNCNCNCu pathways.     

Phosphoserine and specific types of its coordination in copper(II) and adenosine nucleotides systems - Potentiometric and spectroscopic studies

Ternary systems of copper(II) complexes with phosphoserine and adenosine 5′-monophosphate or adenosine 5′-diphosphate or adenosine 5′-triphosphate have been investigated. The studies have been performed in aqueous solution using the potentiometric method with computer analysis of the data, ¹³C and ³¹P nuclear magnetic resonance, visible and electron paramagnetic resonance spectroscopies. The overall stability constants of the complexes have been determined. Analysis of the equilibrium constants of the reaction have allowed determination of the effectiveness of the phosphate groups and donor atoms of heterocyclic rings in the process of complex formation. The potential reaction centres are the nitrogen atoms N(1) and N(7) and the phosphate group of the nucleotides as well as phosphate, carboxyl and amine groups from phosphorylated serine. Coordination sites of investigated ligands have been identified on the basis of the equilibrium constants analysis and spectroscopic studies.     

Synthesis and spectroscopic investigations of four-coordinate nickel complexes supported by a strongly donating scorpionate ligand

Two four-coordinate nickel complexes, HB(^tBuIm)₃NiBr and HB(^tBuIm)₃NiNO, were prepared by reaction of a bulky tris(carbene)borate ligand with NiBr₂(PPh₃)₂ and NiBr(NO)(PPh₃)₂, respectively, and structurally and spectroscopically characterized. In addition to standard techniques, high-frequency and -field electron paramagnetic resonance (HFEPR) was employed to understand the spin triplet (S = 1) ground state of the bromo complex. HFEPR, combined with electronic absorption spectroscopy allows comparison of this novel complex with other paramagnetic four-coordinate Ni(II) species. The tris(carbene)borate ligand is a stronger σ-donor than corresponding tris(pyrazolyl)borates (traditional “scorpionate” ligands). The tris(carbene)borate ligand may also act as a π-acceptor, in contrast to tris(pyrazolyl)borates, which show relatively little π-bonding interactions. The influence of tris(carbene)borate substituents on the donor strength of the ligand have been elucidated from IR spectroscopic investigations of {NiNO}¹⁰ derivatives. HFEPR spectra of HB(^tBuIm)₃NiBr exhibit hyperfine coupling from Br, which indicates the strong electronic interaction between Ni(II) and this halide ligand, consistent with studies on tris(pyrazolyl)borate Ni(II) complexes.     

Novel six-coordinate oxorhenium(V) ‘3+2’ mixed-ligand complexes carrying the SNO/SN donor atom set

Ligand exchange reactions of oxorhenium(V) precursors with bidentate SN and tridentate Schiff bases derived from the condensation of ketones or aldehydes with dithiocarbazic acid methyl ester (H₂NNHC(S)SCH₃) produce novel ‘3+2’ mixed-ligand complexes carrying the SNO/SN donor atom set. Thus, reactions of either [NBu₄][ReOCl₄] or Na[ReO(Gluconate)₂] with SNO ligands (H₂Lⁿ) or a mixture of bidentate SN (HL^m) and tridentate SNO (H₂Lⁿ) in methanol solutions lead, respectively, to the six-coordinated mixed ligand oxorhenium(V) compounds of types [ReO(Lⁿ)(HLⁿ)] and [ReO(Lⁿ)(L^m)], combining one tridentate dianionic S⁻NO⁻ donor Schiff base (L) and one bidentate anionic S⁻N donor ligand (HL). Coordination geometry around rhenium is distorted octahedral with the two SN donor atom sets of each ligand defining the equatorial plane, while apical positions are occupied by the oxo group and the oxygen atom of the tridentate SNO ligand (L), as shown by single-crystal X-ray diffraction structure of [ReO(L¹)(HL¹)] 1.     

On the electron delocalization in cyclopropenylidenes - An experimental charge-density approach

The extent and nature of cyclic electron delocalization in free and coordinated cyclopropenylidene carbenes has been analyzed by combined experimental and theoretical charge-density studies. The significant asymmetry of the C-C bond lengths in substituted cyclopropenylidene carbenes was identified as cooperative effect which depends on contributions of both σ- and π-bonding. We show that analyses of (i) the topology of the Laplacian of the electron density distribution and (ii) the out-of-plane atomic quadrupole moments - the charge-density analogues of p_π occupation - allow to distinguish between the influence of σ- and π-electrons on cyclic electron delocalization. These studies hint for pronounced electron localization in the carbene lone pair region which dominates the electronic structure of free cyclopropenylidene carbenes and hinders the establishment of true aromaticity. We further investigated the electron donating/withdrawing ability of cyclopropenylidene ligands relative to N-heterocyclic carbenes. The experimental benchmark systems LCr(CO)₅ (L = 2,3-diphenylcyclopropenylidene and 1,2-dimethylimidazol-2-ylidene) show that the cyclopropenylidene ligand clearly displays the higher π-acceptor capability relative to N-heterocyclic carbenes.     

Synthesis and characterisation of ferrocene-containing β-diketonato complexes of rhodium(I) and rhodium(III)

The synthesis of new β-diketonato rhodium(I) complexes of the type [Rh(FcCOCHCOR)(CO)₂] and [Rh(FcCOCHCOR)(CO)(PPh₃)] with Fc=ferrocenyl and R=Fc, C₆H₅, CH₃ and CF₃ are described. ¹H, ¹³C and ³¹P NMR data showed that for each of the non-symmetric β-diketonato mono-carbonyl rhodium(I) complexes, two isomers exist in solution. The equilibrium constant, K_c, which relates these two isomers in an equilibrium reaction, are concentration independent but temperature and solvent dependent. Δ_rG, Δ_rH and Δ_rS values for this equilibrium have been determined and a linear relationship between solvent polarity on the Dimroth scale and K_c exists. The relationship between RhP bond lengths, d(RhP), and ³¹P NMR peak positions as well as coupling constants ¹J(³¹P¹⁰³Rh) has been quantified to allow calculation of approximate d(RhP) values. Variations in d(RhP) for [Rh(RCOCHCOR′)(CO)(PPh₃)] complexes have also been related to the group electronegativities (Gordy scale) of the terminal β-diketonato R groups trans to PPh₃. A measure of the electron density on the rhodium centre of [Rh(RCOCHCOR′)(CO)(PPh₃)] may be expressed in terms of the IR carbonyl stretching wave number, ν(CO), the sum of the group electronegativities of the R and R′ groups, (χ_R+χ_R′), or the observed pK_a^′ values of the free β-diketones RCOCH₂COR^′. An empirical relationship between ν(CO) and either pK_a^′ or (χ_R+χ_R′) has also been quantified.     

Specific cation effect on quenching reactions of excited tris(α,α′-diimine) ruthenium(II) and chromium(III) complexes by cyanide complexes in aqueous solutions

Specific salt effects were studied on the quenching reaction of excited [Ru(NN)₃]²⁺ (NN=2,2′-bipyridine(bpy), 1,10-phenanthrorine(phen)) and [Cr(bpy)₃]³⁺ by [Cr(CN)₆]³⁻, [Fe(CN)₆]³⁻ and [Ni(CN)₄]²⁻ in aqueous solutions as a function of alkali metal ions which were added for adjustment of ionic strength. The quenching rate constants in [Ru(NN)₃]²⁺-[Cr(CN)₆]³⁻ and [Cr(bpy)₃]³⁺-[Cr(CN)₆]³⁻ systems are changed by the cations as Li⁺>Na⁺>K⁺≈Rb⁺≈Cs⁺. On the other hand, the rate constants in [Ru(NN)₃]²⁺-[Fe(CN)₆]³⁻ and [Ru(NN)₃]²⁺-[Ni(CN)₄]²⁻ systems, which are diffusion-controlled reactions, are not varied by the alkali metal cations. The obtained order (Li⁺>Na⁺>K⁺≈Rb⁺≈Cs⁺) of the quenching rate constant is quite different from salt effects, Li⁺<Na⁺<K⁺<Rb⁺<Cs⁺, which have been obtained in the electron transfer reactions between complex anions.     

Stabilization of the cobalt coordination site in transmetalation processes on dinuclear salen derivatives

Two dinuclear cobalt/copper compounds have been isolated from the reaction between N,N′-ethylenebis(salicylideniminato)cobalt(II), [Co(salen)], and copper(II) chloride in different conditions. The first one is a dinuclear cobalt(III)/copper(II) derivative, [Co(salen)Cl₂Cu(EtOH)₂Cl], 1, that have the cobalt atom six-coordinated to the four donor atoms of the salen ligand and to two chlorine atoms in a slightly distorted octahedral environment and the copper atom five-coordinated to the two bridging oxygen atoms of the salen ligand, two ethanol molecules and one extra chlorine atom. This compound is the only reported example of a cobalt/copper derivative with the cobalt maintaining the salen coordinative site, since the usual reaction takes place by a transmetalation process. This reaction is observed in the second derivative, [Cu(salen)CoCl₂], 2, where the copper atom displaces the cobalt from the salen cavity. The copper atom adopts a square-planar coordinative environment, while the cobalt is tetrahedrically coordinated to the two bridging oxygen and two chlorine atoms. Both compounds present several intermolecular contacts that increase the dimensionality in the crystal and some of which can transmit magnetic interactions. The magnetic properties confirm the structural picture, with isolated copper(II) centres in 1, where the cobalt(III) is in the low spin form, and with antiferromagnetically coupled S = ¹/₂ and S = ³/₂ centres in 2.     

Asymmetric synthesis of (S)-α-(octyl)glycine via alkylation of Ni(II) complex of chiral glycine Schiff base

Over last decade, the use of Ni(II) complexes, derived from of glycine Schiff bases with chiral tridentate ligands, has emerge as a leading methodology for preparation of structurally diverse Tailor-Made Amino Acids, the key structural units in modern medicinal chemistry, and drug design. Here, we report asymmetric synthesis of derivatives of (S)-α-(octyl)glycine ((S)-2-aminodecanoic acid) and its N-Fmoc derivative via alkylation of chiral nucleophilic glycine equivalent with n-octyl bromide. Under the optimized conditions, the alkylation proceeds with excellent yield (98.1%) and diastereoselectivity (98.8% de). The observed stereochemical outcome and convenient reaction conditions bode well for application of this method for large-scale asymmetric synthesis of (S)-2-aminodecanoic acid and its derivatives.     

Esterification of streptol — a cyclitol derivative — by Candida rugosa lipase: influence of the acyl donor on regioselectivity

The influence of the nature of acyl donors on the regioselectivity of Candida rugosa lipase for the esterification of streptol — a cyclitol derivative — was investigated. Excellent regioselectivity for the formation of 3,7-disubstituted derivatives was observed for vinyl butyrate (100% 3,7-derivative, 68% yield) and vinyl propionate (100% 3,7-derivative, 46% yield) as acyl donors. In contrast, for vinyl methacrylate as acyl donor, a mixture of 71% 3,7-derivative and 29% 1,7-derivative was obtained. Varying the chain length, a certain dependency of regioselectivity on the acyl donor was observed, however, no logical correlation satisfying all cases was found. Mono-substituted streptol derivatives were obtained by employing Novozym 243.     

Synthesis and characterization of novel Cu(II)-bipyridine complexes having functional groups and their application toward molecular recognition

Two kinds of Cu(II) complexes having 2,2′-bipyridine derivatives with two 1-naphthoylamide groups or two ethyl dimethylmalonylamide moieties at 6 and 6′ positions as ligands were prepared and characterized by X-ray crystallography and spectroscopic methods. Those ligands bound to the Cu(II) centers in a tetradentate fashion including two amide oxygen atoms in the equatorial planes. Those complexes were found to recognize carboxylic acids as guest molecules by coordination and additional non-covalent interactions, including intramolecular π-π interactions or hydrogen bonding.     

Synthesis, characterization and coordination chemistry of dibenzofuran derivatives of 1,4,7,10-tetraazacyclododecane

A 1,4-disubstituted dibenzofuran derivative of 1,4,7,10-tetraazacyclododecane (cyclen), L1, has been prepared by the direct reaction of cyclen and chloroacetyldibenzofuran and the mono-substituted derivative, L2, by reaction of chloroacetyldibenzofuran and 1,4,7-tris(t-butoxycarbonyl)-1,4,7,10-tetraazacyclododecane followed by deprotection with trifluoroacetic acid. The ligands were characterized by ¹H and ¹³C NMR spectroscopy, IR spectroscopy and mass spectrometry. The reaction of the 1,4-disubstituted dibenzofuran cyclen, L1, with Cu(ClO₄)₂·6H₂O in methanol yielded crystals of [CuL1](ClO₄)₂·MeOH·1/2H₂O that were suitable for single crystal structural analysis. The X-ray structure confirmed that the 1,4-disubstituted dibenzofuran cyclen had been formed. The copper(II) coordination sphere in the complex cation, [CuL1]²⁺, is occupied by four nitrogen atoms from the macrocycle and an amide oxygen donor from one dibenzofuran pendant group. As is typical for copper(II)-cyclen complexes, the Cu(II) centre sits above the plane of the macrocycle nitrogen towards the oxygen donor, in this case by 0.5 Å. Fluorescence emission studies indicate that coordination of the macrocycle to either copper(II) or zinc(II) results in a decrease in emission with respect to the emission of the pure ligand.     

Synthesis and X-ray crystal structures of dinuclear silver(I) complexes of heteroaryl thioethers

The X-ray crystal structures of four complexes, obtained by reaction of silver nitrate with four different heteroaryl thioethers, are described. In these compounds the ligands act as dinuclear bridges between silver atoms, with coordination exclusively through the nitrogen donor atoms. All ligands form dinuclear complexes, either as discrete species or as higher aggregates involving additional nitrate bridges. π–π Stacking interactions provide extra stabilisation in some of the structures.     

Synthesis, structural characterization and antiproliferative and toxic bio-activities of copper(II) and nickel(II) citronellal N-ethylmorpholine thiosemicarbazonates

This paper reports the syntheses and characterization of ethylmorpholine substituted citronellal thiosemicarbazone copper(II) and nickel(II) metal complexes. The compounds were characterized through elemental analyses and spectroscopic (IR, UV-Vis, NMR, MS) methods. The X-ray analysis of the two complexes shows that both Ni and Cu derivatives present a square planar coordination, where the coordinating homologous donor atoms bind in trans to each other. The compounds were tested for their biological activity after determination of their octanol-saline partition coefficients, followed by their radical scavenging properties. Eventually the complexes were tested for their proliferation inhibition on human histiocytic lymphoma U937 cell line. The GI₅₀ values resulted to be 2.3 μM for the copper derivative and 12.3 μM for the nickel derivative.     

1,3-Dipolar cycloaddition to the FeSC fragment 20. Preparation and properties of carbonyliron complexes of di-thiooxamide. Reactivity of the mononuclear (di-thiooxamide)Fe(CO)3 towards dimethyl acetylenedicarboxylate

Reaction of Fe₂(CO)₉ at room temperature in THF with the di-thiooxamides (L), SC{N(R,R′)}C{(R,R′)N}S [R=Me, R′-R′=(CH₂)₂ (a); R=H, R′=ⁱPr (b); R=H, R′=iPr (c), R=H, R′=benzyl (d); R=H, R′=H (e)], results for ligands a-d initially in the formation of the mononuclear σ-S, σ-S′ chelate complexes Fe(CO)₃(L) (7a-d), which could be isolated in case of 7a and 7d. Under the reaction conditions, complexes 7a-d react further with [Fe(CO)₄] fragments to give three types of Fe₂(CO)₆(L) complexes (8a-d) in high yields, depending on the di-thiooxamide ligand used together with traces of the known complex S₂Fe₃(CO)₉ (14). The molecular structures of these complexes have been established by the single crystal X-ray diffraction determinations of 8a, 8b and 8d. In the reaction with ligand e the corresponding complex 7e was not detected and the well-known complexes 14 and S₂Fe₃(CO)₉ (15) were isolated in low yield. In situ prepared 7a reacts in a slow reaction with 1 equiv. of dimethyl acetylene dicarboxylate in a 1,3-dipolar cycloaddition reaction to give the stable initial ferra [2.2.1] bicyclic complex 10a in 60% yield. In complex 10a an additional Fe(CO)₄ fragment is coordinated to the sulfido sulfur atom of the cycloadded FeSC fragment. When a toluene solution of 10a is heated to 50 °C it loses two terminal CO ligands to give the binuclear FeFe bonded complex 11a in almost quantitative yield. The molecular structures of 10a and 11a have been confirmed by single crystal X-ray diffraction. Reaction of 7d at room temperature with 2 equiv. of dimethyl acetylene dicarboxylate results in the mononuclear complex 12d in 5% yield. The molecular structure of 12b has been established by single crystal X-ray diffraction and comprises a tetra dentate ligand with two ferra-sulpha cyclobutene, and a ferra-disulpha cyclopentene moiety. When the reaction is performed at 60 °C a low yield of 2,3,4,5-thiophene tetramethyl tertracarboxylate is obtained besides complex 12d.     

Chiral P-monodentate phosphoramidite and phosphite ligands for the enantioselective Pd-catalyzed allylic alkylation

The improved synthesis of the chiral phosphoramidite Ia, based on a biphenol backbone and bearing chiral bis(1-phenylethyl)amine group on the phosphorus atom, is described together with its Pd(II) complex. The chiral complex cis-PdCl₂L₂ (L = Ia) has been characterized by X-ray crystal structure analysis and spectroscopic data. The series of the chiral P-monodentate phosphoramidite and phosphite ligands was tested in Pd-catalyzed enantioselective allylic substitution of different substrates. In the palladium-catalyzed asymmetric allylic alkylation of 1,3-diphenylallyl acetate with dimethylmalonate, up to 79% ee was achieved with [Pd₂(dba)₃] × CHCl₃ as precatalyst.     

New palladacyclopentadiene complexes containing an N,P-donor setting. Crystal structure of [Pd{C4(COOMe)4}(o-Ph2PC6H4CHNPr)]

The synthesis of palladacyclopentadiene derivatives with the mixed-donor bidentate ligands o-Ph₂PC₆H₄CHNR (N^∧P) has been achieved. The new complexes of general formula [Pd{C₄(COOMe)₄}(o-Ph₂PC₆H₄CHNR)] [R=Me (1), Et (2), ⁱPr (3), ^tBu (4), NHMe (5)] have been prepared by reaction between the precursor [Pd{C₄(COOMe)₄}]_n and the corresponding iminophosphine. The polymer complex [Pd{C₄(COOMe)₄}]_n also reacts with pyridazine (C₄H₄N₂) to give the insoluble dinuclear complex [Pd{C₄(COOMe)₄}(μ-C₄H₄N₂)]₂ (6), which has been successfully employed as precursor in the synthesis of pyridazine-based palladacyclopentadiene complexes. The reaction of 6 with tertiary phosphines yielded complexes containing an N,P-donor setting of formula [Pd{C₄(COOMe)₄}(C₄H₄N₂)(L)] (L=PPh₃ (7), PPh₂Me (8), P(p-MeOC₆H₄)₃ (9), P(p-FC₆H₄)₃ (10)). The new complexes were characterized by partial elemental analyses and spectroscopic methods (IR, ¹H, ¹⁹F and ³¹P NMR). The molecular structure of complex 3 has been determined by a single-crystal diffraction study, showing that the iminophosphine acts as chelating ligand with coordination around the palladium atom slightly distorted from the square-planar geometry.     

Cationic palladium(II) complexes of the sterically hindered bis(4-methylthiazolyl)isoindoline (4-Mebti) with neutral group XVI donor ligands

A series of cationic palladium complexes [(4-Mebti)PdL]⁺ with 4-Mebti = anion of bis(4-methylthiazolylimino)isoindoline and L = neutral ligand with group 16 donor atom has been prepared from the chlorido derivative [(4-Mebti)PdCl] and NaBAr^F (BAr^F = tetrakis(3,5-bis(trifluoromethyl)phenyl)boranate) in the presence of the respective donor ligand. Crystallographic and spectroscopic analyses were achieved for species with L = SMe₂, SeMe₂, dmf, acetamide, diphenylurea, and formiate. The latter two complexes represent products from hydrolyses of phenyl isocyanate and dmf, respectively, which occur during the ligand exchange reactions. Several other O-donor ligands like thf, acetone, Me₂O, water, and others are not bound to the palladium ion, and the dinuclear μ-chlorido derivative [{(4-Mebti)Pd}₂Cl]⁺ is isolated in these cases instead. The crystallographic analyses prove the expected presence of distorted, pseudo-planar palladium chelates, and the degree of distortion correlates well with the chemical shifts observed for the proton nuclei of the terminal methyl groups in the ¹H NMR experiment.     

Energy-minimized structures and MO levels of catalysts related to [RuO(hpsd)(bpy)] that competently hydroxylate benzene (hpsd(2-)=(2-hydroxyphenyl)salicyldiminato)

Two series of complexes with formal oxidation state assignments of {Ru^V(O²⁻)} have been examined by molecular mechanics and molecular orbital methods at the level of PM3 calculations in order to assess the origin of differences in the activity of these complexes in the conversion of benzene to phenol by oxygen transfer. The first series includes complexes of general formula [RuO(hpsd)(XY)]ⁿ⁺ with hpsd²⁻ (also known in the literature as amp²⁻)=(2-hydroxyphenyl)salicyldiminato; XY=bpy(2,2^′-bipyridine) and other py-X, wherein the second pyridyl group of bpy is changed to X=-CH₂N(CH₃)₂ (stronger σ-donor X), -CH₂P(CH₃)₂ (better π-acceptor X), -CO₂ ⁻ (weak π-donor X), -CH₂S⁻ (strong π-donor X), and -CH₂C(CH₃)₂ ⁻ (very strong σ-donor X).A second series of complexes, [RuO(TDL)(bpy)]ⁿ⁺ was also studied with TDL=(tridentate ligand) of the parent hpsd²⁻ (or amp²⁻); cpsd²⁻=(N-(2-carboxyphenyl)salicylaldiminato); cppc⁻=(N-2-carboxyphenylpyridine-2-carboxaldiminato); and hppc⁻=(2-hydroxyphenyl)2-pyridylcarboxaldiminato (or app⁻). Experimentally, the activity order based upon the percentage yields of oxygenated products for [RuO(TDL)(bpy)]ⁿ⁺ is as follows for TDL’s=hpsd²⁻ (91%) > cppc⁻ (87%) > cpsd²⁻ (84%) > hppc⁻ (80%). The rates approach toward saturation in reactivity as a function of the fractional positive charge on the apical O center: cppc⁻ (0.233) > hpsd²⁻ (0.166) > cpsd²⁻ (0.105) > hppc⁻ (0.041). The reactivity order follows chelate ring strain influences of the TDL, with 5,6-membered chelate rings; hpsd²⁻ and cppc⁻ > 6,6; cpsd²⁻ > 5,5; hppc⁻.It was determined that the general structures of these complexes are best described as pentagonal pyramidal (rather than pseudo-octahedral) with the RuO unit apical, the three donors of hpsd²⁻, cpsd²⁻, cppc⁻ or hppc⁻, and the two donors of XY ligands adopting a waffled arrangement around the Ru center as the remaining donors of the pentagonal set. The donor most trans to the apical RuO is approximately at 140°, rather than 180°. Ligands such as hpsd²⁻ (amp²⁻) are not retained in a single planar array, but rather with one of the aromatic donors turned upward to shield the approach of the RuO unit from one side. The ligand series [RuO(hpsd)(XY)]⁺ averages angles between adjacent atoms of the pentagonal set of 75.4° instead of a theoretical 72.0°; angles between the apical RuO and adjacent donors average 111° but with wide deviations (±30°) depending upon the donors of the TDL.Small changes in the donor atom positions, and in the capability of the “trans” donor’s σ-donor strength, and whether it is a π-acceptor or a π-donor, modulate the degree of mixing of ligand orbitals and the LUMO/SOMO energy gap which influences reactivity. The presence of a π-acceptor ligand provides the most destabilization of Ru-O π bonding, and this appears to be the best way to increase the activity of these catalysts toward oxidation of C₆H₆ to C₆H₅OH. Also, implicated in the activity of the catalysts is the need for two non-innocent phenolate donors that raise the energy of orbitals on the apical O atom. This increases the oxenoid character of the terminal O, and makes the insertion into a C-H bond more favored.