Driving forces in substitution reactions of octahedral complexes: The influence of the competitive effect

The reaction of cis-[RuCl₂(P–P)(N–N)] type complexes (P–P = 1,4-bis(diphenylphosphino)butane or (1,1′-diphenylphosphino)ferrocene; N–N = 2,2′-bipyridine or 1,10-phenantroline) with monodentate ligands (L), such as 4-methylpyridine, 4-phenylpyridine and benzonitrile forms [RuCl(L)(P–P)(N–N)]⁺ species. Upon characterization of the isolated compounds by elemental analysis, ³¹P{¹H} NMR and X-ray crystallography it was found out that the type of the L ligand determines its position in relation to the phosphorus atom. While pyridine derivatives like 4-methylpyridine and 4-phenylpyridine coordinate trans to the phosphorus atom, the benzonitrile ligand (bzCN), a good π acceptor, coordinates trans to the nitrogen atom. A ³¹P{¹H} NMR experiment following the reaction of the precursor cis-[RuCl₂(dppb)(phen)] with the benzonitrile ligand shows that the final position of the entering ligand in the complex is better defined as a consequence of the competitive effect between the phosphorus atom and the cyano-group from the benzonitrile moiety and not by the trans effect. In this case, the benzonitrile group is stabilized trans to one of the nitrogen atoms of the N–N ligand. A differential pulse voltammetry experiment confirms this statement. In both experiments the [RuCl(bzCN)(dppb)(phen)]PF₆ species with the bzCN ligand positioned trans to a phosphorus atom of the dppb ligand was detected as an intermediate complex.     

Cluster core controlled reactions of substitution of terminal bromide ligands by triphenylphosphine in octahedral rhenium chalcobromide complexes

Reactions of rhenium chalcobromides Cs4[{Re6(mu3-S)8}Br6].2H2O, Cs3[{Re6(mu3-Se)8}Br6].2H2O, Cs3[{Re6(mu3-Q)7(mu3-Br)}Br6].H2O (Q = S, Se), and K2[{Re6(mu3-S)6(mu3-Br)2}Br6] with molten triphenylphosphine (PPh3) have resulted in a family of novel molecular hybrid inorganic-organic cluster compounds. Six octahedral rhenium cluster complexes containing PPh3 ligands with general formula [{Re6(mu3-Q)8-n(mu3-Br)n}(PPh3)4-nBrn+2] (Q = S, n = 0, 1, 2; Q = Se, n = 0, 1) have been synthesized and characterized by X-ray single-crystal diffraction and elemental analyses, 31P{1H} NMR, luminescent measurements, and quantum-chemical calculations. It was found that the number of terminal PPh3 ligands in the complexes is controlled by the composition and consequently by the charge of the cluster core {Re6Q8-nBrn}n+2. In crystal structures of the complexes with mixed chalcogen/bromine ligands in the cluster core all positions of a cube [Q8-nBrn] are ordered and occupied exclusively by Q or Br atoms. Luminescence characteristics of the compounds trans-[{Re6Q8}(PPh3)4Br2] and fac-[{Re6Se7Br}(PPh3)3Br3] (Q = S, Se) have been investigated in CH2Cl2 solution and the broad emission spectra in the range of 600-850 nm were observed.     

An apparent violation of microscopic reversibility: mechanisms for ligand substitution reactions of oxorhenium(V) dithiolate complexes

The first theoretical investigation on the mechanism for ligand substitution in five-coordinate square pyramidal oxorhenium dithiolate complexes, CH3ReO(SCH2C6H4S)-X, has been carried out with the Perdew-Burke-Ernzerhof density functional and the Stuttgart relativistic effective-core-potential basis sets. In the mechanism proposed in the experimental kinetic studies, the entering ligand Y attacks the vacant lower axial coordinate site trans to O, and the resulting six-coordinate intermediate rearranges through a turnstile twist (a trigonal prismatic intermediate or transition state) or pentagonal pyramid to allow the leaving ligand X to exit from the same site. These workers proposed this rearrangement to avoid a violation of microscopic reversibility. The computed energy barriers in this reaction pathway show that the turnstile or pentagonal pyramidal transition states are too high in energy to make this pathway accessible. Although the vacant lower axial site in the rhenium complex is the site most easily attacked by Y, transition states for X leaving from the site cis to the attack have quite low energy barriers. Although this direct-exchange mechanism was thought to lead to a violation of microscopic reversibility, we show that these direct-exchange pathways provide low-energy routes for ligand exchange and clarify this apparent violation of microscopic reversibility. Furthermore, computed results of different entering and leaving ligand pairs are analyzed for their effect on the choice of reaction pathways. In the direct-exchange mechanism, the replacement of 4-tBu-pyridine by PPhMe2 is monophasic without an intermediate, but the replacement of PPh3 by PPhMe2 is biphasic (proceeds by a two-stage pathway) and generates the observed intermediate, an isomer of product. These predictions are completely consistent with the observed experimental phenomena. The accuracy of the particular functional/basis set used for the study is compared to 10 functionals in 19 basis sets and to large basis set coupled cluster calculations on model systems.     

Spin-orbit electronic states of octahedral Pt-group d 6 complexes as derived from reflectance spectra and ligand field calculations

The 90 K diffuse reflectance spectra of some octahedral Rh(III), Ir(III) and Pt(IV) complex compounds with chloride, thiocyanate and amine ligands are reported. The spectra show in the visible and near ultraviolet distinct bands due to spin-orbit components of singlet-triplet d-d transitions which are explained by ligand field calculations including spin-orbit coupling considering all possible d-electron configurations. Model parameter sets are obtained by fitting the measured band peaks to possible transitions between calculated energy levels. For some of the bands the vibrational structure could be resolved which is assigned to metal-ligand stretching vibrations of the electronically excited complex octahedra with vibrational fundamentals lower than those of the ground state.     

New chelate ring opening reactions leading to the synthesis of novel mixed ligand complexes of the octahedral metal carbonyls

Reactions of the four-membered ring chelate CH₃N(PF₂)₂Cr(CO)₄ with trivalent phosphorous ligands (L) at 80°C leads to facile opening of the chelate ring to give the mixed ligand complexes trans-CH₃N(PF₂)₂Cr(CO)₄L (L=C₆H₅)₃P or monodentate C₆H₅N(PF₂)₂) containing a monodentate CH₃N(PF₂)₂ ligand.     

Substitution in labile octahedral metal complexes – a retrospective view

Transition Metal Chemistry -     

High pressure effects on ligand substitution reactions of molybdenum(0) carbonyl complexes

Summary Substitution reactions ofcis-Mo(CO)₄(py)₂, Mo(CO)₅-(4-Mepy) and Mo(CO)₅(py) with biacetylbis(phenylimine), bipyridine and 1,10-phenanthroline in toluene and 1,2-dichloroethane were studied as a function of temperature and pressure. The volumes of activation are +4 cm³ mol^–1 and almost zero for the tetra- and penta-carbonyl systems, respectively, and independent of entering ligand and solvent. Furthermore, these values closely parallel the trend in S^#. The results are discussed in terms of earlier suggested mechanisms and relevant data recently reported in the literature.     

Gas-phase ligand loss and ligand substitution reactions of platinum(II) complexes of tridentate nitrogen donor ligands

The source of protons associated with the ligand loss channel of HX((n - 1)+) from [Pt(II)(dien)X](n+) (X = Cl, Br and I for n = 1 and X = NC(5)H(5) for n = 2) in the gas phase was investigated by deuterium-labelling studies. The results of these studies indicate that these protons originate from both the amino groups and the carbon backbone of the dien ligand. In some instances (e.g. X = Br and I), the protons lost from the carbon backbone can be even more abundant than the protons lost from the amino groups. The gas-phase substitution reactions of coordinatively saturated [Pt(II)(L(3))L(a)](2+) complexes (L(3) = tpy or dien) were also examined using ion-molecule reactions. The outcome of the ion-molecule reactions depends on both the ancillary ligand (L(3)) as well as the leaving group (L(a)). [Pt(II)(tpy)L(a)](2+) complexes undergo substitution reactions, with a faster rate when L(a) is a good leaving group, while the [Pt(II)(dien)L(a)](2+) complex undergoes a proton transfer reaction.     

X-ray fluorescence analysis from the information theory point of view

Information theory is a fundamental concept for a general comparison of different methods of chemical analysis. It delivers the information content of an analysis in binary digits (bits). In contrast to the normal interpretation of the result of an analysis, information theory demonstrates the gain of information about the composition of a sample before and after the analysis has been made. In the first section of this paper, the useful information content of a quantitative analysis of a completely unknown sample with an X-ray spectrometer is derived. It is shown that the decision for the discrimination steps has to be made on the basis of a geometrical rather than an arithmetical series, as is recommended in the present literature. According to this thesis, an XRF-equipment can deliver an information quantity of about 940 bits.In the second section, examples are given of the information content obtained in the field of process control. Here the analyst has a lot of information on the composition of the sample in advance and the analysis provides much less information content than in case of an unknown sample. In three examples, concerning the manufacturing of stainless steel brass, and cement, it is shown, that the gain in information by the XRF-equipment lies between 3 and 10 bits.     

Quantitative densitometry from the point of view of information theory.

The process of quantitative densitometry is analyzed with methods developed in information theory. It is shown that the steps involved in densitometry, e.g. gel staining, mechanical, optical and electronic processing, as well as all the steps of data processing, can be viewed as communication channels. The factors affecting both the relevant and irrelevant part of the total information passed through these channels are discussed in the consistent frame provided by information theory. This view leads to a unifying context for analyzing the performance of quantitative densitometers.     

Expanded redox accessibility via ligand substitution in an octahedral Fe6Br6 cluster

Oxidation of the nominally all-ferrous hexanuclear cluster ((H)L)(2)Fe(6) with six equivalents of ferrocenium in the presence of bromide ions results in a six-electron oxidation of the Fe(6) core to afford the nominally all-ferric cluster ((H)L)(2)Fe(6)Br(6). The hexabromide cluster is also structurally characterized in a 4+ core oxidation state. A structural comparison of these two clusters provides an insight into the Fe(6) core electronic structure.     

The cation influence on substitution reactions in aqua dioxalate uranyl complexes

The cation influence on substitution reactions in aqua dioxalate uranyl complexes with the participation of a fluoride ion as an attacking ligand and two-charge cations of ethylene diammonium and propylene diammonium was considered. The structure of (C₂H₁₀N₂)₃[UO₂(C₂O₄)₂F]₂ · 6H₂O and (C₃H₁₂N₂)[UO₂(C₂O₄)₂(H₂O)] complexes was determined by X-ray crystallography.     

Promoting effect of ionic liquids on ligand substitution reactions

Ionic liquid solvents N-hexylpyridinium bistrifylimide ([C₆pyr][Tf₂N]] and 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄mim][PF₆]) promoted the displacement of anionic ligands by pyridine derivatives in trans-(Ph₃P)₂Rh(CO)NO₃ to a much greater extent than did dichloromethane. Thus, addition of a slight excess of 2-fluoropyridine to trans-(Ph₃P)₂Rh(CO)NO₃ in [C₄mim][PF₆] gave a 29:71 product mixture of trans-(Ph₃P)₂Rh(CO)NO₃:[trans-(Ph₃P)₂Rh(CO)(2-fluoropyridine)][NO₃], while the ratio was 91:9 in dichloromethane.     

The kinetics and mechanisms of aromatic nucleophilic substitution reactions in liquid ammonia

The rates of aromatic nucleophilic substitution reactions in liquid ammonia are much faster than those in protic solvents indicating that liquid ammonia behaves like a typical dipolar aprotic solvent in its solvent effects on organic reactions. Nitrofluorobenzenes (NFBs) readily undergo solvolysis in liquid ammonia and 2-nitrofluorobenzene is about 30 times more reactive than the 4-substituted isomer. Oxygen nucleophiles, such as alkoxide and phenoxide ions, readily displace fluorine of 4-NFB in liquid ammonia to give the corresponding substitution product with little or no competing solvolysis product. Using the pK(a) of the substituted phenols in liquid ammonia, the Br?nsted β(nuc) for the reaction of 4-NFB with para-substituted phenoxides is 0.91, indicative of the removal of most of the negative charge on the oxygen anion and complete bond formation in the transition state and therefore suggests that the decomposition of the Meisenheimer σ-intermediate is rate limiting. The aminolysis of 4-NFB occurs without general base catalysis by the amine and the second-order rate constants generate a Br?nsted β(nuc) of 0.36 using either the pK(a) of aminium ion in acetonitrile or in water, which is also interpreted in terms of rate limiting breakdown of the Meisenheimer σ-intermediate. Nitrobenzene and diazene are formed as unusual products from the reaction between sodium azide and 4-NFB, which may be due to the initially formed 4-nitroazidobenzene decomposing to give a nitrene intermediate, which may then give diazene or be trapped by ammonia to give the unstable hydrazine which then yields nitrobenzene.