P.A. Cox - Inorganic chemistry (793955), страница 53
Текст из файла (страница 53)
Most examples with coordination compounds have chelating (e.g. bidentate) ligands (see Topic E3).Structures 10 and 11 show respectively the delta and lambda isomers of a tris(chelate) complex, with the bidentateligands each denoted by a simple bond framework. As discussed in Topic C3, optical isomerism is possible only when aspecies has no improper symmetry elements (reflections or inversion).
Structures 10 and 11 have the point group D3, withonly C3 and C2 rotation axes.Section H—Chemistry of transition metalsH7COMPLEXES: KINETICS AND MECHANISMKey NotesLigand exchangeOctahedral complexesSquare-planarcomplexesElectron transferreactionsRelated topicsLigand exchange mechanisms may be associative (A), dissociative (D)or interchange (Ia or Id) in nature. Kinetically inert complexes areformed by Cr3+ and Co3+, and by 4d and 5d ions with d6 and d8configurations.The mechanism becomes more dissociative for 3d ions later in theseries.
Substitution rates may be increased by the conjugate basemechanism.Exchange in square-planar complexes is associative, and is influencedby the trans effect, whereby some ligands facilitate substitution of transligands.Inner sphere mechanisms involve a bridging ligand, whereas in outersphere mechanisms ligand coordination remains intact. Thereorganization of metal-ligand distances is important in determiningouter sphere electron transfer rates. These can be very slow for redoxreactions involving CoIII complexes.Complex formation (E3)Complexes: structure andLigand field theory (H2)isomerism (H6)Ligand exchangeLigand exchange reactions are of the kindand are effectively nucleophilic substitutions. The possible mechanisms are classified as associative (A) ordissociative (D) according to whether the new bond is formed before or after the old one is broken, or interchange(I), the intermediate case, which can be subdivided into Ia or Id according to the degree of associative or dissociativecharacter.
Kinetic studies of ligand exchange can sometimes distinguish between the mechanisms although these resultscan be misleading. Determination of the volume or entropy of activation (i.e. the volume or entropy change in thetransition state) can often give guidance.For many metal ions ligand exchange is an extremely fast reaction, with rate constants close to the limit of diffusioncontrol (around 1010 M−1 s−1 in water). There is a correlation with the charge and size, and outside the transition seriesBe2+ and Al3+, which have large charge/size ratio, have significantly slower exchange. With transition metals theinfluence of ligand field effects is apparent (see Topic H2). Complexes of Cr3+ (d3) and Co3+ (d6) and of many d6 and d8SECTION H—CHEMISTRY OF TRANSITION METALS231ions in the 4d and 5d series are kinetically inert and undergo ligand substitution many orders of magnitude more slowlythan comparable nontransition ions.
These ions have a ligand field stabilization energy (LFSE) that contributes a barrierto the geometrical change required in the transition state. A large LFSE value also gives shorter bond lengths, which enhanceother contributions (electrostatic, etc.) to the metal-ligand bond strength.The existence of kinetically inert complexes is useful in mechanistic studies, and important for the separation ofdifferent isomers (see Topic H6).Octahedral complexesMost M2+ ions of the 3d series undergo ligand exchange at a rate comparable with that for nontransition metal ions ofsimilar size.
V2+ (d3) and Ni2+ (d8) are somewhat slower, these being the electron configurations that give maximumoctahedral LFSE for high-spin ions. Entropies and volumes of activation suggest a change from predominantly Iamechanisms early in the series (e.g. V2+) to Id towards the end (e.g. Ni2+). Both decreasing size and increasing d orbitaloccupancy may contribute to this trend. Incoming ligands in the Ia mechanism must approach an octahedral complexalong directions where the t2g orbitals normally point (see Topic H2, Fig.
1). Filling these orbitals will tend to inhibit theapproach of ligands and favor the dissociative pathway.For the kinetically inert low-spin CoIII complexes the mechanism of exchange is certainly dissociative although kineticstudies can give results that are super-ficially misleading. For example, the base hydrolysis reactionhas a rate proportional to the concentrations of both complex and OH−.
This is not indicative of an associativemechanism, but of a conjugate base mechanism where the first reversible step is deprotonation of the complex:Deprotonation trans to the leaving group is especially effective at promoting the dissociation step. The conjugate basemechanism cannot operate if a tertiary amine with no ionizable proton is placed trans to the leaving group; as expectedthe rate of substitution is then slower and does not depend on [OH−].Square-planar complexesKinetically inert square-planar complexes are formed by d8 low-spin ions, especially Pt2+. Ligand substitution isassociative and correlated with the ease of forming a five-coordinate transition state (or intermediate).
Substitution ismuch faster with Ni2+ where five-coordinate complexes such as [Ni(CN)5]3− are more stable than for Pt. For a givenmetal, the rate of substitution is controlled by:• the nature of the incoming and leaving ligands; more polarizable groups are generally faster in both bond-making andbreaking processes;• the trans effect, which is the ability of some ligands to facilitate the substitution of the ligand trans to them in thecomplex. Some ligands in order of increasing effectiveness are:The trans effect is a kinetic phenomenon and is influenced by different factors that operate either in the ground state orin the five-coordinate transition state.
Some ligands weaken the bond trans to them in the original complex. This ground-232H7—COMPLEXES: KINETICS AND MECHANISMstate phenomenon is called the trans influence, and depends mostly on the σ bonding capability and the polarizabilityof the ligand. Some ligands such as CN− do not show much trans influence but nevertheless have a large kinetic transeffect, because their π-acceptor properties help in the stabilization of the transition state.The trans effect is useful in synthesis. For example, different isomers are formed in the reactions below by the greatertrans directing ability of Cl− compared withElectron transfer reactionsElectron transfer is the simplest type of redox process, an example beingA majority of reactions of this type are very fast, but oxidation by some complexes (especially of CoIII) is much slower.In an inner sphere process, the coordination sphere of one complex is substituted by a ligand bound to the othercomplex, which then acts as a bridge and may be transferred during the redox process.
For example, isotopic labelingstudies show that the oxidation of aqueous Cr2+ with [CoIII(NH3)5Cl]2+ proceeds via a bridged species Cr-Cl-Co, thechlorine not exchanging with free labeled Cl− in solution but remaining attached to the kinetically inert CrIII product.An inner sphere mechanism requires one of the reactants to be substitutionally labile, and a ligand that can act as abridge. One test is to compare the rates of reaction with the ligands azideand (N bonded) thiocynanate NCS−;azide is generally better at bridging and so gives faster rates if the inner sphere route is operating.The outer sphere mechanism involves no disruption of the coordination of either complex, and is always availableas a route to electron transfer unless the inner sphere rate is faster. The Marcus theory shows that the rate of outersphere transfer depends on:(i) the orbital interaction between the two metal centers involved, a factor that decreases roughly exponentially withthe distance between them;(ii) the change in metal-ligand distances resulting from electron transfer, the effect that provides most of the activationenergy for the reaction;(iii) an enhancement term, which depends on the difference of redox potentials of the two couples involved.Reactions of complexes containing unsaturated ligands such as bipyridyl are generally fast because the π systemfacilitates transfer, and because the change in geometry is small (as significant charge is distributed over the ligand).
Onthe other hand, oxidation by [Co(NH3)6]3+ is often very slow. The orbital interaction term is small because the reactionis ‘spin forbidden’, the ground state of Co changing from low-spin d6 with no unpaired electrons to high-spin Co2+ d7with three. The activation energy is also large because the number of eg electrons increases by two, which gives asignificant change of LFSE and so causes a large increase in the metal-ligand distances. The inner sphere route isunavailable as NH3 does not normally act as a bridging ligand.Section H—Chemistry of transition metalsH8COMPLEXES: ELECTRONIC SPECTRA AND MAGNETISMKey NotesElectronic transitionsd-d spectraCharge transfer spectraParamagnetismRelated topicsElectronic absorptions, in which an electron is excited to a higherenergy orbital, occur in the visible and neighboring parts of thespectrum. Transitions are classified as d-d, ligand-to-metal chargetransfer (LMCT), metal-to-ligand charge transfer (MLCT), or ligandbased.d-d transitions are weak, especially in centro-symmetric complexes.The number of transitions depends on the d-electron configuration.The energies provide information about ligand field splittings andelectron repulsions.Charge transfer energies may be correlated with redox potentials.LMCT is at low energy if the metal ion is easily reduced or the ligandeasily oxidized.Paramagnetism depends on the number of unpaired electrons and canprovide information about spin states and metal-metal bonding.Methods of characterizationLigand field theory (H2)(B7)Electronic transitionsIn an electronic transition an electron is excited from an occupied to an empty molecular orbital (MO).
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
