P.A. Cox - Inorganic chemistry (793955), страница 54
Текст из файла (страница 54)
The energy ofsuch transitions normally corresponds to photons in the near IR, visible or UV region of the electromagnetic spectrum.Electronic absorption bands give rise to the colors of compounds, including ones without transition metals (seeTopic D7).In d-block complexes various types of MO can be involved. In d-d transitions both the lower and upper MOs arethose based on the d atomic orbitals, split by interaction with the ligands (see Topic H2). Charge transfertransitions involve ligand-based MOs as well, and may be divided into ligand-to-metal charge transfer (LMCT,the commonest type) or metal-to-ligand charge transfer (MLCT).
There may also be transitions between twoligand MOs (e.g. n to π* in unsaturated ligands). Charge transfer and ligand-based transitions often appear at higherenergy than d-d transitions, and are generally also more intense. Figure 1 shows the absorption spectrum of [Ti(H2O)6]3+. The d-d transition peaks at around 20 000 cm−1 (500 nm) corresponding to green light, giving a violet color to thecomplex (transmitting red and violet light). The strong absorption rising beyond 25 000 cm−1 is due to LMCT.234SECTION H—CHEMISTRY OF TRANSITION METALSFig. 1.
Absorption spectrum of [Ti(H2O)6]3+.d-d spectrad-d transitions are weak because of atomic selection rules, which make transitions between d orbitals forbidden.They remain forbidden in complexes with a center of symmetry (e.g. octahedral or square planar, see Topic C3),and appear only because of vibrational motions that break this symmetry. In complexes without a center of symmetry(e.g. tetrahedral) the transitions are stronger but are still weak compared with charge transfer. There are also spinselection rules, the strongest transitions being spin-allowed ones where there is no change in the number ofunpaired electrons.In a d1 octahedral complex such as [Ti(H2O)6]3+ excitation of an electron from t2g to eg gives a single absorption bandat an energy equal to the ligand field splitting Δo (see Topic H2).
The theory is more complicated for ions with many delectrons because the energy of a state is now determined by the repulsion between electrons as well as the occupancy oft2g and eg orbitals. The predicted number of spin-allowed d-d transitions in high-spin octahedral or tetrahedral complexesis shown below. Not all transitions may be visible in all cases, because bands may overlap or some may be obscured bycharge transfer:one for d1, d4, d6 and d9;three for d2, d3, d7 and d8;none for d0, d5 and d10.The absence of spin-allowed transitions for high-spin d5 can be understood from the fact that in ground state all dorbitals are singly occupied by electrons having parallel spin (see Topic H2, Fig.
3). This is the only possible state withfive unpaired electrons, and any d-d transition must involve a change of spin, d-d transitions in high-spin Mn2+ and Fe3+complexes are indeed very weak compared with other ions, which have spin-allowed transitions.H8—COMPLEXES: ELECTRONIC SPECTRA AND MAGNETISM235A mathematical analysis of the transition energies in dn ions allows Δo to be determined as well as electronrepulsion parameters. Electron repulsion between d electrons in complexes is found to be less than in the free gasphase dn ions. This reduction is called the nephelauxetic effect (meaning ‘cloud expanding’) and arises because ‘dorbitals’ in complexes are really MOs with some ligand as well as metal contribution, so that electrons are on averagefurther apart than in the pure d orbitals of the uncombined ions.
Larger nephelauxetic reductions are observed incomplexes with ‘soft’ ligands such as I− than with ‘hard’ ones such F−, reflecting the greater degree of covalent bondingin the former case.Charge transfer spectraCharge transfer is analogous to an internal redox reaction, and the absorption energies can be correlated withtrends in redox properties (see Topics E5 and H3). In LMCT an electron is transferred to the metal, which is thereforereduced in the excited state.
The more positive the redox potential concerned, the easier such reduction will be, and sothe lower the LMCT energy. LMCT transitions in the visible region of the spectrum give intense color, as is found withpermanganatea d0 complex, which therefore has no d-d transitions. The energy trends in some d0 species are:(i)(ii)which follow the trends towards less strongly oxidizing compounds, (i) towards the left in the 3d series (see Topic H3)and (ii) down each transition metal group (see Topic H5). The above orders of LMCT energy are reflected in thechanging colors of the ions (e.g.deep purple,deep yellow,pale yellow, as the transition movesprogressively to higher energy out of the visible spectrum into the UV).LMCT energies also follow expected trends as the ligand is changed, for example, O>S, and F>Cl>Br, as theheavier ions in each group are more easily oxidized (see Topic F1).
With different metal ions, there is a general decreasein energy towards the right in each series. For ions in lower oxidation states, LMCT often occurs in the UV rather thanthe visible part of the spectrum.MLCT is less common, as it requires the existence of empty ligand orbitals of suitable energy. Many of these ligandsare π acceptors (see Topics H2 and H9). With changing metal ions and oxidation states, MLCT bands often follow thereverse of the trends found with LMCT.ParamagnetismIn diamagnetism substances are repelled by a magnetic field: this property is associated with all closed electron shells.Paramagnetic substances are attracted into a magnetic field, the force being related to the magneticsusceptibility. Paramagnetism normally arises from the spin of unpaired electrons (see Topic A3).
The Curie lawfor the susceptibility per mole (χm) iswhere NA is Avogadro’s number, µ0 the magnetic permeability of free space, µeff the effective magnetic moment ofthe paramagnetic species, k is Boltzmann’s constant and T the temperature in kelvin. The inverse temperaturedependence arises because thermal agitation acts against the alignment of moments in an applied field. For many d-blockcompounds the spin-only formula is a fairly good approximation to the effective magnetic moment:236SECTION H—CHEMISTRY OF TRANSITION METALSwhere S is the spin quantum number, equal to half the number of unpaired electrons n, and µB the Bohr magneton,equal to approximately 9.274×10−24 J T−1. The most straightforward application of magnetic measurements is thereforeto establish the number of unpaired electrons, and so to distinguish between high- and low-spin states.
For example,most Co3+ complexes have µeff=0 as expected for low-spin d6; however, [CoF6]3− has µeff around 5µB, corresponding tofour unpaired electrons and a high-spin state (see Topic H2).Magnetic measurements are sometimes used to give information about metal-metal bonding. For example,dimeric Cr2+ complexes such as [Cr2(CH3CO2)4] (see Topic H6, Structure 2) have µeff=0, suggesting that all four delectrons of Cr2+ are paired to form a quadruple bond. However, there are many other factors that can complicatemagnetic properties.
The oxygen-bridged complex [(RuCl5)2O]4− (Topic H6, Structure 1) also has µeff=0. In this case,there is no metal-metal bond and the electrons are paired as a consequence of the Ru-O bonding.Section H—Chemistry of transition metalsH9COMPLEXES: π ACCEPTOR LIGANDSKey NotesDefinition and evidenceBinary carbonylsThe 18-electron rule16-electron complexesRelated topicsπ-acceptor ligands such as CO have empty π antibonding orbitals thatcan accept electron density from filled metal d orbitals.
The CO bondis weakened as result. Other π-acceptor ligands include NO andphosphines.Many transition metals form carbonyl compounds where theoxidation state of the metal is zero. Polynuclear compounds are alsoknown with metal-metal bonds, and sometimes with bridging COgroups.In many carbonyls and related compounds, the metal atoms have atotal valence count of 18 electrons. This rule can break down forsteric reasons with early transition metals, and is less often obeyed inlater groups.Elements of groups 9–11 form many 16-electron square-planarcomplexes. These undergo various reactions including oxidativeaddition.Ligand field theory (H2)Organometallic compounds(H10)Definition and evidenceMost ligands have a nonbonding electron pair that can act as a donor to empty orbitals on the metal atom (see TopicsC9 and H2).
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
