P.A. Cox - Inorganic chemistry (793955), страница 49
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Large values are found for octahedrald3, d8 and low-spin d6 complexes. A large LFSE leads to smaller ionsand higher lattice or solvation energies.Different ligand geometries give characteristic patterns of ligand fieldsplitting. The square-planar (D4h) geometry is common for some d8ions, and a Jahn-Teller distortion from a regular octahedron isnormal for d9 and high-spin d4 ions.Atomic orbitals (A2)Lewis acids and bases (C9)Molecular symmetry andComplexes:electronicpoint groups (C3)spectra and magnetism (H8)Octahedral splittingsThe five d orbitals with different values of the magnetic quantum number (m) have the same energy in a free atom or ion(see Topic A2). In any compound, however, they interact differently with the surrounding ligands and a ligand fieldsplitting is produced.
The commonest coordination is octahedral (Oh point group) with six surrounding ligands (seeFig. 1). Then two of the d orbitals ( andknown together as the eg set) are found at higher energy than theother three (dxy, dxz and dyz, known as t2g). Such a splitting (denoted Δo) occurs in any transition metal compound withoctahedral coordination, including aqua ions and many solids. Electronic transitions between t2g and eg orbitals give riseto colors, which are a familiar feature of transition metal complexes, and allow Δo to be measured experimentally (seeTopic H8).Although originally explained in terms of electrostatic repulsion between d electrons and the ligands, it is nowrecognized that ligand field splittings come from the same type of orbital overlap effects as donor-acceptor interactions214SECTION H—CHEMISTRY OF TRANSITION METALSFig. 1.
The five d orbitals, showing eg and t2g sets in an octahedral complex, with ligands along the x, y and z axes.(see Topic C9). Most ligands coordinate to the metal ion using nonbonding electrons (see Topic C1). A ligand lone-pairorbital pointing directly towards the metal overlaps with the eg orbitals (1) but has the wrong symmetry to interact witht2g. The overlap gives rise to σ bonding and antibonding molecular orbitals (see Fig.
2. and Topics C4 and C5). Thebonding orbitals are occupied by the electrons from the ligand, and it is the σ antibonding levels that form the ‘metal’ egset, available for the d electrons of the metal ion. A strong σ-donor ligand will produce a large splitting Δo by raisingthe eg energy, π bonding arises when ligands have orbitals directed perpendicular to the metal-ligand axis, which caninteract with the metal t2g orbitals (2). Ligands such as halide ions have occupied pπ orbitals and act as π-donors. Thisinteraction raises the energy of the metal t2g orbitals, and decreases Δo. On the other hand, π-acceptor ligands such asCO have empty antibonding π orbitals (see Topic H9).
Overlap with the metal in this case causes the t2g orbitals to belowered in energy so that A0 is increased (see Fig. 2b and c).The order of Δo values produced by different ligands is known as the spectrochemical series. A partial series inorder of increasing splitting is:As expected, strong a donors are generally high in the series, π donors are low, and π-acceptor ligands such as CN− andCO are among the highest, and known as strong field ligands. The major trends with different metal ions are (i) Δoincreases with charge on the ion, and (ii) splittings are larger for 4d and 5d series elements than in the 3d series.Fig. 2.
Partial MO diagram showing an octahedral complex with (a) σ-donor only, (b) π-donor, and (c) π-acceptor ligands.H2—LIGAND FIELD THEORY215Fig. 3. Electron configurations for d5 in (a) high-spin and (b) low-spin octahedral complex.Table 1. Electron configurations for dn high- and low-spin octahedral complexes, with corresponding ligand field stabilization energiesaConfigurations susceptible to Jahn-Teller distortion.High and low spinAssignment of the electron configuration of an octahedral complex involves (i) finding the d electron number of the ion(see Topic H1) and (ii) determining the occupation of the t2g and eg orbitals. Electron repulsion effects are important,and other things being equal the ground state will be formed with the maximum number of electrons in differentorbitals and with parallel spin (see Topic A3).
Two and three d electrons will occupy the t2g orbitals with parallel spin,but with four or more there are different possibilities. If the extra repulsion coming from spin-pairing is large enough,the ground state will be of the high-spin type formed by keeping electrons in separate orbitals as far as possible. Onthe other hand, if Δo is larger than the spin-pairing energy, the favored configuration will be low-spin formed byplacing as many electrons as possible in t2g even though they must be paired. As shown in Fig.
3. the high- and low-spinconfigurations for d5 are (t2g)3(eg)2 (five unpaired electrons) and (t2g)5 (one unpaired electron), respectively.The spin state of a transition metal ion can often be measured from the paramagnetic susceptibility (see Topic H8).For ions of the 3d series it is found that most complexes with ligands such as halides, water or ammonia are high-spincompounds, the notable exception being Co3+, a d6 ion that normally forms lowspin compounds. Low-spin complexesare found with strong field ligands such as CN−, and nearly always with 4d and 5d elements whatever the ligand.Ligand field stabilization energyThe ligand field stabilization energy (LFSE) of an ion is calculated by summing the orbital energies of the delectrons present, measured relative to the average energy of all five d levels.
In octahedral coordination, each electronin a t2g orbital is assigned an energy −(2/5)Δo, and each eg electron an energy +(3/5)Δo. LFSE values in terms of Δo areshown for high- and low-spin configurations in Table 1. LFSE is zero for ions with the d10 and high-spin d5 configurationswhere all d orbitals are equally occupied.216SECTION H—CHEMISTRY OF TRANSITION METALSMaximum values of octahedral LFSE in high-spin states occur with the d3 and d8 configurations, and for low-spin withThese patterns of LFSE influence thermodynamic, structural and kinetic aspects of complex formation (see TopicsH6 and H7). They also have an effect on ionic radii and on lattice and solvation energies. Superimposed on a generaldecrease of radius along the 3d series, the ions with the largest LFSE have smaller radii (and also larger lattice orsolvation energies) than otherwise expected (see Topic H1, Fig.
2). One interpretation of this effect is that in an ionwith large LFSE, the repulsion between closed shells is decreased by the predominance of metal electrons in t2g orbitalsthat do not point directly towards the ligands.d6.Other geometriesThe pattern of ligand field splitting depends on the coordination geometry; generally those d orbitals that point moststrongly towards the ligands are raised in energy relative to the others.
Figure 4 shows the splittings produced by someother ligand coordination geometries. Tetrahedral (Td) coordination gives a splitting in the opposite direction (andabout half the magnitude) to that found with octahedral. Tetragonally distorted octahedral (D4h) coordinationarises where two opposite ligands are further from the metal than the other four. In this and in square-planarcoordination (also D4h), the d orbital pointing towards ligands in the xy plane is higher in energy than the others. (The maindifference from the octahedral case is the lowering in energy ofas this interacts less strongly with the ligands).Ligand field splitting is sometimes important in understanding the geometrical preferences of an ion, although otherfactors may play a part.
The splitting in tetrahedral coordination is only about half that for octahedral, and so incompetition between octahedral and tetrahedral geometry the octahedral LFSE is more important; thus ions such as Cr3+ (d3) and Co3+ (d6 low-spin) are nearly always found in octahedral coordination and are notably resistant to formingtetrahedral complexes. Square-planar complexes are found for d8 ions when the ligand field splitting is large enough forthe electrons to pair in the four lowest orbitals (see Fig. 4c); examples are Ni2+ with strong-field ligands, and Pd2+ and Pt2+ in nearly all situations (see Topics H5 and H6).The geometry of Fig. 4b arises from a so-called Jahn-Teller distortion of the octahedron. The eg orbitals are split inenergy, and this allows stabilization of a complex if these two orbitals are unequally occupied.
Thus in d9 (Cu2+) twoelectrons occupy theand one the. Nearly all Cu2+ compounds show this type of distortion, as do many high42+spin d ions such as Cr .Fig. 4. Ligand field splitting patterns for (a) tetrahedral, (b) tetragonally distorted octahedral, and (c) square-planar complexes.Section H—Chemistry of transition metalsH33d SERIES: AQUEOUS IONSKey NotesOxidation statesEffect of pHComplex formationRelated topicsElements from Sc to Mn can form oxidation states up to the group number).
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