Octahedral complexes of transition metals

A complete study of the molecular orbitals for an octahedral complex sue as [Cr(CN)6] or [Co(NH3)6] would require linear combinations of all the valence atomic orbitals of the metal and of the ligands. An approximation isl to take the metal valence a.o.s (nine a.o.s for a metal of the first transition series (five 3d orbitals, one 4s and three 4p orbitals)) together with six a.o.s from the ligands, one for each atom directly bonded to the metal atom. Ini general, these six a.o.s are quasi-localized molecular orbitals (see Chapter 8), which point from the ligand to the metal and have essentially non-bonding character  

They can be, for example, a carbon hybrid orbital, n, of the cyanide ion 

As with other symmetrical systems (Chapters 7 and 9), the form of the m.o.s can easily be found by establishing intermediate linear combinations of the n orbitals contributed by the ligands. These are symmetry-adapted linear combinations that can be constructed using group theory (see, for example, ref. 40)  

The various combinations are named according to the degree of degeneracy and to the g ( gerade , i.e. even) or u ( ungerade , i.e. odd) parity (see page 75) the aig and the two Cg combinations have an even number of nodes, whereas each of the three tiu combinations have just one node. 

Confirm that the symmetry-adapted combinations above are 

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In general, octahedral complexes of transition-metal ions possessing 0, 1, or 2 electrons beyond the electronic configuration of the preceding noble gas, ie, i/, (P configurations, are labile. The (P systems are usually inert the relative lability of vanadium(II) may be charge and/or redox related. 

The molecular orbital model as a linear combination of atomic orbitals introduced in Chapter 4 was extended in Chapter 6 to diatomic molecules and in Chapter 7 to small polyatomic molecules where advantage was taken of symmetry considerations. At the end of Chapter 7, a brief outline was presented of how to proceed quantitatively to apply the theory to any molecule, based on the variational principle and the solution of a secular determinant. In Chapter 9, this basic procedure was applied to molecules whose geometries allow their classification as conjugated tt systems. We now proceed to three additional types of systems, briefly developing firm qualitative or semiquantitative conclusions, once more strongly related to geometric considerations. They are the recently discovered spheroidal carbon cluster molecule, Cgo (ref. 137), the octahedral complexes of transition metals, and the broad class of metals and semi-metals. 

For the octahedral complexes of transition metals studied in Chapter 11, important transitions are those of energy A (the ligand field splitting parameter) between the non-bonding tzg orbitals and the anti-bonding Cg orbitals. The simplest example is provided by the hexaquotitanium(in) ion, [Ti(H20)6] , whose fundamental valence configuration is (t2g) The absorption band at 493 nm, responsible for the purple colour of the complex, is assigned to the t2g e transition (Fig. 12.9). 

Problem 12.11 The octahedral complexes of transition metals show, in general, weak colours, in comparison with the permanganate ion, for instance. In the first case, the relevant transition is t2g e, whereas in the latter it is a charge transfer transition between the metal and the oxygen atoms. Justify the different transition probabilities. 

In paramagnetic complexes, such as some octahedral complexes of transition metal ions, the metal -ligand interactions are responsible for transfer of electron spin from the metal to the ligands. The existence of net electron spin population in the hydrogen Is orbitals of the ligands causes very large chemical shifts, either for high or low frequency. Three of the several mechanisms involve tt interactions as follows. 

Octahedral complexes of transition metal ions with bidentate ligands can display chirality ... 

Although square-planar configuration is customarily considered classical for v/c-dioximate of nickel(II), attempts have been made repeatedly over the years for preparing the above complexes in other configurations also. By employing weakly polar solvents and some other variations, success has been claimed in the preparation of mono(dioxime) complexes of nickel(II).42,43 The dichloro-bis(l,2-cyclohexanedione dioximato)nickel(II) has been shown to have an octahedral vie structure.44 Examples of tris(dioxime) complexes of transition metals in general45"18 and of bivalent atoms40,47 in particular are rare and structural details of only a tris(dioxime) complex of cobalt(III) are known.48 In a more recent publication,49 the crystal structure of tris(l,2-cyclohexanedione dioximo)nickel(II) sulfate dihydrate has been elucidated. 

The first-order JT effect is important in complexes of transition metal cations that contain nonuniformly filled degenerate orbitals, if the mechanism is not quenched by spin-orbit (Russell-Saunders) coupling. Thus, the JT effect can be expected with octahedrally coordinated and high spin d cations, and tetrahedrally coordinated and d cations. The low-spin state is not observed in tetrahedral geometry because of the small crystal field splitting. Also, spin-orbit coupling is usually the dominant effect in T states so that the JT effect is not observed with tetrahedrally coordinated d, d , d, and d ions. 

Fig. 11.3 Typical m.o. energy diagram for an octahedral complex of a metal of the first transition series.

THE RELATIVE STABILITY OF OCTAHEDRAL AND TETRAHEDRAL COMPLEXES OF TRANSITION METAL IONS,... 

A very important class of organometallic compounds is complexes of transition metals with carbonyl (CO) ligands (see examples in Tables S3.13) [228-231]. The coordination number in these complexes is determined by the FAN mle, hence the stable complexes are tetrahedral Ni(CO)4 and Pd(CO)4, trigonal-bipyramidal [Mn(CO)5] andFe(CO)s, octahedral Cr(CO)e, Mo(CO)6 and W(CO)6. Formally CO is an inorganic ligand, the bond in it is shorter (1.128 A in the gas phase) than in CO2 (1.160 A) and is nearly triple (C O) in character. Usually CO coordinates with metal in a linear fashion which can be formally described by the valence-bond scheme M=C=0. However, metal carbonyls are typical k complexes in their... 

Kyba EP, Davis RE, Liu ST et al (1985) Structural characterization of tridentate 11-membered phosphino macrocyclic complexes of transition metals. Examples of octahedral, square-pyramidal and tetrahedral geometries. Inorg Chem 24 4629 634... 

Figure 19.14 Molecular orbital diagram for an octahedral complex of a first series transition metal (only a interactions are considered in this simplified diagram).

Octahedral Having the symmetry of a regular octahedron. In an octahedral species, a central atom is surrounded by six other atoms, one above, one below, and four at the comers of a square, 176 complex in transitional metals, 418-420 geometric isomerism, 415 Octane number, 584... 

The nature of the transition between high spin and low spin octahedral complexes of the transition metals. R. L. Martin and A. H. White, Transition Met. Chem. (N.Y.), 1968,4,113-198 (115). 

The coordination of transition metal ions in acidic chloroaluminate melts has not been firmly established. However, in the case of AICb-EtMelmCI. the E0 values of simple redox systems that are electrochemically accessible in both acidic and basic melt, e.g., Hg(II)/Hg [51], Sb(III)/Sb [52], and Sn(II)/Sn [53] exhibit a large positive potential shift on going from basic melt, where metal ions are known to exist as discrete anionic chloride complexes, to acidic melt. Similar results were observed for Cu(I) in AlCh-NaCl [48]. This dramatic decrease in electrochemical stability isprima facie evidence that metal ions in acidic melt are probably only weakly solvated by anionic species such as AICI4 and AECI-. Additional evidence for this is derived from the results of EXAFS measurements of simple metal ions such Co(II), Mn(II), and Ni(II) in acidic AlCh-EtMelmCl, which indicate that each of these ions is coordinated by three bidentate AICI4 ions to give octahedrally-coordinated species such as [ M (AIC14) 2 ] [54]. Most transition metal chloride compounds are virtually... 

A persistent feature of qualitative models of transition-metal bonding is the supposed importance of p orbitals in the skeletal hybridization.76 Pauling originally envisioned dsp2 hybrids for square-planar or d2sp3 hybrids for octahedral bonding, both of 50% p character. Moreover, the 18-electron rule for transition-metal complexes seems to require participation of nine metal orbitals, presumably the five d, one s, and three p orbitals of the outermost [( — l)d]5[ s]1[ p]3 quantum shell. 

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