Octahedra, distorted

The observation of more wetikly a regated oligomeric species for the heavier metals is in accord with observations made for related Cp M compounds (M = Al-Tl). Thus, although all four compounds vaporize as monomeric species [145-148], the aluminum compound crystallizes as a tetramer, with mean Al-Al distances (2.769 A), which are not markedly different from 28 (2.739 A), and which are shorter than the intermettJlic separations in aluminum metal (2.86 A) [131]. By contrast, both the gtdlium and indium compounds crystallize as loosely bound hexamers (Cp M)g, which have structures that can be viewed as octahedra, distorted by compression along a C3 axis [147, 149]. In addition, and in contrast to (Cp Al)4, the r -Cp centroid-M vectors lie not radial to the metal core, but tilted to give overtJl 5j symmetry. One interpretation of these structural distortions is in terms of a second order Jtihn-Teller effect, which maximizes M-M orbital overlap ... 

Reduction of X Me from 8 to 6 leads to a reduction of the metal coordination number from 8 to 6, respectively, and to the formation of octahedrons (distorted octahedrons) as a basic unit. 

There are three kinds of Mn-0 distances within each oxygen octahedron surrounding a central manganese ion two of them are shorter (0.191 and 0.195 nm for PrMnOj, and 0.194 and 0.190 for TbMnOj) and the other is a little longer (0.222 nm for PrMnOj and 0.223 nm for TbMnOj). Thus each oxygen octahedron distorts and tilts each other, which are caused by the Jahn-Teller effect of trivalent manganese ion. Each rare... 

There is one result of the general theory which should be mentioned. It can be shown that in a regular octahedron the Jahn-Teller effect only operates by way of vibrations of symmetry when the electronic state is "Eg (the value of n is irrelevant because the Jahn-Teller theorem applies only to space functions, not spin). For electronic states of either or "T2g symmetries then the Jahn-Teller effect operates through vibrations of either 6g or t2g symmetries a vibration of the latter symmetry is shown in Fig. 8.11. Of course, the orbital degeneracy in an octahedral complex may be relieved by distortions other than those shown in Figs. 8.8 and 8.11. However, in such cases we may conclude that whatever is responsible for the distortion it is not the Jahn-Teller effect. In particular, all of the Jahn-Teller-active vibrations of an octahedron carry the g suffix and this means that they cannot give rise to a distortion which destroys the centre of symmetry of an octahedron. Distorted octahedral complexes which lack a centre of symmetry cannot owe their distortion to the operation of the Jahn-Teller effect. This account of the Jahn-Teller effect indicates why it is of little importance when the t2g orbitals are unequally occupied. Occupation of these orbitals... 

A really improved agreement was obtained using the optimized structures. The shifts of the atomic positions lead to reduced octahedron distortions. The optimized structures provided a reliable assignment of the Al quadrupolar parameters to the aluminium sites in the four studied compounds, which contain two sites with the same multiplicity [15,16,124]. The correlation between experimental and calculated EFG tensor elements (Figure 5.12) allowed the determination of the value of the Al nuclear quadrupole moment with an improved accuracy [124]. Moreover the DFT calculations provided the orientation of the... 

A1 EFG tensors in the crystal frame and allowed a quantitative interpretation of the tensor element orientations and magnitudes in terms of electron densities and octahedron distortions. Electron density maps (Figure 5.13) suggested that the magnitude and orientation of the A1 EFG tensors in fluoroaluminates mainly result from the asymmetric distribution of the A1 3p orbital valence electrons. 

Cobalt II) halides can be obtained by direct combination of the elements, or by dehydration of their hydrates. Anhydrous cobalt(II) chloride is blue, and the solid contains octahedrally-coordinated cobalt the hydrated salt C0CI2. bHjO is pink, with each cobalt surrounded by four water molecules and two chloride ions in a distorted octahedron. 

One atom above the face of an octahedron, which is distorted chiefly by separating the atoms at the corners of this face. 0.592... 

Titanium Trifluoride. The trifluoride (121) is a blue crystalline soHd, density 2980 kg/m, ia which the titanium atoms are six-coordinate at the center of a slightly distorted octahedron, where the mean Ti—F distance is 197 pm. Titanium trifluoride [13470-08-1] is stable ia air at room temperature but decomposes to titanium dioxide when heated to 100°C. It is insoluble ia water, dilute acid, and alkaUes but decomposes ia hot concentrated acids. The compound sublimes under vacuum at ca 900°C but disproportionates to titanium and titanium tetrafluoride [7783-63-3] at higher temperatures. 

In a similar fashion [Mc4N][TeF5] reacts with trimethylsilyl azide to produce [Mc4N][Te(N3)5] (Eq. 5.7), which has been stmcturally characterized in the solid state. The pentaazidotellurite anion is a distorted pseudo-octahedron with Te-N bond lengths in the range 2.07-2.26 A. 

In fact the vast majority of 6-coordinate complexes are indeed octahedral or distorted octahedral. In addition to the twist distortion just considered distortions can be of two other types trigonal and tetragonal distortions which mean compression or elongation along a threefold and a fourfold axis of the octahedron respectively (Fig. 19.8). 

The most usual type of coordination in compounds of Hg with other donor atoms is a distorted octahedron with 2 bonds much shorter than the other 4. In the extreme, this results in linear 2-coordination in which case the bonds are largely covalent. Hg(CN)2 is actually composed of discrete linear molecules (C-bonded CN ), whereas crystalline Hg(SCN)2 is built up of distorted octahedral units, all SCN groups being bridging ... 

According to crystal analysis performed by Stomberg [173], Na2NbOF5 is made up of sodium ions and isolated NbOF52 complex ions and is similar in structure to FeWC>6. NbOFs2" polyhedrons comprise slightly distorted octahedrons that are located in one of two equivalent positions. The niobium atom is shifted 0.234 A from the equatorial plane towards the oxygen atom. 

The niobium atom has a slightly distorted octahedral coordination. Interatomic distances between the niobium atom and the two oxygen atoms in trans positions, O-Nb-O are 1.81 and 2.14 A. The niobium atom is shifted from the base plane of the octahedron by 0.23 A, and this shift, in adjacent chains, is in opposite directions. Pakhomov and Kaidalova [204] concluded that the shorter Nb-O bond (1.81 A) is an intermediate between a single and double bond. 

The structure of the low-temperature modification of the compound, Na2Ta205F2 (I), has not yet been determined. The high-temperature modification, Na2Ta205F2 (II), can be conceived as two sub-lattices Tai6X52, which is composed of TaX6 octahedrons and Na Xt, which contains Na X2 tetrahedrons [192]. Fig. 38 shows the structure of Na2Ta205F2 (II). Two additional sodium atoms occupy the centers of two bi-pyramids with distorted hexagonal bases. 

CoNbOF5 [129] can also be considered an MeX3 type compound due to the steric similarity of cobalt and niobium ions. This compound crystallizes in tetragonal syngony with cell parameters a = 7.81 and c = 9.02 A (Z = 4 p = 3.19 g/cm3), and can be considered to have a distorted cubic Re03 structure. Both cobalt and niobium occur in the center of oxyfluoride octahedrons that are linked via their vertexes. 

S Tantalum and niobium are present in the crystal structure in the form of complex ions. The lowest coordination number, 6, corresponds to the formation of slightly distorted octahedrons. The linking and packaging of the octahedrons depends on the X Me ratio, where X is the total number of oxygen and fluorine atoms, and Me is the total number of tantalum or niobium ions as well as other metals that can replace tantalum or niobium in the octahedral polyhedron. The crystal structure type can be defined based on the X Me ratio, as follows ... 

---