Octahedral coordination structures

Quite different octahedrally coordinated structures are observed in the bromides, iodides (and some chlorides) of divalent metal ions with suitable radius ratios. In these, the MX6 octahedra are not linked in a three-dimen-... 

We now consider the consequences of continuously reducing the size of the central cation in an octahedrally coordinated structure. First, there may be another structure, perhaps quite unrelated and involving tetrahedral or other coordination, which becomes stable relative to the octahedral structure before the latter becomes unstable with respect to small displacements, that is, before the maximum contact distance is reached. In this case there is a sudden change from regular octahedral to regular tetrahedral coordination, as shown in Fig. 26a. The oxides of the divalent transition-metal ions have structures in accord with this model, for, in the absence of Jahn-Teller effects, they are either octahedrally or tetrahedrally coordinated. 

In the face-centred cubic structure tirere are four atoms per unit cell, 8x1/8 cube corners and 6x1/2 face centres. There are also four octahedral holes, one body centre and 12 x 1 /4 on each cube edge. When all of the holes are filled the overall composition is thus 1 1, metal to interstitial. In the same metal structure there are eight cube corners where tetrahedral sites occur at the 1/4, 1/4, 1/4 positions. When these are all filled there is a 1 2 metal to interstititial ratio. The transition metals can therefore form monocarbides, niU ides and oxides with the octahedrally coordinated interstitial atoms, and dihydrides with the tetrahedral coordination of the hydrogen atoms. 

High-temperature neutron diffraction studies have shown that this latter phase has the cubic ordered Re03-type structure (p, 1047) with octahedral coordination of both types of Sn atoms by F (Sn -F 229 pm, Sn" -F 186 pm). The fi-phase also features octahedral coordination in a structure closely related to that of rhombohedral LiSbF6. 

Tin(IV) halides are more straightforward. Snp4 (prepared by the action of anhydrous HF on SnCU) is an extremely hygroscopic, white crystalline compound which sublimes above 700°. The structure (unlike that of CF4, SiF4 and GeF4) is polymeric with octahedral coordination... 

Figure 21.3 Two representations of the structure of perovskite, CaTi03, showing (a) the octahedral coordination of Ti, and (b) the twelve-fold coordination of Ca by oxygen. Note the relation of (b) to the cubic structure of Re03 (p. 1047).

LCo(H20)6] ion, and bidentate /V-donor ligands such as cn, bipy and phen form octahedral cationic complexes [Co(L-L)3] , which are much more stable to oxidation than is the hexaammine [Co(NH3)6l . Acac yields the orange [Co(acac)2(H20)2] which has the tram octahedral structure and can be dehydrated to [Co(acac)2l which attains octahedral coordination by forming the tetrameric species shown in Fig. 26.3. This is comparable with the trimeric [Ni(acac>2]3 (p. 1157), like which it shows evidence of weak ferromagnetic interactions at very low temperatures. fCo(edta)(H20)] is ostensibly analogous to the 7-coordinate Mn and complexes with the same stoichiometry, but in fact the cobalt is only 6-coordinate, 1 of the oxygen atoms of the cdta being too far away from the cobalt (272 compared to 223 pm for the other edta donor atoms) to be considered as coordinated. 

Violet, easily hydrolysed, PdFp is produced when Pd [Pd F(sl s refluxed with SCF4 and is notable as one of the very few paramagnetic compounds of Pd. The paramagnetism arises from the configuration of Pd which is consequent on its octahedral coordination in the rutile-type structure (p. 961). The dichlorides of both Pd and Pt are obtained from the elements and exist in two isomeric forms which form i.s produced depends on the exact experimenial conditions used. The more usual a form of PdCb is a red material with... 

Black-brown RuBr3 has roughly octahedral coordination of ruthenium (Ru-Br 2.46-2.54 A) with short Ru-Ru contacts (2.73 A) [17]. Black Rul3 has a similar structure. Neither is particularly soluble in water. 

It exists in two stable forms, of which the a-form has the corundum (a-A12Oj) structure with octahedrally coordinated rhodium (Rh-0 2.03-2.07 A) the /3-form and a high-temperature form also have octahedral coordination. Black Rh02 has the rutile structure (Rh-0 1.95-1.97 A) and is best made by heating rhodium or Rh203 at 400-900°C under oxygen pressures up to 3500 atm. 

Other binary compounds include MAs3 (M = Rh, Ir), which has the skutterudite (CoAs3) structure [33] containing As4 rectangular units and octahedrally coordinated M. The corresponding antimonides are similar. M2P (M = Rh, Ir) has the anti-fluorite structure while MP3 has the CoAs3 structure. In another compound of this stoichiometry, IrSi3, 9-coordination exists for iridium. 

PdF2 is that rare substance, a paramagnetic palladium compound, explicable in terms of (distorted) octahedral coordination of palladium with octahedra sharing corners [15], It exists in two forms, both having /zeff 2.0 /xB, rather below the spin only value for two unpaired electrons. Bond lengths are Pd-F 2.172 A (two) and 2.143 A (four) in the tetragonal form (rutile structure). 

PdF4 is the only stable palladium(IV) halide [18] (testimony to the oxidizing nature of palladium(IV)) and is a very moisture-sensitive diamagnetic red solid the structure is based on Pd6F24 hexameric units linked three-dimensionally. It has octahedrally coordinated palladium with two terminal (cis) fluorines and four bridging ones. Despite the absence of other tetra-halides, the complete series of PdX - exist (cf. Ir). 

Axe, J. D. "Electronic Structure of Octahedrally Coordinated Protactinium(IV) in Cs2ZrCl6", UCRL-9293,... 

Fig. 35.—(a) Stereo view of about a turn of the 3-fold double helix of potassium gellan (41). The two chains are drawn in open and filled bonds for distinction. Both intra- and inter-chain hydrogen bonds stabilize the helix. The vertical line is the helix axis. Octahedrally coordinated potassium ions (crossed circles) and triply hydrogen-bonded water molecules (open circles) located above the ions are integral components of the structure of 41. 

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