Octahedral compounds

The most common oxidation states and the corresponding electronic configuration of mthenium are +2 and +3 (t5 ). Compounds are usually octahedral. Compounds in oxidations states from —2 and 0 (t5 ) to +8 have various coordination geometries. Important appHcations of mthenium compounds include oxidation of organic compounds and use in dimensionally stable anodes (DSA). 

The most common oxidation states, corresponding electronic configurations, and coordination geometries of iridium are +1 (t5 ) usually square plane although some five-coordinate complexes are known, and +3 (t7 ) and +4 (t5 ), both octahedral. Compounds ia every oxidation state between —1 and +6 (<5 ) are known. Iridium compounds are used primarily to model more active rhodium catalysts. 

The most common oxidation states and corresponding electronic configurations of platiaum are +2 which is square planar, and +4 which is octahedral. Compounds in oxidation states between 0 and +6 [t) exist. Platiaum hydrosilation catalysts are used in the manufacture of siHcone polymers. Several platiaum coordination compounds are important chemotherapeutic agents used for the treatment of cancer. 

This is of most importance in square-planar and octahedral compounds where ligands, or more specifically donor atoms, can occupy positions next to one another (cw) or opposite each other (trans) (Fig. 19.11). 

A few cases of optical isomerism are known for planar and tetrahedral complexes involving unsymmetrical bidentate ligands, but by far the most numerous examples are afforded by octahedral compounds of chelating ligands, e.g. [Cr(oxalate)3] and [Co(edta)] (Fig. 19.13). 

Numerous carbonyl halides, of which the best known are octahedral compounds of the type [M(C0)4X2] are obtained by the action of halogen on Fe(CO)5, or CO on MX3 (M = Ru, Os). Stepwise substitution of the remaining CO groups is possible by X or other ligands such as N, P and As donors. 

In tetrahedral fields the splitting of the free ion ground term is the reverse of that in octahedral fields so that, for d ions in tetrahedral fields A2g(F) lies lowest but three spin-allowed bands are still anticipated.In fact, the observed spectra usually consist of a broad, intense band in the visible region (responsible for the colour and often about 10 times as intense as in octahedral compounds) with a weaker one in the infrared. The only satisfactory interpretation is to assign these, respectively, as, wj = 7 i (P)-i A2(F) and ut = i(F)- A2(F) in which case U = ) should be... 

Isothermal a—time curves for the decomposition at 363—464 K of the pseudo octahedral compounds NiL2(NCS)2 (L = py, 3-picoline or quinoline) to NiL(NCS)2 and volatilized L, obeyed the contracting volume equation [eqn. (7), n = 3]. E values decreased in the sequence L = py > 3-picoline > quinoline and this order was ascribed to the effect of increasing ligand volumes [1128]. 

The poor data on PuF6 are probably best interpreted as a very small TIP of about 150 x 10-6 emu indicating a singlet ground state and a large crystal field splitting of the octahedral compound ( 5). 

Draw ball-and-stick models of all possible isomers of the octahedral compound [Cr (NH3)3 CI3 ]. 

SOME OCTAHEDRAL COMPOUNDS WHICH UNDERGO SLOW SUBSTITUTION... 

In the case of replacement of CO from the group VI carbonyl compounds there is additional evidence to the effect that the type A ligands labilize CO whereas the type B do not, but rather promote a second-order reaction. For the group VII octahedral compounds there is no strong evidence in favour of an associative activation step, except when interpretation is obscured by subsequent or concurrent reaction (but see ref. 146). There is, however, good reason to believe that such an associative reaction does occur in certain of the group VI compounds. 

At present only one Re(IV) dithiocarbamato complex has been described, a paramagnetic octahedral compound Cl2Re(Et2C tc) (59). Obviously, the reported formula is in error. 

The spectrum of the tetrahedrally coordinated tetraphenyl titanate does not resemble the type IV spectrum, primarily due to enlargement of the 20 ev. absorption peak. The second tetrahedral coordination sphere in this compound, consisting of carbon atoms, may account for this change. An analogous change, also attributed to the second coordination sphere, is observed for the octahedral hexacyanides as compared to the other octahedral compounds. 

X-ray proofs of octahedral compounds showing this composition are scarce. More numerous are reports on ternary fluorides AMeF4, in which coordination numbers other than 6 are exhibited. Both groups shall be briefly discussed here. 

The spectrum of the compound VCl3(NMe3)2 in the solid state is shown in Fig. 16. Fowles and coworkers have proposed that the presence of two bands (vi and 2) in the range 5—10 kK should be considered diagnostic of trigonal bipyramidal stereochemistry (76). In fact, in this region the octahedral compounds of V(III) are quite transparent (76). 

Treatment of UCLt with 3Li[N (CH2)2PPr 2 2] (= 3Li[2]) in thf adventitiously afforded the crystalline tran,s-octahedral compound 59, which was isolated in a better yield when the reaction was conducted in the presence of the stoichiometric amount of O2." ... 

LCAO MO approximation, 201, alternative notation, 217 for octahedral compounds, 244 for sandwich oompounds, 252 for tetrahedral compounds, 251 ... 

The valence bond picture for six-coordinate octahedral complexes involves dispi hybridization of the metal (Fig. I i.lc. d). The specific d orbitals that meet the symmetry requirements for the metal-ligand o bonds are the four-coordinate d complexes discussed above, the presence of unpaired electrons in some octahedral compounds renders the valence level ( — l)J orbitals unavailable for bonding. This is true, for instance, for paramagnetic [CoFJ3- (Fig. I I.lc). In these cases, the VR model invokes participation of -level dorbitals in the hybridization. 

Octahedral compounds with an odd number of electrons in the eg orbitals, for example manganese(III) (f32 e1 ) and copper(II) (t62ge3g), are often observed to be distorted. 

As mentioned above (Section 13.2.2.2) the steric effect of these ligands especially the ligands of the phen type, can be signficant in preventing the formation of square-planar or frans-octahedral compounds (Figures 2 and 4). Substituents at positions 2 and 9 may increase the preference for tetrahedrally distorted species this has also been found in other systems, such as in spartein (59), and also in combinations of five- and six-membered rings (Section 13.2,6.4). 

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