The trans- octahedral complex

Consider an octahedral complex of the type [ML4CI2], with two doubleface r-donor ligands (Cl) in trans positions and four other ligands which only have a interactions with the metal. Three of the symmetry elements in the complex are shown in 3-19 the planes Pi (xz) and P2 (yz) and the inversion centre i, located on the central atom. 

Four lone pairs of r-type are available, described by the px and py orbitals on each chlorine atom. It must be noticed that the two Cl ligands are equivalent by symmetry, that is, they are interchanged by at least one symmetry element (e.g. the horizontal plane xy or the inversion centre i). 

Rather than consider each ofthep c mdpy orbitals individually, it is therefore preferable to use linear combinations of these orbitals that properly 

Electronically speaking, these four orbitals are doubly occupied, since they are formed from atomic orbitals that describe r-type lone pairs on each of the chloride ligands. They therefore provide a delocalized description of these lone pairs, which is adapted to the symmetry of the complex. 

Before constructing the diagram for the interaction between the 7T orbitals on the ligands and the d orbitals on the metal centre, we must analyse the symmetry properties of these orbitals. We shall describe two methods, in which we use either some selected symmetry elements, or the full set of these elements and the machinery of group theory. 

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Quite a few structural determinations have been made on dialkyl dithiophosphate complexes they have been recently reviewed by Haiduc." In the square-planar complexes [Ni (RO)2PS2 2] (R = Me, Et, Pr ) the coordination is symmetrical (isobidentate) with virtually equivalent P—S distances at 1.95-1.98 A and the Ni—S bonds are also identical at 2.21-2.22 A. In the trans octahedral complex [Ni (EtO)2PS2 2(py)2] the Ni—S distance is much longer (2.49 A). In the cis octahedral complexes [Ni((RQ)2PS2 2(phen)] (R = Me, Et) and [Ni (MeO)2PS2 2(bipy)] the Ni—S bonds trans to the sulfur atoms are some 0.04 A longer than those trans to the nitrogen atoms. 

NiS4 core is conserved giving trans octahedral complexes, while with bidentate amines cis octahedral complexes are obtained. Linear bidentate amines can result in a ID chain like in compound (341) where 4,4 -bipyridine molecules bridge the square planar units. 

Analogous to the dithiophosphate complexes, cis octahedral complexes are formed with bident-ate ligands like phenanthroline, 2,2 -bipyridine, or tmeda and trans octahedral complexes are formed with monodentate ligands like pyridine and its derivatives. Some examples are mentioned with linear bidentate ligands like 4,4 -bipyridine, which build up ID chains by bridging the [NiL2] molecules.926-933... 

Ni11 in a meridional fashion with pairs of S- and pyridine N atoms in cis positions, while the thiocarbazone N atoms are trans to each other as it is usually found in the related octahedral complexes with thiosemicarbazones.1273,1274... 

The complex anions [Mo(CN)5(NO)]3 and [MoCU(NO)(OH2)]2 have been prepared by oxidation of [Mo(CN)5(NO)]4 with air (in cone. HC1 in the latter case) and have been structurally characterized as the expected octahedral complexes, with NO trans to H20 in the dianion.56 A compound formulated as [Mo(NCS)4(NO)]2 has also been prepared56 by treatment of [Mo04]2 with NH2OH in presence of SCN. ... 

The properties of nickel(II) complexes with unsaturated macrocycles which contain pyridyl groups are included in a very comprehensive review article.2626 The complexes are usually square planar with the exceptions of the trans octahedral NiX2(CR)2743 (CR = 2,12-dimethyl-3,7,ll,17-tetraazabicyclo[11.3.1]heptadeca-l(17),2,ll,13,l5-pentaene) and of the diamagnetic square pyramidal NiBr(CR)]Br-H20 (381).2747 The diamagnetic complexes Ni(CR)(C104)2 give rise to square planar octahedral equilibria in coordinating solvents,2744,2746 whereas... 

To overcome this problem we switched to [Re(CO)5Br] as the metal precursor, because it is one of the few octahedral complexes that undergoes an exclusive cis ligand exchange, because of the remarkable trans effect of the carbonyl ligands. The replacement of two equatorial cis-CO groups gave the desired neutral... 

Aliphatic phosphines add to CrX2 (X = Cl, Br), yielding adducts of uncertain composition, for example [Cr(PR3) X2] (n = 1-3). With chelating diphosphines, for example dmpe, well-characterized six-coordinate complexes have been prepared. The latter have low-spin electronic confignrations. A spin-crossover situation has been found in the complex [Cr(depe)2l2] (depe = l,2-bis(diethylphospino)ethane). At ca. 170 K, this trans octahedral complex exhibits a discontin-nous spin transition from S = I (2.8 /u-b) to S = 2 (4.8 /xb). 

A number of neutral complexes of 0-donor Lewis bases have been made, such as 10-coordinate Ce(N03)4(Ph3P0)2, and octahedral CeCl4L2 (L = PhsPO, PhsAsO, HMPA, urea, t-Bu2SO). The Ph3PO complex is cis, like the uranium analog, while X-ray diffraction study of the trans) HMPA complex shows a close structural correspondence to the corresponding compound. 

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