Non-octahedral complexes

So far, this chapter has been concerned entirely with octahedral complexes. Whilst the majority of complexes are octahedral, almost all of them display some slight deviation from the ideal geometry. Other complexes have quite different geometries, as was seen in Chapter 3. For the moment the discussion will be extended in a more limited way, so as to include only tetrahedral and square planar complexes. 

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The extension to non-octahedral complexes is possible, but must be carried out with great care The orbitals chosen can be expressed as a linear combination of the usual orbitals for an octahedron, and electron-electron repulsions can then be calculated from those for the octahedral case. It is not necessarily adequate for tetrahedral complexes of the first row transition elements, to use ligand field theory in the strong field limit, even for powerful ligands in V(mesityl)4 the ligand field splitting is only 9250 cm ... 

The vast majority of MoO + core complexes are six-coordinate and mostly of distorted octahedral structure. Some non-octahedral complexes are known. In the octahedral structure the two Mo—O, bonds are invariably cis to each other. The strong o- and jr-donor nature of the oxo ligands makes it favorable for them to avoid competing for the same p and d orbitals. If the oxo groups were tram they would be forced to share two d orbitals and one p orbital. By residing on adjacent coordination sites they are forced to share only a single d orbital. 

The only non-octahedral complex of cobalt(ni) whose structure has been described during the period of this Report is that of the paramagnetic biuret complex KCo(3-propbi)2,2(l-propbiH2), where 3-propbi = HN-CO-NPr --CO NH and 1-propbi = Pr HN-CONH CO NH2. The crystals contain [Co(3-propbi)2] anions (13) in which the cobalt atom is in a square-planar co-ordination. The Co-N distances are equal at 1.88 A. ... 

Aqueous solutions of salts with non-coordinating anions contain the pale-pink, [Mn(H20)6] +, ion which is one of a variety of high-spin octahedral complexes which have been... 

The most extensively studied family of non-labile complexes is the cobalt(III) ammine series. These are octahedral systems and all those to be considered are low spin <7 systems. The subtle variations that can be achieved synthetically make... 

The related octahedral non-labile complexes which have received some attention will be grouped together simply because there is much less information... 

Picolylphenylketone S-benzyldithiocarbazate, 48, yielded paramagnetic [ Ni(48-H)A 2] (A = Cl, Br) and diamagnetic [Ni(48-H)I] [207]. All three compounds are non-electrolytes and the iodo complex is planar while the other two complexes involve sulfur bridging atoms and five-coordinate nickel(II) centers. All three complexes can be converted to monomeric, octahedral complexes by addition of pyridine, 2-picoline or quinoline. 

Group IV Donors. A H n.m.r. study of the cis-trans isomerization of [OslCO) -(SiMe3)2] indicates that the process is non-dissociative. These species may therefore provide a rare example of stereochemical non-rigidity in nonchelate octahedral complexes. Similar non-rigidity was observed for [Os(CO)4-(SiMeCl ) ] and [Os(CO)4(SnMe3)2]. ... 

There are several complexes now known to possess a cis dioxo structure and to be non-octahedral in nature. In a number of these the coordination number about Mo is lower. 

In describing a stereoisomer, it is perhaps most important initially to define whether or not it is chiral. The origins of chirality (optical activity) in coordination compounds and important experimental results have been recently reviewed.112,113,121,122 The classical example of chirality or enantiomerism in coordination chemistry is that of octahedral complexes of the type [M-(bidentate)3]. These exist in the propeller-like,123 non-superimposable, mirror-image forms (13a) and (13b). Synthesis of this type of complex from M and the bidentate ligand in an achiral environment such as water results in an equimolar mixture of the two stereoisomers. The product... 

Optical isomers are special kinds of stereoisomers they are non-superimposable mirror images of each other (Fig. 16.27). Both geometrical and optical isomerism can occur in an octahedral complex, as in [CoCI2(en)2]+ the trans isomer is green (14a) and the two alternative cis isomers (14b) and (14c), which are optical isomers of one another, are violet. 

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