Structures of Coordination Complexes

FIGURE 8.14 Octahedral structure of the Co(NH3)i ion. All six corners of the octahedron are equivalent. 

FIGURE 8.15 (a) The c/s-[Co(NH3)4Cl2] and (b) t/-ar7s-[Co(NH3)4Cl2] ions. The c/s complex is purple in solution, but the trans complex is green. 

FIGURE 8.16 The complex ion [CoCl2(en)2] is an octahedral complex that has c/s and trans Isomers, according to the relative positions of the two cr ligands. Salts of the c/s Isomers are purple, and salts of the trans Isomers are green. 

FIGURE 8.17 Two structural isomers of the coordination compound Co(NH3)3Cl3. 

How many geometric isomers does the octahedral coordination compound [Co(NH3)3Cl3] have.  

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EIGURE 43. Solid state structure of coordination complex 76... 

Although a variety of structures of coordination complexes containing the phosphole moiety have been determined, they are beyond the scope of this chapter. 

The overall shape of a coordination compound is the product of several interacting factors. One factor may be dominant in one compound, with another factor dominant in another. Some factors involved in determining the structures of coordination complexes include the following ... 

Different names have been used for the theoretical approaches to the electronic structure of coordination complexes, depending on the preferences of the authors. The labels we will use are described here, in order of their historical development ... 

The failure of crystal field theory and VB theory to explain the spectrochemical series stimulated the development of ligand field theory, which applies qualitative methods of molecular orbital theory to describe the bonding and structure of coordination complexes. The terms ligand field theory and molecular orbital theory are often used interchangeably in inorganic chemistry today. 

Various theoretical approaches to the electronic structure of coordination complexes have been developed. We will discuss three of these bonding models. 

Page 1093 (Figure 26 9c) is adapted from crystallographic coordinates deposited with the Protein Data Bank PDB ID ICLE Ghosh D Wawrzak Z Pletnev V Z Li N Kaiser R Pangbom W Jornvall H Erman M Duax W L Structure of Un complexed and Linoleate Bound Candida Cholesterol Esterase To be published... 

Liquids are able to flow. Complicated stream patterns arise, dependent on geometric shape of the surrounding of the liquid and of the initial conditions. Physicists tend to simplify things by considering well-defined situations. What could be the simplest configurations where flow occurs Suppose we had two parallel plates and a liquid drop squeezed in between. Let us keep the lower plate at rest and move the upper plate at constant velocity in a parallel direction, so that the plate separation distance keeps constant. Near each of the plates, the velocities of the liquid and the plate are equal due to the friction between plate and liquid. Hence a velocity field that describes the stream builds up, (Fig. 15). In the simplest case the velocity is linear in the spatial coordinate perpendicular to the plates. It is a shear flow, as different planes of liquid slide over each other. This is true for a simple as well as for a complex fluid. But what will happen to the mesoscopic structure of a complex fluid How is it affected Is it destroyed or can it even be built up For a review of theories and experiments, see Ref. 122. Let us look into some recent works. 

Figure 13.12 Schematic representation of the structure of the complex anion LSbjCIiiO] " showing the two different coordination geometries about Sb and the unique quadruply bridging Cl atom.

The structure of the complex of (S)-tryptophan-derived oxazaborolidine 4 and methacrolein has been investigated in detail by use of H, B and NMR [6b. The proximity of the coordinated aldehyde and indole subunit in the complex is suggested by the appearance of a bright orange color at 210 K, caused by formation of a charge-transfer complex between the 7t-donor indole ring and the acceptor aldehyde. The intermediate is thought to be as shown in Fig. 1.2, in which the s-cis conformer is the reactive one. 

A chiral titanium complex with 3-cinnamoyl-l,3-oxazolidin-2-one was isolated by Jagensen et al. from a mixture of TiCl 2(0-i-Pr)2 with (2R,31 )-2,3-0-isopropyli-dene-l,l,4,4-tetraphenyl-l,2,3,4-butanetetrol, which is an isopropylidene acetal analog of Narasaka s TADDOL [48]. The structure of this complex was determined by X-ray structure analysis. It has the isopropylidene diol and the cinnamoyloxazolidi-none in the equatorial plane, with the two chloride ligands in apical (trans) position as depicted in the structure A, It seems from this structure that a pseudo-axial phenyl group of the chiral ligand seems to block one face of the coordinated cinnamoyloxazolidinone. On the other hand, after an NMR study of the complex in solution, Di Mare et al, and Seebach et al, reported that the above trans di-chloro complex A is a major component in the solution but went on to propose another minor complex B, with the two chlorides cis to each other, as the most reactive intermediate in this chiral titanium-catalyzed reaction [41b, 49], It has not yet been clearly confirmed whether or not the trans and/or the cis complex are real reactive intermediates (Scheme 1.60). 

An X-ray structure of the complex formed between 3-cinnamoyl-l,3-oxazohdin-2-one and a chiral TADDOL-Ti(IV) complex (see Chapters 1 and 6 by Hayashi and Gothelf, respectively) has been characterized [16]. The structure of this complex has the chiral TADDOLate and cinnamoyloxazohdinone ligands coordinated to titanium in the equatorial plane and the two chloride ligands in the axial plane and is similar to A in Fig. 8.8. The chiral discrimination was proposed to be due to... 

Fig. 6. Coordination geometry at silicon structures of the complexes 4 (left) and 5 (right)...

The coordination theory and the principles governing coordinated structures provide the foundation for an interpretation of the structure of the complex silicates and other complex ionic crystals which may ultimately lead to the understanding of the nature and the explanation of the properties of these interesting substances. This will be achieved completely only after the investigation of the structures of many crystals with x-rays. To illustrate the clarification introduced by the new conception the following by no means exhaustive examples are discussed. 

The X-ray absorption fine structure (XAFS) methods (EXAFS and X-ray absorption near-edge structure (XANES)) are suitable techniques for determination of the local structure of metal complexes. Of these methods, the former provides structural information relating to the radial distribution of atom pairs in systems studied the number of neighboring atoms (coordination number) around a central atom in the first, second, and sometimes third coordination spheres the... 

The five-coordinate iridium complexes may be protonated by glacial acetic acid, yielding [Ir(PPh3)2(CNR)3H] + the structure of this complex is determined by PMR measurements to be (XXIX). However, in the analogous HCl reaction [Ir(PPh3)2(CNR)2Cl2] is obtained. The reaction of [Ir(PPh3)2(CNR)3] with methanol also proved quite out of the ordinary. 

The reaction with PPh2CCH leads to the formation of [Au(QF5)(PPh2CCH)[ [53] whose P H NMR spectrum shows a singlet at 17.2ppm, in the H NMR spectrum the resonance of the C = CH proton is observed at 3.46 ppm. The IR spectrum shows, besides the pentafluorophenyl absorptions, a band at 3271 cm due to the V(Csch) and another absorption at 2056 cm for the asymmetric C = C stretch. The structure of this complex was studiedby X-ray diffraction, the Au(I) atom is an almost linearly coordinated and the Au—C and Au—P distances are in the range of the values found for similar complexes. The excitation and the emission data in the solid state at 77 K are 331 and 445 nm. 

The H and C NMRspectmm of the thione complex does not provide any due to identify the coordinated heteroatom although the signal of the NCS-carbon is shifted upheld in the complex. The structure of the complex was studied by X-ray diffraction, conhrming the almost linear coordination around the gold atom and showing that the ligands are almost co-planar. 

The structure of the complex [Au(pdma)2][Au(C6F5)2] was studied by X-ray crystallography and was the first full X-ray analysis of a four-coordinate Au(I) complex and also the first of an [AUR2] anion where R is an organic group. 

The crystal structure of the complex [PhCH2PPh3][Au(C6F5)Cl] has been determined by X-ray diffraction it consists of an anion-cation pair where the anion shows a two coordinate Au(I) atom bonded to a Cl atom and a C Fs ring and the coordination is almost linear (molecules with two different angles were found but the differences between the two molecules in the asymmetric unit are not significant 177.8(5) and 179.8(4) A) [78]. 

The crystal structure of this complex was determined by X-ray diffraction and the sulfur coordination to the Au atom was confirmed. In this structure all the distances are in the ranges found for similar molecules but inter- and intramolecular Au Au contacts are longer that those in other similar molecules. 

When [AuTl(C6F5)2]n reacts with DMSO the complex [Tl2 Au(C6F5)2 2 lt-DMSO 3]n [126] is obtained. The crystal structure of this complex shows unsupported Au - Tl interactions that range from 3.2225(6) to 3.5182(8) A but there are no Tl- - - Tl interactions. There are Au- - - Au interactions of3.733 A and the gold centers are almost linearly coordinated to two pentafluorophenyl groups. The complex is strongly luminescent both at room temperature (emits at 440 nm (exc.390 nm)) and at 77 K (emits at 460 nm (exc. 360 nm)). 

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