Structures of Coordination Compounds

In studying the geometry of coordination compounds, we sometimes find that there is more than one way to arrange the ligands around the central atom. Such compounds in which ligands are arranged differently, known as stereoisomers, have distinctly different physical and chemical properties. Coordination compounds may exhibit two types of stereoisomerism geometric and optical. 

Optical isomers are nonsuperimposable mirror images. Superimposable means that if one structure is laid over the other, the positions of all the atoms will match.) Like geometric isomers, optical isomers come in pairs. However, the optical isomers of a compound have identical physical and chemical properties, such as melting point, boiling point, dipole moment, and chemical reactivity toward molecules that are not themselves optical isomers. Optical isomers differ from each other, though, in their interactions with plane-polarized light, as we will see. 

In general, stereoisomers are compounds that are made up of the same types and numbers of atoms, bonded together in the same sequence, but with different spatial arrangements. 

Optical isomers are described as chiral (from the Greek word for hand ) because, like your left and right hands, chiral molecules are nonsuperimposable. Isomers that are superimposable 

Stereoisomers are compounds that are made up of the same types and numbers of atoms bonded together in the same sequence but with different spatial arrangements. There are two types of stereoisomers geometric isomers and optical isomers. Coordination compounds may exhibit one or both types of isomerism. Note, however, that many coordination compounds do not have stereoisomers. 

Polaroid sheets are used to make Polaroid glasses. 

Optical isomers are described as chiral (from the Greek word for hand ) because, like your left and right hands, chiral molecules are nonsuperimposable. Isomers that are superimposable with their mirror images are said to be achiral. Chiral molecules play a vital role in enzyme reactions in biological systems. Many drug molecules are chiral. It is interesting to note that frequently only one of a pair of chiral isomers is biologically effective. 

Fi re 22.13 The (a) cis and p) trans isomers of dichiorobis(ethyienediamine)cobait(lll) ion and their mirror images, if you couid rotate the mirror image in p) 90° ciockwise about the verticai position and piace the ion over the trans isomer, you wouid find that the two are superimposabie. No matter how you rotated the cis isomer and its mirror image in (a), however, you couid not superimpose one on the other. 

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Gas phase U. V. photoelectron spectroscopy as a tool for the investigation of electronic structures of coordination compounds. C. Cauletti and C. Farlani, Comments Inorg. Chem., 1985, 5, 29 (95). 

Structures of coordination compounds suggest that smaller chelate rings promote higher coordination numbers. Twelve coordination... 

CNR Laboratory of Theory and Electronic Structure of Coordination Compounds, Rome. 

There are further possible advantages from the use of P.E. spectroscopy in the elucidation of the electronic structure of coordination compounds, and information not otherwise available or only indirectly available may thus come to light. The energy position of ionization bands of nonbonding orbitals, such as 6 s2 in (T1(I),... 

Alfred Werner received the Nobel Prize in Chemistry in 1913 for his research into the structure of coordination compounds. 

It is especially important to investigate the molecular structure of coordination compounds in the vapor phase because the relatively weak coordination interactions may be considerably influenced by intermolecular interactions in solutions and especially in crystals. It has been shown that the geometrical variations can be correlated with other properties of the molecular complexes ). In particular the structural changes in the F3B N(CH3)3 and CI3B N(CH3)3 molecules ) relative to the respective monomeric species unambiguously indicated boron trichloride to be a stronger acceptor than boron trifluoride. Data on the geometry and force field have also been correlated ). 

Weroer. in formulating his ideas about the structure of coordination compounds, had before hint facts such as the following. Four complexes of cobaltflll) chloride with ammonia had been discovered and named according to their colors ... 

The previous chapter described the bonding principles responsible for the energetics and structure of coordination compounds. In this chapter the resulting structures will be examined in more detail with particular regard to (he existence of various coordination numbers and molecular structures, and the effect of these structures on their chemical and physical properties. 

Like infrared spectrometry, Raman spectrometry is a method of determining modes of molecular motion, especially the vibrations, and their use in analysis is based on the specificity of these vibrations. The methods are predominantly applicable to die qualitative and quantitative analysis of covalently bonded molecules rather than to ionic structures. Nevertheless, they can give information about the lattice structure of ionic molecules in the crystalline state and about the internal covalent structure of complex ions and the ligand structure of coordination compounds both in the solid state and in solution. 

Winter and co-workers used the proton NMR spectra of niobium(v) complexes with 5-phenyltetrazolate ligand recorded at temperatures between —80 and +40°C in toluene-structural analysis <2001IC6451>. One- and two-dimensional 111 NMR methods in solution and solid state were used for analyzing the structure of irbesartan 83 <1998J(P2)475> as well as for the analysis of the structure of coordination compounds of ruthenium(n) and iron(n) with 5-aryltetrazolates <2002OM3774, 2003JOM(669)135, 2006IC695>. 

In this work we have paid attention to the most frequently reported ligand systems from within the enormous number of ligands used in modern coordination chemistry, as reported in the many pieces of literature dedicated to this topic. In the material that follows we have separated inorganic and organic compounds. In general, the data summarized in monographs [1,9] and our previous reviews, [10-17a] forms the basis for the presentation of this work. It must be said that not only neutral molecules are examined throughout, but also anions. Considerable attention is paid to the reviewed literature of the last decade. Due to the synthetic direction of this book, classic theories on the structure of coordination compounds [17b] are not examined here. 

As a result of interaction of 843 and pyridine, the adduct 845 is formed [53], The structures of coordination compounds 844 and 845 were proved by x-ray diffraction. As shown above (Sec. 3.4.3.2), the direct ammonia synthesis [55,56] with participation of various ligands (especially aliphatic, aromatic, and heterocyclic amines, aminoalcohols), elemental metals (or their oxides), and NH4SCN in mostly non-aqueous media, opens definite possibilities for obtaining thiocyanate complexes. In this respect, transformation (4.9) should be mentioned [57] ... 

CHARACTERISTIC FEATURES OF THE ELECTRONIC STRUCTURE OF COORDINATION COMPOUNDS... 

To give more breadth to our discussion of the connections between molecules and solids we now consider examples of systems in which one can see transition-metal complexes. They are of pedagogical interest because their simple electronic structures can be used to good effect in elementary treatments of the electronic structure of coordination compounds. 

The structure of coordination compounds can vary with a change of the state of aggregation The strength of donor-acceptor bonds, both intra- and intermolecular,... 

Until now we considered the topology of various organic molecules. In inorganic chemistry the topological methods are not so popular. Therefore, in this chapter we shall confine ourselves to only two topics, the topological theory of borane structure and the discussion of the interrelationship between topology, symmetry, and electronic structure of coordination compounds. 

Pfeiffer s application of Werner s theory to crystals and the advent of new experimental techniques led a number of scientists in various countries to use X-ray diffraction to determine crystal structures of coordination compounds. American chemist Ralph W. G. Wyckoff(l 897-1994) chose ammonium hexachloroplatinate(IV) as a crystal that should... 

When considering the structures of coordination compounds it is worth noting that transition element complexes are usually formed from reactions between their salts and Bronsted bases in solution. However, the structures of the compounds formed are usually determined in the solid state using samples crystallized from solution. While it may usually be assumed that the solid state structures are similar to the solution structures, this may not always be so. and some complexes may adopt different structures in solution and the solid state. 

The proposals of Werner (49) about the structure of coordination compounds were amply justified by the chemical reactions exhibited by them, by the identification of geometrical isomers, and by the resolution of certain compounds into their optical antipodes. Modern methods for the determination of structures and for studying the nature of charged or neutral species in solution have demonstrated the fundamental soundness of Werner s views. 

The structures of coordination compounds are governed largely by the coordination number of the metal. Many have structures similar to the simple molecules and ions we studied in Chapter 8. Unshared pairs of electrons in d orbitals usually have only small influences on geometry because they are not in the outer shell. Table 25-7 summarizes the geometries for common coordination numbers. 

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