Four-coordinate system

The reaction of a 1,10-phenanthroline complex of iridium, [Ir(cod)-(phen)]+, with dioxygen in methanol solution has been studied (38). When the anion for this cationic complex is chloride, no anion-cation interaction occurs, and the iridium system remains four-coordinate. However, when either iodide or thiocyanate is present due to the addition of their sodium salts (or in the presence of added triphenylphos-phine when the anion is chloride), the iridium system becomes five-coordinate because of the interaction between I", SCN", or PPh3 and the iridium center. These five-coordinate systems react more rapidly with dioxygen than did the four-coordinate system at both normal and elevated pressures. An end-on oxidative addition of the dioxygen moiety, with displacement of the , SCN, or PPh3 ligands, was postulated. 

An important geometric parameter for four-coordinate systems is the tetrahedral twist angle 9 (Fig. 17.7.3). We can use this parameter to inspect the geometry around one of the coordinated amine donors. Define a new reference plane with one of the N donors and two of the atoms bound to it (e. g., the Co center and one of the FI s). Then, click on the Angle between two planes button and follow the instruction to define the other plane (the same N donor again and the other two substituents, i. e., the second H and the C atom bound to the N). The value of the tetrahedral distortion (0 = 88.5°) will then be displayed (the value of 9 obviously depends on the choice of planes). 

Before specifying the form for the kinetic energy operator, we will comment on the various coordinate systems that are used in studies of molecular dynamics. Four coordinate systems are frequently used they are defined as follows for 21-mode planar benzene ... 

Conversions between these four coordinate systems are defined by the following tensorial or linear transformations (160) ... 

To conclude this review, we should like emphasize the fact that no serious argument can be presented for an exclusive use of pure atomic orbitals in quantum-chemical calculations, except that of the separation of the radial and angular parts of the wave function in the Hartree-Fock picture of the atoms themselves [80]. To the defenders of the traditional s, p, d. .. orbitals, we wish to reply that there are four coordinate systems for which the SchrQdinger equation of the hydrogenic atom can be solved [81], instead of eleven for a wave... 

An important geometric parameter for four-coordinate systems is the tetrahedral twist angle 0 (Figure 17.17). We can use this parameter to inspect the geometry arormd one of the coordinated amine donors. 

Unlike four-coordinated systems, six-coordinated complexes afford many examples of optical isomerism. These are very common among compounds or ions of the type [M(AA)3]. For example, the optical isomers of trioxalatochromate(lll) are complexes XX and XXL... 

The most common coordination number is six and such complexes have an octahedral structure. The next most common four-coordinated systems have either tetrahedral or square planar structures. Other complexes are known having different coordination numbers and structures. The stereochemistry of metal complexes is a fascinating subject. Several different types of isomeric structures are possible and have been demonstrated in these systems. For our purpose here it is sufficient to cite examples of geometrical (ds-trans) and optical isomerism. This can readily be iUustrated by the cis (III) and trans (IV) isomers of QCo(en)2Cl2]+. Note that the... 

Electi ocyclic reactions are examples of cases where ic-electiDn bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are fomred along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HCl + H system. The reaction is now Mdbius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mdbius-Huckel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

To illustrate, again consider the H2O molecule in the coordinate system described above. The 3N = 9 mass weighted Cartesian displacement coordinates (Xl, Yl, Zl, Xq, Yq, Zq, Xr, Yr, Zr) can be symmetry adapted by applying the following four projection operators ... 

Glass-forming systems other than siUca have been examined. The fraction of three- and four-coordinated boron in borate glasses can be determined by nmr (29). Both nmr and x-ray diffraction (30) results led to the suggestion that the boroxyl ring is the stmctural unit of vitreous The... 

Examples of the C2N3S ring system containing three- or four-coordinate sulfur are also well known. The monohalogenated derivatives 12.2 (E = S, Se), are best prepared by the condensation of imidoyl amidines with SCI2 or SeCU, respectively (Scheme 12.1)." In the case of the selenium derivative, the initial product is heated at 60°C and then at 120°C in order to convert it to 12.2 (E = Se) via (PhC)2(NH)N2SeCl2. In the solid state this intermediate is a weakly associated, centrosymmetric dimer with Se-Cl and Se Cl distances of 2.42 and 3.39 A, respectively. ... 

Ring Systems Containing Four-coordinate Chalcogen... 

---