Generalizations about Coordination Numbers

Coordination Numbers and Radii. In the transition metal ions, the interaction of the ligand orbitals with the d orbitals of the metal ions generally determines the coordination number and geometry of the oordination sphere about the metal. The... 

From a structural point-of-view the bulk metallic state, that is, fee lattice (with varying densities of defects such as twins and stacking faults) is generally established in gold nanoparticles of about 10 nm diameter and upwards. However, such particles still display many unusual physical properties, primarily as the result of their small size. Shrinking the size of gold particles has an important effect it increases both the relative proportion of surface atoms and of atoms of even lower coordination number, such as edge atoms [49] and these atoms in turn are relatively mobile and reactive. 

From Adams (193). In all cases, the radii apply to a coordination number of six (radii for coordination number eight are generally about 0.12 A larger). 

Here, as in other branches of inorganic chemistry, interatomic distances show a considerable variation and, although some correlation with bond order is possible, attempts to do so should be regarded with caution.For metals with close-packed structures, the coordination number of any atom is 12 for cubic or hexagonal structures, and 14 (8 plus 6 more neighbors at about 15% further away) for body-centered cubic structures. In general, this number exceeds the number of electrons per atom available for metal-metal bond formation and precludes the formation of localized, two-electron bonds between metal atoms. Bond orders of less than 1 are therefore commonly recorded. For metal clusters, it is necessary to consider the variety of ways in which valence electrons may be utilized in chemical bonding within the Mm... 

Chapter 12 Coordination Chemistry Structure 472 Coordination Number 472 Coordination Number 2 473 Coordination Number 3 474 Coordination Number 4 474 Coordination Number 5 479 Coordination Number 6 488 Coordination Number 7 503 Coordination Number 8 507 Higher Coordination Numbers 509 Generalizations about Coordination Numbers 511 Linkage Isomerism 513 Other Types of Isomensm 52] The Chelate Effect 522... 

The crux of organic mechanistic stereochemistry may be the Walden inversion, the inversion of stereochemistry about a four-coordinate carbon atom by nucleophilic attack of, for example, a hydroxide ion on an alkyl halide. Many reactions of inorganic molecules follow the same mechanism. In contrast, the dissociative mechanism of tertiary halides to form tertiary carbocatanion intermediates is essentially unknown among the nonmetallic elements silicon, germanium, phosphorus, etc. The reason for this is the generally lower stability of species with coordination numbers of less than 4, together with an increased stability of five-coordinate intermediates. This difference is attributable to the presence of d orbitals in the heavier elements (Chapter 18). 

In general, the coordination numbers of the elements of the second and third transition series lend to be greater than for the first series because the ionic radii are larger by about 15-20 pm (0.15-0.20 A) for corresponding species.20 Thus tetrahedral coordination is considerably less frequent although observed in species such as [WO/-, [ReOJ-, and OsO<- Square planar coordination is found in dH species such as Rh(I), Pd(II), Pt(II). and Auflll), which are especially stabilized by LFSE. Octahedral species are quite common, and the occurrence of coordination numbers 7. 8, 9, and 10 is fairly common. 

Divalent and tetravalent Pt probably form as many complexes as any other metal. The platinum(II) complexes are numerous with IV. S, halogens, and C. The letranitritoplatinum complexes are soluble in basic solution. Tetranitntoplatinum(II) ion is formed when a solution of plat-inum(II) chloride is boiled, at about neutral pH, with an excess of NaNO f. The ammonium salt may explode when heated. Generally, platinum-metal nitrites should be destroyed in solution. They never should be heated in the dry form. Pladnum(II) complexes most often have a coordination number of 4. Many compounds have been prepared with olefins, cyanides, nitriles, halides, isonitnles, amines, phosphines, arsines, and nitro compounds. 

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