Higher Coordination Numbers

Coordination numbers higher than 6 are rare and in some cases are known to involve chelating N03 ions which not only have a small bite but, may also be coordinated asymmetrically so that the coordination number is not well defined. 

The familiar standard de carbonyl at ion mechanism ( 3, 5) involving a concerted oxidative-addition of aldehyde, CO migration (with subsequent elimination), and reductive-elimination of product, would seem with metalloporphyrins to require coordination numbers higher than six, and in this case Ru(IV) intermediates. Although this is plausible, the data overall strongly suggest a radical mechanism and Ru(III) intermediates. 

The geometry of the ferrous iron is trigonal bipyramidal. Thus, this structure confirms the tendency of iron complexes to coordination numbers higher than four. The two N-donors and one of the carboxylates of 5 occupy the equatorial positions. In good agreement... 

XXX-bridge hydrogens) two coordination numbers higher than otherwise present, or adjacent to bridge hydrogens ... 

As illustrated in the following sections, there are many examples wherein carbon occupies sites 1 coordination number higher than is otherwise available but never (to date) has an isomer been observed in which carbon occupies a site that is 2 coordination numbers higher than is otherwise available. Thus, no candidate structures in Pig. 10 are illustrated wherein carbons might be placed in the six-coordination situations when four-coordination alternatives are available. When such isomers are produced they should be less stable than those illustrated and will be cataloged as structures that violate rule 3p. 

Attention will be restricted here to fluorine compounds of the three d-transition series elements, in which the metal ion is octahedrally coordinated. Octahedral coordination, however, is found in nearly all these cases, which is quite reasonable considering the sizes of the ions in question. A close-packed octahedron of fluoride ions of radius 1.33 A adapts a size of its octahedral interstice appropriate to a sphere of radius 0.55 A. Cations having this size and larger ones meet the conditions of a contact between cations and anions. Thus stability is predicted for octahedral coordination until such contacts of ions become possible for coordination numbers higher than 6. For a coordination of 8 fluoride ions this is only the case if the radii of the cations are as large as 0.86 A (square antiprism) or 0.97 A (cube) resp. 

Most commonly, metal ions M2+ and M3+ (M = a first transition series metal), Li+, Na+, Mg2+, Al3+, Ga3+, In3+, Tl3+, and Sn2+ form octahedral six-coordinate complexes. Linear two coordination is associated with univalent ions of the coinage metal (Cu, Ag, Au), as in Ag(NH3)2+ or AuCL Three and five coordination are not frequently encountered, since close-packing considerations tell us that tetrahedral or octahedral complex formation will normally be favored over five coordination, while three coordination requires an extraordinarily small radius ratio (Section 4.5). Coordination numbers higher than six are found among the larger transition metal ions [i.e., those at the left of the second and third transition series, as exemplified by TaFy2- and Mo(CN)g4 ] and in the lanthanides and actinides [e.g., Nd(H20)93+ as well as UC Fs3- which contains the linear uranyl unit 0=U=02+ and five fluoride ligands coordinated around the uranium(VI) in an equatorial plane]. For most of the metal complexes discussed in this book a coordination number of six may be assumed. 

Monomeric Sr—N compounds with coordination numbers higher than six have been reported, as in the benzamidinate shown in Figure 3.9. ... 

In addition to complexes containing the WO, W02 and WO3 structural units, the element shows quite a varied chemistry in this oxidation state, undoubtedly because the hexahalides are reasonably stable and give rise to a variety of substitution products of the type [WX6 L ] (n = 1-6). In addition, coordination numbers higher than six can also be obtained. WVI also shows considerable tendency to form bonds of order higher than one with good jz donor atoms such as S, Se, N, NR, CHR and CR. 

Y-0 2.277(7)-2.391(6), Y-Y 3.4295(19) A] (Fig. 5b) [52], It is important to note that the smaller Cd11 ion seldom adopts coordination numbers higher than six in these and related systems, while the larger Ym ion normally shows higher coordination numbers of 8 or 9 in its complexes. 

Although coordination numbers higher than six were not explicitly considered above, certain species, such as the seven-coordinate 14-1-7 molecule IF7 in a pentagonal bipyramid geometry, contain a linear F-I-F 3c/4e hypervalent bond. 

Studies of the complex chemistry of the tervalent lanthanides show that coordination numbers higher than six are common, for example in the tetrakistropolonates 79), the complexes of the perchlorates with N,N-dimethylacetamide (DMA) (77) or octamethylpyrophosphoramide 61), of the iodides with N,IV-dimethylformamide (DMF) (76), the nitrates with triphenylphosphine (or arsine) oxide 46), and the -diketone... 

Optical activity was also demonstrated for the corresponding complexes of Zn(II) and Cu(II), although in these cases coordination numbers higher than 4 are more likely than for Be. In all three cases the resolving agent could not be removed in less time than required for complete racemization of the complexes. The complex bis(benzoylacetonato)beryllium(II) has... 

The different coordinations of the H2O molecules observed in solid hydrates are due to the fact that water molecules can act both as proton donors and proton acceptors (via the two lone-pairs). The most frequent coordinations are tetrahedral surroundings, i.e., types A, B, E, G, and H (class 2) (see Table 1). Trigonal and trigonal-pyramidal coordinations such as types C, D, and F (class 1) and I, J, and K (class T), respectively, are less frequent. Coordination numbers higher than 4, e.g., trigonal bipyramidal coordination. 

In summary, we conclude that in contrast to phosphinomethanides A, anionic phosphines of type B can only form intra- or intermolecular phosphorus silicon donor-acceptor complexes under particularly favorable circumstances. These are seen primarily in suitable ligand geometries. Multidentate phosphine ligands which strongly impose coordination numbers higher than four to silicon seem to be especially suitable. In accord with previous findings [4], chlorine atoms as further silicon substituents favor the phosphine coordination additionally. With some caution we also state that hypervalent silicon with phosphorus donor atoms seems to prefer hexacoordination over pentacoordination. 

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