Coordination Numbers Higher than Six

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. 

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... 

Thus, a contribution by the coordinative mechanism cannot be exluded. It seems that such a possibility was taken into account by Eastham, as indicated by the dotted lines in Scheme 4.3. The proposed structure would then require a coordination number higher than six for the tin atom. 

Coordination numbers higher than six are postulated for lanthanide complexes of tren but no X-ray structural data are available. A number of live-coordinate systems have been characterized using divalent central metal ions, e.g. Mn(tren)X+ (X = iVCS or ACO ), Cu(tren)X"+ (X = NCO, ACS," or and Zn(tren)X+ (X = NCS, Cl "). If a suitable fifth... 

The program was tested on 639 molecules. A detailed analysis of the quality of 3D structures for various 3D structure generators was performed. To explore the scope of the program the entire public domain database of the National Cancer Institute (126705 structures) was submitted to CORINA. 126 148 connection tables could successfully be converted into 3D structures 327 structures had atoms with a coordination number higher than six and were thus outside the scope of CORINA 230 structures could not be converted into 3D. The entire conversion took 34 856 s (0.28 s per structure) on a Silicon Graphics Indigo 2 Workstation. 

Sc(acac)3] crystallizes in the orthorhombic space group Pbca (Andersen et al., 1973). The stmcture consists of discrete Sc(acac)3 molecules. The scandium(III) ion is six-coordinate, and the coordination polyhedron can be described as a slightly distorted octahedron (fig. 7). The site symmetry is close to D3. Another example of a six-coordinate scandium(III) )3-diketonate complex is [Sc(dbm)3] (Zaitseva et al., 1990). Two different crystal structures are described, one with a triclinic and one with a monochnic cell. So far, no scandium(lll) )3-diketonates with coordination number higher than six have been described. Because of the small ion size of Sc +, coordination number six is the general rule fortius rare-earth ion. Also in the trinuclear scandium(III) disiloxanediolate complex [(Ph2Si0)20]2Sc3(acac)5], the coordination number of Sc + is six as well (Lorenz et al., 2001) (fig. 8). 

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