Maximum coordination number

Considering the interesting bissilylene complexes, the question arises how many silylenes can be coordinated to a transition metal. In the series of stannylene complexes, a maximum coordination number of at least 3 could be established. 

Table 1. The Measured and Calculated Maximum Coordination Number of CO, and the Predicted Structure for each of Co a...

On the other hand, however, Sajeevkumar and Singh found the average coordination number to be 4.7 (141), whereas Megyes et al. found up to six acetonitrile molecules bound to the Li+ ion in the gas phase by means of mass spectrometry (142). Therefore, we reinvestigated the maximum coordination number of [Li(CH3CN)n]+ and also of [Li(NCH),J 1 purely by quantum chemical calculations, which resulted in [Li(NCH)4]+ and [Li(CH3CN)4]+ (93). 

Table 4.2. Lewis-like oxidation numbers (n07L), formal d count, metal electron configuration (eep, Chp) and minimum and maximum coordination numbers (ncs) for low-spin normal-valent compounds of group 6-11 transition metals...

A guiding principle for the solvent extraction chemist is to produce an uncharged species that has its maximum coordination number satisfied by lipophilic substances (reactants). Eor trivalent lanthanides and actinides (Ln and An, respectively), the thermodynamic data suggest a model in which addition of one molecule of TBP displaces more than one hydrate molecule ... 

Solutes containing metals can further be classified according to the type of ligand N refers to the maximum coordination number of the metal relative to the hgand ... 

In eqn (6.2) i o = 220 pm, Z) = 34pm, and i min is the smallest 0-0 distance that can be achieved with an effective valence of i. The maximum coordination number that can be observed must therefore satisfy the inequality (6.3) obtained from eqns (6.1) and (6.2) ... 

Table 2.1 Maximum coordination numbers for cations based on valence considerations...

Complexes of both selenium(IV) and tellurium(IV) with sulfur-containing ligands are known although the structural properties appear to have been determined only for the tellurium(IV) species. However, due to the smaller size of selenium(IV) and its poorer acceptor properties, as compared with tellurium(IV), its maximum coordination number appears to be six whilst it has been found to be eight for tellurium.78 Furthermore, the greater tendency of tellurium to form secondary bonds may cause the structures of similar selenium(IV) and tellurium(IV) compounds to be different. 9 It is also pertinent to note that the reduction of tellurium(IV) to... 

Why should the early transition metals form so many polyoxoanions The answer lies in the size of the M5/6+ cations and their -acceptor properties.1,5 The effective ionic radii of V5+ (0.68 A), Mo6+ (0.77 A) and W6+ (0.74 A) are consistent with the observation that these cations adopt four-, five- and six-fold coordination by oxide ion. With very few exceptions V, Mo and W atoms in heteropolyanions are six-coordinate. On the other hand Cr6+ (0.58 A) hap a maximum coordination number of four in oxides and oxoanions. Few isopoly- and heteropoly-chromates are known and they are all based on groups of corner-shared Cr04 tetrahedra [Cr207]2-, [Cr3O10]2-, [Cr4Oi3]2-, [03SOCrO3]2-, [02I0Cr03]-,... 

The largest nonmetals show coordination numbers as high as eight in the oc-tafiuoroanions, ]Fjj" and XeFg- (see Chapter 17). The corresponding oxyacids and oxyanions show a maximum coordination number of six [Sb(OH)fi]-, Te(OH)6, OI(OH)s, and [XeO, ]4- Of these, apparently only iodine shows a maximum oxidation state with a coordination number as low as four Periodic acid can exist as either 01(0H)S or HI04. 

Maximum coordination numbers cf the nonmefals shown by the lluarides... 

The next two series of nnnmctals, silicon through chlorine and germanium through krypion. show a maximum coordination number of six in hexafluoro anions. SF, and TuF . Even here the oxyucids and uxyunions typically show a coordination... 

Two additional points that are self-evident are discussed below for the sake of completeness. The coordination number of a metal ion counting all ligands other than the adjacent metal involved in multiple metal bonding is less than the maximum coordination number possible for that metal center. Furthermore, the number of d-electrons on the metal is nearly equal to the number of metal valence orbitals not involved in metal-ligand bonding to optimize metal-metal bond formation by filled bonding MOs. 

It may be noted that, for d2, d4, d6, rf8 and d ° transition metal ions, octa-, hepta-, hexa-, penta-and tetra-cyano complexes represent the species of maximum coordination number. It should thus be possible, so far as electronic factors are concerned, to prepare an ennea complex of a d° system (e.g. Revl1 or La111). No ennea cyano complex has, however, yet been reported. 

Coordination chemists are, however, generally interested in those systems with definite coordination numbers. The maximum coordination number is often achieved in a transition-metal ammine complex, especially where synthetic routes involve the use of excess ligand, as the gas, aqueous solution or the anhydrous liquid. Even so, forcing conditions or catalysts are required to form such complexes as Cu(NH3)62+ or Co(NH3)63+. 

After Werner 76b and more recently Iiundle 17K one is led to suggest that the maximum coordination number of an atom may be considered as having reference to the space around the surface of the kernel of the atom 78),... 

Aside from the CuF 3 complex, the maximum coordination number for ions of the coinage metals is 4 the complexes of Cu(II) and Au(III) nearly always are tetracoordinated. Coordination numbers of 4 and 3 are known for the univalent ions, but coordination number 2 is much the more usual. 

We see (Table 7) that the quantity of energy is a maximum for surrounding by six ions. Should we wish, therefore, to add a further one or two to this number then energy would no longer be gained by doing so, rather the contrary. Thus six can be regarded as the maximum coordination number for mono-... 

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