Larger Coordination Numbers

Mossbauer spectroscopy has also been widely used to investigate the structures of dialkylstannylene derivatives of carbohydrates in the solid state. The usual magnitude of A = 2.78-3.07 mms indicated a coordination number larger than four, with Sn centers in a penta- or hexacoordinated environment. 

The first experimental information on the kinetic parameters for water exchange on a tetravalent metal ion was published in 2000 for U4+ and Th4+ (265,268,271). The coordination numbers for these two complexes were determined by EXAFS to be 10 1. Based on the high coordination number (there are no complexes known with unidentate ligands and coordination numbers larger than 10) a limiting associative mechanism (A) is unlikely and a d-activated mechanism is probable. Surprisingly,... 

During the early sixties Thompson and Loraas (77) reported the formation of mixed complexes of reasonable stability (log K 3.0—5.3) between lanthanide—HEDTA and ligands such as EDDA (N,N -ethylenediaminediacetic acid), HIMDA (N-hydroxyethyliminodiacetic acid) and IMDA (iminodiacetic acid). This observation together with the remarkably large formation constants (72) for the bis-EDDA complexes [log A2 =4.73 (La) 8.48 (Lu)] suggested a coordination number larger than six for the tripositive lanthanide ions in aqueous solution, in view of the fact that mixed chelates of the t5q>e M (HEDTA) (IMDA) axe not formed when M =Co(II), Ni(II) or Cd(II). 

There are few structures known with coordination numbers larger than 8. The existence of coordination number 12 in some crystal lattices was mentioned above. Discrete nine-coordinate structures are known for complexes such as (LnlH.O) )1 and for the hydnde complexes [MHJ2- (where M = Tc or Re). These structures are formed by adding a ligand to each of the rectangular feces of a trigonal prism (Fig. 12.40). 

The complexes in which cobalt has a coordination number larger than six are too rare to justify a complete treatment here. They will be discussed briefly in Sect. 4. 

Figure 4.8 Idealized structures for coordination numbers larger than 8...

Although silylenes have been known for a long time as transient intermediates in thermal, photochemical or dehalogenation reactions, stable species have become available only recently. Si(II) compounds with coordination number larger than two have already been isolated in the late eighties, i.e., decamethylsilicocene prepared by Jutzi et al [1] and the tetracoordinated phosphine derivative obtained by Karsch et al [2]. 

Silicon compounds with coordination number larger than four are the object of many studies first with respect to their application as catalysts in organic and inorganic syntheses and second as starting materials for the preparation of a broad variety of organosilicon compounds [1]. Additionally, hypervalent silicon hydride compounds can successfully be used as model compounds to study, for instance, the mechanism of nucleophilic substitution reactions, which is of great interest since the silicon atom is able to easily extend its coordination number [1]. Moreover, hypervalent silanes are suitable as starting materials for the synthesis and stabilization of low-valent silanediyl transition metal complexes [2-5]. 

V(C0)7]+ the addition of two disadvantages, a coordination number larger than 6 and the cationic nature of a binary metal carbonyl leads to the non-inexistence of this complex in spite of its 18-electron structure. 

Seppelt, K., "Coordination Numbers Larger Than Six Among Electronegative Elements," Comments Inorg. Chem. 1991,12,199. 

The Valence Shell Electron Pair Repulsion model correctly predicts the structures of main group compounds and efi transition metal compounds in most cases. The structures of less symmetric molecules can be predicted semi-quantitatively. The model is particularly successful if the coordination number (including the count of nonbonding electron pairs) does not exceed six. A peculiarity of the model is the stereochemical activity of the free electron pair. Whether the VSEPR model can be successfully extended to coordination numbers larger than 6 is the topic of the following discussion. 

It is clear that the crystal-field parameters do not have the same sign. B and B are positive, Bq is negative. It is possible to generalize the PCEM expressions of the Bg parameters for planar coordination polyhedra with a coordination number larger than 3. The number of B parameters with will of course be symmetry-dependent. All these planar polyhedra have a D h symmetry. The B parameters are ... 

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