Coordination number three

Coordination Number Three. These include planar complex ions such as boric acid B(OH)3 and out-of-plane pyramidal complexes (including SnCl3 ). 

The reaction of SiF-coupled six-membered rings with lithium organyls first produces ring contraction and subsequently eliminates LiF. As discussed in Section II, we have now obtained an intermediate ylide that contains silicon atoms with the coordination numbers three and five. This gives rise to a nucleophilic migration of a methanide ion. A fused bicyclic compound is formed (Scheme 24). This compound has also been characterized by an X-ray analysis and the structure is shown in Fig. 13. Both the four-membered rings are planar.42,46,50... 

Elimination of LiF leads to an intermediate ylide containing silicon atoms with the coordination numbers three and four as well as nitrogen atoms with the... 

Dithietane derivatives having one or two sulfur atoms with coordination number three have puckered structures. 

We will now examine some examples of rare earth complexes spanning coordination numbers three to twelve. The examples given are illustrative and no claim is made as to coverage of an exhaustive list of compounds. It is also necessary to bear in mind that the coordination polyhedron for complexes with large coordination numbers is rarely uniquely determined and that it is generally possible to describe the polyhedron as related to more than one of the ideal structures. The description of the polyhedron is usually influenced by the compounds with which comparisons are made [21]. 

The commonest coordination number of FeIi is six but a range of other coordination numbers — three, four, five, seven and eight — are also well established. Examples of all of these will be given in the following pages along with discussion of the structural and electronic factors which determine them. 

Consequently the corresponding silylene transition metal complexes with coordination number three at silicon are accessible. In contrast the reaction of dichlorosilanes with carbonylate dianions in the presence of a donor solvent yields donor stabilized silylene complexes (coordination number at silicon 3+1). 

A similar coordirmtion sphere at silicon is found in unsaturated Si=E compounds (E = element of group 14-16) and their donor adducts (coordination numbers 3 and 3+1). Attached to coordination number three at silicon the existence of silicenium cations is still controversely discussed in the literature. While some experiments are interpreted as evidence for their existence in the condensed phase, theoretical calculations do not support the formulation of donor free silicenium cations. However, there seems to be no doubt about the verfication of inter- and intramolecular donor stabilized cationic species (coordination numbers 3+1 and 3+2, respectively). 

Complexes of coordination number three are rare but there are many of coordination number four. They are mostly tetrahedral and will contain sp3 hybridised orbitals, which are characteristically disposed in this way. An example of a tetrahedral complex is the cyano complex of the cuprous ion [Cu(CN)4F. 

The additional ligands and coordination number three at silver are highly probable, but they are not detectable in the NMR spectra. 

Well-defined 1 1 and 1 2 adducts with triphenylphosphine are formed with ease 42,45, 46). The yellow 1 1 adduct is more stable than bis(acrylo-nitrile)-nickel itself (dec. p. 185° C) and represents one of the first examples of a nickel complex with the coordination number three. The 2 1 adduct is monomeric in benzene and has a dipole moment of 6 Debye units (46). Unstable bis(pyridine) adducts were also isolated, but were found to decompose at room temperature, depositing metallic nickel (46). 

Reduction of nickel acetylacetonate in the presence of phosphines and olefins produces complexes of zero-valent nickel with the coordination number three. The ligands in (LIII) may be replaced by others under mild... 

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