Coordination number one

Pyridine derivatives with additional donor functions and sterically demanding substituents have been used with the intention of producing complexes of Cd (and of other metals) with low coordination number one of these ligands is the tridentate, planar-bonding 2,6-bis[(2,6-dimethyl-phenylimino)methyl] pyridine (pydim a Schiff base derived from 2,6-pyridine dialdehyde), which with Cd(BF4)2 and thiocyanate gives a dinuclear complex [(pydim)Cd(/x-NCS-S,N)]2(BF4)2 with N-dominated coordination sphere.191 As centrosymmetric Plijc, Z= 2), the complex has an antiparallel /x-1,3 NCS double bridge with Cd—N and Cd—S bonds (224.6 pm and 255.5 pm, respectively) the Cd—N(py) bond is clearly shorter than the Cd—N(imino) bonds (225.6 pm and 245.0 pm,... 

The data analysis in Table 9.3 summarizes the crystallographic information of the Co-Mo-S phase active for hydrodesulfurization. The Co-S distance in Co-Mo-S is 0.22 nm, with a high sulfur coordination of 6.2 1.3. Each cobalt has on average 1.7 0.35 molybdenum neighbors at a distance of 0.28 nm. Based on these distances and coordination numbers one can test structure models for the CoMoS phase. The data are in full agreement with a structure in which cobalt is on the edge of a MoS2 particle, in the same plane as molybdenum. 

To illustrate the potential diversity of structure and bonding of phosphorus, the classic Lewis representations for each possible coordination number one to six are shown in Fig. 15.4.1. Many of these bonding types have been observed in stable compounds, which are discussed in the following sections. 

Occasionally, very bulky ligands enforce two-coordination on metals with fewer d electrons that would usually prefer other higher coordination numbers. One of very few such compounds is the d6 Fe(II) complex of the amido anion N(SiR3)2. The particular two-coordinate example [Fe N(Si(CH3)(C6Hs)2)2 2] approaches linear shape (Figure 4.5), but the N—Fe—N angle is only 170°. As the bulk of the substituents on the Si atoms decrease, there is a switch to higher coordination number the anion N(Si(CH3)3)2, for example, forms a three-coordinate [Fe N(Si(CH3)3)2 3] complex. 

Coordination number zero corresponds to an isolated atom coordination number one occurs for very simple molecule combinations such as Ni-NN that are stable only in very cold matrices such as argon. Coordination... 

Knowing the coordination numbers, one can solve a number of crystal chemical problems. Thus, 80 years ago Goldschmidt discovered the dependence of the chemical bond length on the coordination number of a cation [245], At the same time Pauling introduced the concept of bond strength [246], according to which the valence of an atom i, which is linked to atoms j, obeys the relationship... 

Today inorganic chemists do not use such terms as "primary" (Haupt) and "secondary" Neberi) valence but instead speak of "oxidation state" and "coordination number." One modem popular definition for the term "oxidation state" is "The oxidation state of a metal in a complex is... 

Another more familiar example of two isomeric forms in solution is Eu(EDTA) (Fig. 8.3a). The two different excitation peaks purportedly arise from Eu(EDTA) complexes with two different coordination numbers, one with two water hgands and one with three ligands. This assignment is based in part on the equilibrium behaviour of the two species as a function of pressure [48,49]. 

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