Ligand-lone pair coordination number

Sterically demanding ligands and high coordination numbers can shift the lead geometry to a more spherically symmetric, holodirected structure, in which the lone pair of electrons is stereochemicaUy inactive. 

VB concept of hybridization proposes the mixing of particular combinations of s, p, and d orbitals to give sets of hybrid orbitals, which have specific geometries. Similarly, for coordination compounds, the model proposes that the number and type of metal-ion hybrid orbitals occupied by ligand lone pairs determine the geometry of the complex ion. Let s discuss the orbital combinations that lead to octahedral, square planar, and tetrahedral geometries. 

A coordination compound, or complex, is formed when a Lewis base (ligand) is attached to a Lewis acid (acceptor) by means of a lone-pair of electrons. Where the ligand is composed of a number of atoms, the one which is directly attached to the acceptor is called the donor atom . This type of bonding has already been discussed (p. 198) and is exemplified by the addition compounds formed by the trihalides of the elements of Group 13 (p. 237) it is also the basis of much of the chemistry of the... 

Because of the comparatively large space requirements of a lone pair, low coordination numbers favor the expression of a stereochemically active lone pair [25]. Specific ligand design can trigger the stereochemical activity of a lone pair in complex compounds. 

By definition, the coordination number includes the adjacent atoms, and lone electron pairs are not counted. On the other hand, now we are considering lone pairs as occupying polyhedron vertices. To take account of this, we regard the coordination sphere as including the lone pairs, but we designate them with a //, for example -octahedral = octahedron with two lone electron pairs and four ligands. 

Molecules of the elements of period 3 and beyond may have higher LLP coordination numbers than four, and therefore considered to be hypervalent, because their atoms are larger than those of the period 2 elements. In other words, more than a total of four ligands and lone pairs can pack around a central atom if it is from period 3 and beyond. 

The geometries in Figs. 4.86 and 4.87 suggest an important distinction in the multicenter hapticity character of ligand attachment to the metal atom. Hapticity refers to the number of atoms in a ligand that are coordinated to the metal. In the Ir+ diammine complex (Fig. 4.86(a)), the metal attaches to each of two nN donor lone pairs in simple monohapto (one-center, q1) fashion. However, in the Ir+ complexes with HCCH or CML the metal attaches to the face of the pi bond or three-center allylic pi system in dihapto (two-center, r 2) or trihapto (three-center, q3) fashion, respectively. The hapticity label q" therefore conveniently denotes the delocalized n -center character of the donated electron pair(s) and the geometry of the resulting coordination complex. 

Microbial resistance to established organic antibiotics is a potentially serious problem and provides an impetus for the development of novel antimicrobial metal compounds. The potency of Ag(I) ions is well known—but how does Ag(I) kill a bacterium Much current attention is focused on Bi(III) on account of its ability to kill Helicobacter pylori, an organism which prevents ulcers from healing. Bis-muth(III) chemistry has many unusual features a variable coordination number, strong bonds to alkoxide ligands, the stereochemical role of its 6s2 lone pair, facile formation of polymers, and dual hard and soft character. 

The high electrophilicity of the positively charged element can be modified by intramolecular donation from remote donor substituents. This interaction leads to solvent-free cations with coordination numbers for the positively charged element > 3 and to a considerable electron transfer from the donor group to the element. Frequently used donor substituents utilize heteroatoms with lone pairs (e.g. amino, hydrazino, methoxy, carboxy, phosphino, etc.), in many cases in combination with pincer-type topology of the ligand, for the stabilization of the cationic center. These strongly stabilized cations are beyond the scope of this review and instead we will concentrate on few examples where we have weak donors such as CC multiple bonds, which stabilize the electron-deficient element atom. 

In the simpler cases, such as ethylene, the n-electron pair can be donated to the metal just like a lone pair on, say, the nitrogen of ammonia. This results in a contribution of unity to the coordination number, but two carbon atoms are bound to the central atom. The hapticity of the ethylene is defined as two and is denoted formally by the symbol rf. In general, the hapticity of a ligand is the number of ligating atoms, n, in the ligand that bind to the metal, and is represented by the symbol t ". 

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