Covalent bond, coordinated

HC1 is the acid, because it is donating an H+ and the H+ will accept an electron pair from ammonia. Ammonia is the base, accepting the H+ and furnishing an electron pair that the H+ will bond with via coordinate covalent bonding. Coordinate covalent bonds are covalent bonds in which one of the atoms furnishes both of the electrons for the bond. After the bond is formed, it is identical to a covalent bond formed by donation of one electron by both of the bonding atoms. 

MONOD-WYMAN-CHANGEUX MODEL COORDINATE-COVALENT BOND COORDINATION NUCLEARITY... 

B) In this case, a lone electron pair on oxygen forms a bond to Ff. A coordinate covalent bond (also known as a dative covalent bond) is a special type of covalent bond in which the shared electrons come from one of the atoms only. After the bond has been formed, its strength is no different from that of a covalent bond. Coordinate covalent bonds are formed when a Lewis base (an electron donor) donates a pair of electrons to a Lewis acid (an electron accepter) the resultant componnd is then called an addnct (a compound formed by the addition reaction between two molecules). The process of forming a dative bond is called coordination. 

Coordinate Covalent Bond Coordinate Covalent Bond... 

In 1951, chemists trying to make sense of metal complex optical spectra and color returned to an emphasis on the ionic nature of the coordinate covalent bond. Coordination chemists rediscovered physicists Hans Bethe s and John van Vleck s crystal field theory (CFT),... 

A definition of this kind confines bond types to covalent bonds, coordinate bonds, and electron-deficient bonds. Metallic bonds and ionic bonds are excluded. Associations of atoms built from metallic bonds are not considered to be macromolecules since the electrons could be shared by all atoms, but the bonds are nondirectional. Likewise, ionic crystals such as common salt are not considered macromolecules because, for the ideal ionic bond, the electrons are not common to both atoms and the bond is likewise nondirectional. On the other hand, compounds held together by hydrogen bonds, such as the double helix of DNA, are considered macromolecules and not associates since the electrons are shared by the hydrogen bonds, and the hydrogen bond is directed. 

The perspectives provided by Lehn s supramolecular chemistry (3,7) and Cram s host/guest complexation (4) do indeed broaden the realm of coordination chemistry, but the focus still remains on a molecular coordination entity. On the other hand, the coordination polyhedron has lost its pivotal position in the broad definition of coordination chemistry. Furthermore, all manner of intermolecular interactions and interacting pairs are included, and the forces included range from van der Waals and subtle hydrophobic interactions through strong covalent bonds. Coordination chemistry demands only that the molecular entities that unite to form the complex still be recognizable substructures within the complex (6). It is particularly instructive, at this point, to examine examples of coordination entities formed by various modes of interaction that were not recognized in traditional coordination chemistry. 

The examples given above have included only compounds containing simple covalent bonds. Coordination compounds and metal carbonyls offer many more violations" of the rule of eight. Other examples in which some electrons are not used as bonding electrons (analogous to the lone pair in ammonia) will be discussed subsequently. Turning to a consideration of the valences... 

All the other aluminium halides are covalently bonded with aluminium showing a coordination number of four towards these larger halogen atoms. The four halogen atoms arrange themselves approximately tetrahedrally around the aluminium and dimeric molecules are produced with the configuration given below ... 

The covalently bonded oxygen atom still has two lone pairs of electrons and can act as an electron pair donor. It rarely donates both pairs (to achieve 4-coordination) and usually only one donor bond is formed. A water molecule, for example, can donate to a proton, forming H30, and diethyl ether can donate to an acceptor such as boron trifluoride ... 

There is nothing unique about the placement of this isolated segment to distinguish it from the placement of a small molecule on a lattice filled to the same extent. The polymeric nature of the solute shows up in the placement of the second segment This must be positioned in a site adjacent to the first, since the units are covalently bonded together. No such limitation exists for independent small molecules. To handle this development we assume that each site on the lattice has z neighboring sites and we call z the coordination number of the lattice. It might appear that the need for this parameter introduces into the model a quantity which would be difficult to evaluate in any eventual test of the model. It turns out, however, that the z s cancel out of the final result for, so we need not worry about this eventuality. 

The duoroborate ion has traditionally been referred to as a noncoordinating anion. It has shown Httie tendency to form a coordinate—covalent bond with transition metals as do nitrates and sulfates. A few exceptional cases have been reported (13) in which a coordinated BF was detected by infrared or visible spectroscopy. 

Amine—borane adducts have the general formula R3N BX where R = H, alkyl, etc, and X = alkyl, H, halogen, etc. These compounds, characterized by a coordinate covalent bond between boron and nitrogen, form a class of reducing agents having a broad spectmm of reduction potentials (5). 

The reaction between a trinuclear metal carbonyl cluster and trimetbyl amine borane has been investigated (41) and here the cluster anion functions as a Lewis base toward the boron atom, forming a B—O covalent bond (see Carbonyls). Molecular orbital calculations, supported by stmctural characterization, show that coordination of the amine borane causes small changes in the trinuclear framework. 

If additional, auxiliary constraints are present that are not part of the reaction coordinate (e.g., constraints on covalent bond lengths), the formulas are much more complicated, and the algebra becomes rapidly prohibitive. The same is true when qisa. multidimensional coordinate (e.g., a set of dihedrals). Umbrella sampling approaches (discussed in previous sections) are vastly simpler in such cases and appear to be the method of choice for all but the simplest reaction coordinates. 

It would be desirable to achieve a quantitative version of the Hammond postulate. For this purpose we need a measure of progress along the reaction coordinate. Several authors have used the bond order for this measure.The chemical significance of bond order is that it is the number of covalent bonds between two atoms thus the bond orders of the C—C, C==C, bonds are 1, 2, and 3,... 

The structure of AICI3 is similarly revealing. The crystalline solid has a layer lattice with 6-coordinate Al but at the mp 192.4° the stmcture changes to a 4-coordinate molecular dimer Al2Clg as a result there is a dramatic increase in volume (by 85%) and an even more dramatic drop in electrical conductivity almost to zero. The mp therefore represents a substantial change in the nature of the bonding. The covalently bonded... 

---