Co-ordinate bonds

Co-ordinate bonds are formed by the sharing of electrons, both electrons being donated by the same atom. Thus the hydrogen ion, has no outer electrons whilst ammonia has eight, six shared with hydrogen atoms and one lone-pair. This lone-pair is donated to the hydrogen ion and the ammonium ion is formed ... 

The third main type of bond is the co-ordinate bond, in which both of the shared electrons come from one atom. Examples of interest in polymer science are the addition compounds of boron trifluoride Figure 5.3). 

Interaction of the inhibitor with water molecules Due to the electrostatic and co-ordinate bond interactions described under the previous two headings, the surfaces of metals in aqueous solutions are covered with adsorbed water molecules. Adsorption of inhibitor molecules is a displacement... 

In the case of ions, the repulsive interaction can be altered to an attractive interaction if an ion of opposite charge is simultaneously adsorbed. In a solution containing inhibitive anions and cations the adsorption of both ions may be enhanced and the inhibitive efficiency greatly increased compared to solutions of the individual ions. Thus, synergistic inhibitive effects occur in such mixtures of anionic and cationic inhibitors . These synergistic effects are particularly well defined in solutions containing halide ions, I. Br , Cl", with other inhibitors such as quaternary ammonium cations , alkyl benzene pyridinium cations , and various types of amines . It seems likely that co-ordinate-bond interactions also play some part in these synergistic effects, particularly in the interaction of the halide ions with the metal surfaces and with some amines . 

A dative covalent bond, or co-ordinate bond, is a covalent bond in which both of the shared pair of electrons come from the same atom. 

Carbonic anhydrase is a metalloprotein with a co-ordinate bonded zinc atom immobilized at three histidine residues (His 94, His 96 and Hisl 19) close to the active site of the enzyme. The catalytic activity of the different isoenzymes varies but cytosolic CA II is notable for its very high turnover number (Kcat) of approximately 1.5 million reactions per second. 

You know that a covalent bond involves the sharing of a pair of electrons between two atoms each atom contributes one electron to the shared pair. In some cases, such as the hydronium ion, HsO", one atom contributes both of the electrons to the shared pair. The bond in these cases is called a co-ordinate covalent bond. In terms of the quantum mechanical model, a co-ordinate covalent bond forms when a filled atomic orbital overlaps with an empty atomic orbital. Once a co-ordinate bond is formed, it behaves in the same way as any other single covalent bond. The next Sample Problem involves a polyatomic ion with a co-ordinate covalent bond. 

Assign all bands and performed c a normal co-ordinate analysis. v(M—N) decreases in order Pt > Pd > Ni, suggesting same trend for strength of co-ordinate bonds. 

Preliminary data. Square-planar co-ordination. Bond lengths suggest extensive delocalization in cyanoacetylido-group. trans square-planar structure with bidentate thiol. Pt—As distance (2.354 A) indicated (4-4)n-bonding. 

Figure 7-1. The reaction of a nickel(n) dioximato complex with BF3 gives the nickel(n) complex of a new dianionic, macrocyclic ligand. Note that in the interests of clarity, throughout this chapter we use simple lines to represent co-ordinate bonds...

Of the very stable salts of the trans isomer only the alkali metal salts can be recrystallized from water. The commonest and best known is the potassium salt, known as Croft s salt18 or as red potassium chromioxalate. It crystallizes with two, three, or four molecules of water and is used extensively in tanning.19 Proof of the co-ordinative bonding of the oxalate ion to the chromium atom in this salt has been obtained by measurements of absorption spectra,20 and its structure has been determined by x-ray diffraction.21... 

In the following, a single bond is shown as I , co-ordinate bond I, double bond I , triple bond I , single-electron bond I, 3,4, 5, 6, 7, and 8-membered rings by III, IV, V, VI, VII, VIII, and a naphthalene ring by n ... 

Surface complexation reactions (surface hydrolysis, the formation of co-ordinative bonds at the surface with metals and with ligands). 

W) Restricted rotation about the bond between the surface intermediate and the active site (the co-ordination bond). This is also a steric effect, free... 

Scheme 3 H/D exchange of propene over MoS (ref. 31 ). The proportion of [1- H ]propene (route 1) is greater than [3- H ]propene (route 2) because rotation about the co-ordination bond (required by route 2) is restricted...

The restricted rotation effect has been invoked to explain the induction periods observed for some reactions. An induction period is typical of those reactions that require rotation about the co-ordination bond (e.g., but-l-ene isomerization). It is considered that the edge surface of M0S2 has a high rotational barrier for bulky groups and that the barrier is lowered during the induction period by changes in the conformations of the sites. 

The first compounds containing an intramolecular N — P co-ordinate bond (26) are formed when substituted fluorophosphoranes R1PF4 and 8-trimethylsiloxyquinolines react.295 On decreasing the electronegativity of the... 

Mordanting depends upon the fact that a number of metallic elements can act as acceptors to electron donors to form co-ordinate bonds. In the covalent bond each participant contributes one electron but co-ordination depends upon one atom or radical with one or more lone pairs of electrons, often referred to as ligands, donating one or more yf these lone pairs to the acceptor which has empty orbitals of appropriate energy available. In structural formulae it is not an uncommon practice to use the arrow —> pointing from the donor to the acceptor to signify co-ordination. 

Co-ordinate links are formed by some, or all, of the vacant 3d, 4s and 4p orbitals accepting lone pairs from donors, 6 and 4 pairs being commonest. The simplest illustration of mordant action is seen in the behaviour of the chromium ion towards alizarin. The chromium combines with three of the hydroxyl groups in the 1 position forming salt linkages, as does sodium with phenol in sodium phenate. The adjacent quinonoid oxygen then donates a pair of electrons yielding a co-ordinate bond. The complex therefore consists of three alizarin molecules attached to one chromium atom by three ionic, and also by three co-ordinate, bonds. This would... 

---