Co-ordinate link

Also an atom, molecule, or ion that is electron deficient and which can form a co-ordinate link with an electron donor. Thus in the complex ion [Co(NH3)eP the cobalt(Ill) ion is an acceptor and the ammonia the electron donor. t-acceptors are molecules or atoms which accept electrons into n, p or d orbitals. 

Other compounds containing lone pairs of electrons readily form co-ordinate links and in each case a change in spatial configuration accompanies the bond formation. The oxygen atom in dimethyl ether, CHj—O—CHj, has two lone pairs of electrons and is able to donate one pair to, for example, boron trichloride ... 

Sugden at first thought that the co-ordinate link had a specific parachor, and was able in this way to confirm its presence in many compounds. Mumford and Philips,7 however, showed that many parachor values used by Sugden were unsatisfactory, and the latter, when it was generally recognised that the co-ordinate link behaves like an ordinary covalent single bond, gave it the zero parachor value assumed as standard for the latter, and issued a new list of parachors. 

Mercury forms direct covalent as well as co-ordinate links with nitrogen. Mercury(II) chloride reacts with gaseous ammonia to give fusible white precipitate ... 

When ammonia reacts with Hg ( 104)2, a compound of much greater ionic character than HgClg is formed, a complex tetra-ammine in which there are co-ordinate links between nitrogen and mercury ... 

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... 

Before examining the restdts of some of these investigations, it would be well to consider briefly the nature of the co-ordinate link in different types of donor-acceptor compounds. This has been discussed in terms of the molecular orbital theory by MulUken. In the simplest case, the D A linkage is regarded as a resonance hybrid built up from a Uo-bond structure, (D, A), and an electron... 

Walsh, A.D. (1947) 21. Co-ordinate links formed by bonding electrons a suggestion regarding the stmcture of diborane. J. Chem. Soc.(Resumed), 89-92. 

Chatt J (1950) The co-ordinate link in chemistry. Nature (London) 165 637... 

Chatt J, Willians AA (1951) The nature of the co-ordinate link IV. Complex formation by phosphorus trifluoride. J Chem Soc 3061-3078... 

The lithium atom in hypothetical complexes RLiLs would be carrying too high a negative charge, and the molecule would be in conflict with the electroneutrality principle. The tetramethylethylenediamine (TMED) complexes of butyl-lithium and some other organolithium compounds have already been mentioned in connection with metalation reactions (p 49). The effect of forming two Li—N co-ordinate links is to make the carbon atom boimd to lithium very strongly nucleophilic. 

It is postulated for ligands like ethylene that bonding with the metal involves both a simple o-co-ordinate link and also a back-donation of the metal electrons to the ligand. The ordinate bond is the reason for the classification of ethylene as a 2-electron ligand. 

Chatt J, Hitchcock PB, Pidcock A, Warrens CP, Dixon KR (1984) The nature of the co-ordinate link. Part 11. Synthesis and phosphorus-31 nuclear magnetic resonance spectroscopy of platinum and palladium complexes containing side-bonded ( )-diphenyldiphosphene. X-Ray crystal and molecular structures of [Pd (i )-PhP = PPh (Ph2PCH2CH2PPh2)] and [Pd [( )-PhP = PPh][W(CO)5]2 (Ph2PCH2CH2PPh2)]. J Chem Soc Dalton Trans 2237-2244... 

---