Co-ordination compounds

Complex ions used for electroplating are anions. The cathode tends to repel them, and their transport is entirely by diffusion. Conversely, the field near the cathode assists cation transport. Complex cyanides deserve some elaboration in view of their commercial importance. It is improbable that those used are covalent co-ordination compounds, and the covalent bond breaks too slowly to accommodate the speed of electrode reactions. The electronic structure of the cyanide ion is ... 

The precipitate redissolves in excess soluble cyanide, and the complex ion is probably an ion-dipole co-ordination compound, i.e. 

A substance which results in the chemical inactivation of a metal. The catalytic effect of heavy metals, mainly copper and manganese, on the oxidation of unsaturated compounds such as rubber, results in very rapid deterioration. Chelating agents convert the metal into a chelate co-ordination compound and thus render the metal inactive. The term sequestering agents has been applied to chelating agents but this infers that the metal has been removed and not merely inactivated. 

Theory The limit test for Iron is based on the reaction between iron and thioglycollic acid in a medium buffered with ammonium citrate to give a purple colour, which is subsequently compared with the standard colour obtained with a known amount of iron (0.04 mg of Fe). Ferrous thioglycollate is a co-ordination compound that attributes the purple colour besides thioglycollic acid converts the entire Fe3+ into Fe2+. The reactions involved may be expressed as follows ... 

Y = Cl dp +131 p.p.m.). Formation of six-co-ordinate compounds from spiro-oxyphosphoranes has been reported.41 The chemical shifts are to high field of the corresponding oxyphosphoranes. 

A discussion of metal complexes of biguanides has recently formed part of m extensive review (532) on co-ordination compounds, so that this subject need not be considered here in detail. As in the past, Indian laboratories continue to be the main source of publications on this aspect of biguanide chemistry (38, 193, 195, 281, 282, 300, 435, 527, 528, 529, 533, 586, 649, 650). 

Oxygenated Bronzes , P. Hagenmuller, Progr. Solid State Chem., 1971, 5, 71. The Photochemistry of Transition-metal Co-ordination Compounds , W. L. 

The extended series [PdX Crac-diars)] and [PdX2(meso-diars)] (X = Cl, Br, or I) have been synthesized using the method previously employed " for the chloro-complex. Their configurations were confirmed from n.m.r. and i.r. data. " Also reported are the five-co-ordinate compounds [PdXL]" [(X = Cl, Br, I, or SCN L = (71a)]. Trigonal-bipyramidal geometry was indicated... 

Frowein 3 shows by conductivity measurements and by change of colour of the first isomer that the monaquo-salt gradually passes over into the diaquo-salt, and this in turn into the triaquo-salt. The colour of the solution changes from blue to reddish violet and then to red. In all three eases the change is accompanied by increase in conductivity, and the conclusion is that the compounds contain no water outside the inner sphere—that is, they are purely co-ordinated compounds. 

Later it was found that a number of other metals which can give typical complex salts (Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni and Zn) can form explosive co-ordination compounds. 

Another interesting group of explosive co-ordination compounds is formed by the azides of ammines of general formulae... 

Van Nostrand, NY(1945), Chapter 4 Co ordination Compounds and Inorganic Stereochemistry 37)L.Pauling, "Nature of the Chemical Bond," Cornell Univ Press, Ithaca, NY(1945) SSjMellorV Modern Inorganic Chemistry Longmans, Green,London(1946), pp 841-6(Complex compds including some ammines) 39)A.K.Dey,Nature 158, 95(1946)... 

Double cyanides of trivalent vanadium have been prepared which show the properties of co-ordinated compounds, e.g. Ks[V(CN)g] compare with K3[Cr(CN)6] and K3[Fe(CN)6]. 

The double fluorides of vanadium and other double halogen salts in most cases can be regarded as in accordance with Werner s theory of co-ordinated compounds. The vanadium has a co-ordination number six. 

N. Bohr 3 discussed the fitness of configurations of the electrons in various atoms for the formation of ions. N. V. Sidgwiek has extended Bohr s theory to the electronic structure of atoms in co-ordination compounds. The subject was also discussed by J. D. M. Smith, and others at the Faraday Society s discussion on The Electronic Theory of Valency. A. Job discussed the catalyzed reaction NH3+HC1—NH4CI on the assumption that an unstable electronic system is formed as an intermediate product. 

The terms co-ordination compound, dative covalent compound, complex, or donor-acceptor compound are used synonymously for those compounds formed by the interaction of a molecule containing an empty orbital with one that possesses a filled orbital. The term complex was used to distinguish materials such as CoBr33NH3 from simple salts such as CoBr3. Notice the use of a dot to indicate some kind of a dative covalent interaction. The molecule or ion with the filled orbital is termed the donor and that with the empty orbital is called the acceptor. Examples are known in which only main group compounds are involved (Fig. 1-1). 

However, the most common examples of co-ordination compounds are those in which metals (and particularly transition metals) act as the acceptors. The metal may be in any oxidation state, and the donor-acceptor compounds may be neutral or charged (Fig. 1-2). 

Figure 1-1. The formation of co-ordination compounds may involve only main group donors and acceptors, as shown in the formation of the adduct between trimethylamine and boron trifluoride, and in the formation of the hexafluorophosphate anion.

Figure 1-2. Some examples of transition metal co-ordination compounds. In each case, the metal changes co-ordination number, but not oxidation state.

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