The Coordinate Bond

Compounds whose molecular compositions are multiples of a simple stoichiometry are polymers, stricdy, only if they are formed by repetition of the simplest unit. However, the name polymerization isomerism is applied rather loosely to cases where the same stoichiometry is retained but where the molecular arrangements are different. The stoichiometry PtCl2(NH3)2 applies to the 3 known compounds, [Pt(NH3)4][PtCU], [Pt(NH3)4][PtCl3(NH3)]2, and [PtCl(NH3)3]2[PtCl4] (in addition to the cis and trans isomers of monomeric [PtCl2(NH3)2]). There are actually 7 known compounds with the stoichiometry Co(NH3)3(N02)3. Again it is clear that considerable differences are to be expected in the chemical properties and in physical properties such as conductivity. 

Should a ligand exist in different isomeric forms then of course the corresponding complexes will also be isomers, often described as ligand isomers . In [CoCl(en)2(NH2C6H4Me)]Cl2, for instance, the toluidine may be of the o-, m- or p- form. 

The concept of the coordinate bond as an interaction between a cation and an ion or 

In his valence bond theory (VB), L. Pauling extended the idea of electron-pair donation by considering the orbitals of the metal which would be needed to accommodate them, and the stereochemical consequences of their hybridization (1931-3). He was thereby able to account for much that was known in the 1930s about the stereochemistry and kinetic behaviour of complexes, and demonstrated the diagnostic value of measuring their magnetic properties. Unfortunately the theory offers no satisfactory explanation of spectroscopic properties and so was 

Fioois, Chap. 7 in Comprehensive Coordination Chemistry, Vol. 1, pp. 213-80, Pergamon Press, Oxford, 1987. S. F. A. Kettle, Physical Inorganic Chemistry, A 

Werner s coordination theory, with its concept of secondary valence, provides an adequate explanation for the existence of such complexes as [Co(NH3)6]Cl3-Some properties and the stereochemistry of these complexes are also explained by the theory, which remains the real foundation of coordination chemistry. Since Werner s work predated by about twenty years our present electronic concept of the atom, his theory does not describe in modem terms the nature of the secondary valence or, as it is now called, the coordinate bond. Three theories currently used to describe the nature of bonding in metal complexes are (1) valence bond theory (VBT), (2) crystal field theory (CFT), and (3) molecular orbital theory (MOT). We shall first describe the contributions of G. N. Lewis and N. V. Sidgwick to the theory of chemical bonding. 

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We are interested in the H-H bond length, so we specify the coordinate bonding those two atoms to the AddRedundant option so that its value will be included in the printout of the optimized structure (the Si-H bond lengths will be included by default). 

It is believed [1135,1136] that the decomposition of metal complexes of salicyaldoxime and related ligands is not initiated by scission of the coordination bond M—L, but by cleavage of another bond (L—L) in the chelate ring which has been weakened on M—L bond formation. Decomposition temperatures and values of E, measured by several non-isothermal methods were obtained for the compounds M(L—L)2 where M = Cu(II), Ni(II) or Co(II) and (L—L) = salicylaldoxime. There was parallel behaviour between the thermal stability of the solid and of the complex in solution, i.e. Co < Ni < Cu. A similar parallel did not occur when (L—L) = 2-indolecarboxylic acid, and reasons for the difference are discussed... 

Closed geometries of reaction points are induced by the coordination bonds. 

Fig. 3a,b. Template cyclization reactions of a crown ethers and b CPOs. The coordination bonds are illustrated by black arrows. In the crown ether synthesis, ethylene glycols coordinate toward the metal acting as the template (normal template reaction) however, the template coordinates to the incorporated metals of porphyrin In CPO synthesis (inverse-template reaction)... 

Attempts to realize enzymatic reactions have been reported over the past four decades in the context of host-guest chemistry, presently a well-established research field. In the field of CPOs, much attention has been paid to identical research objectives. The host-guest chemistry based on CPOs holds a special position, because specific selectivity and reactivity will be achieved using the coordination-bond-forming reactions between the substrate and the incorporated metals in the porphyrins, as well as the redox reaction associated with the porphyrin s rr-electron system. 

Smith (91) reported an X-ray crystal structure of a zinc porphyrin polymer (77, Fig. 32) where, unusually, the coordination bond is between a nitro group and the zinc center. The tetranitroporphyrin is highly substituted, and the resulting steric hindrance causes the macrocycle to be noticeably distorted. Adjacent porphyrin planes in the polymer are almost orthogonal. However, there is no evidence of polymerization in solution, and the nitro-zinc interaction is probably too weak to maintain this structure outside the solid state. 

In the case of r)2-coordination of the exocyclic C=C bond, it becomes substantially elongated compared with the double bond of free alkenes, as a result of back donation from the metal to the 7t orbitals of the double bond. For instance, in complex 17b the coordinated bond length is 1.437 A (see Fig. 3.2).18 This is also reflected in the loss of planarity around the quaternary exocyclic carbon, the methylenic carbon being bent out of the ring plane by 10.78°.18 Similar structural features were also observed with other P2Pd conjugated olefin complexes.39... 

The formation of a coordinate bond is the result of the donation and acceptance of a pair of electrons. This in itself suggests that if a specific electron donor interacts with a series of metal ions (electron acceptors) there will be some variation in the stability of the coordinate bonds depending on the acidity of the metal ion. Conversely, if a specific metal ion is considered, there will be a difference in stability of the complexes formed with a series of electron pair donors (ligands). In fact, there are several factors that affect the stability of complexes formed between metal ions and ligands, and some of them will now be described. 

The length of the coordinative bond (2.86 A) corresponds to a normal Sb-Sb single bond. The coordination geometries of the donor or acceptor antimony atoms are distorted tetrahedral for the former and pseudo trigonal bipyramidal for the latter with the iodine atoms in axial (I-Sb-I 169.71°) and the lone pair, the Me3Sb and the methyl group in equatorial positions. 

Oudeman law physchem The law that the molecular rotations of the various salts of an acid or base tend toward an identical limiting value as the concentration of the solution is reduced to zero. od-a-man, I6 ) outer orbital complex phys chem A metal coordination compound in which the d orbital used in forming the coordinate bond is at the same energy level as the s and p orbitals. aud-or 6rb-od-3l kam.pleks) overall stability constant analychem Reaction equilibrium constant for the reaction... 

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