Dative bonding complex ions

A compound which is formed in a complexation reaction between two or more species which are capable of independent existence. Most complexes of analytical importance involve dative bonds between Lewis bases and metal ions. 

The major uses of non-ionizing solvents in chemical analysis are twofold. They may be used simply to provide media for the dissolution and reaction of covalent materials, or they may play a more active part in a chemical process. For example, oxygen-containing organic solvents can be used to effect the solvent extraction of metal ions from acid aqueous solutions the lone pair of electrons possessed by the oxygen atom forming a dative bond with the proton followed by the extraction of the metal ion as an association complex. 

The cations 18a and 19a are predicted to be distorted, the bridge atom (7) being tilted towards the double bond of 18 or one double bond of 19 and the hydrogen atom tilted away this is exactly what would be expected if the ions are it complexes 38) in which one double bond forms a dative bond to the 7-carbon atom. It is perhaps surprising that the saturated cation (18a) shows a similar deformation presumably I-strain at the 7-position favours a 7r-complex-like structure in which a C-C a bond acts as donor. 

The Re—Cl bonds in the complex may be retarded as dative bonds between the i Cl ligands and Re3 (d4) ions. The eight d electrons from the two metals will occupy... 

Hydride abstraction from alkylamines forms the corresponding imi-nium ions, whose coordination to transition metals gives either a ir-complex or cr-bonded three-membered ring (Scheme 15) (26). Ligation of the cationic dehydro amines to Rh is aided by substantial electron donation from the metal to the electron-deficient carbon atom to produce the Rh(IH) complex with a covalent C—Rh bond and an N—Rh dative bond, consistent with the long C—N bond (1.467 A) and the small H— C( 1)-—N—C(2) dihedral angle (124.6°) as well as the noncoplanarity of the CH2—CH bond and a possible CH—NH2 plane seen in the allylam-ine oxidative addition product (24). 

The complex ion octachlorodirhenate(III) ion has short Re Re distance and eclipsed configuration of the chlorine atom Re(III) is a d4 species. The Re Cl bonds may be considered tcrbe dative bonds from Cl ions to Re3+. The formation of one sigma bond, two pi bonds and one delta bond causes the pairing of four electrons in quadruple bond, hence the complex is diamagnetic. 

The ditelluride complex is a centrosymmetric dimer [177] and the Yb is tetrahedrally bonded to two Cp ligands and to the Te - ion. The structure of Se complex [178] has a linear Se-Yb-Se bridge (170.1°) with two Cp Yb units mutually staggered and the compound is seven coordinate. The structure of Ce complex [180] shows a rare example of Ce-S dative bond with Ce-S distances of 3.058 and 3.072 A. 

The classical rules of valency do not apply for complex ions. To explain the particularities of chemical bonding in complex ions, various theories have been developed. As early as 1893, A. Werner suggested that, apart from normal valencies, elements possess secondary valencies which are used when complex ions are formed. He attributed directions to these secondary valencies, and thereby could explain the existence of stereoisomers, which were prepared in great numbers at that time. Later G. N. Lewis (1916), when describing his theory of chemical bonds based on the formation of electron pairs, explained the formation of complexes by the donation of a whole electron pair by an atom of the ligand to the central atom. This so-called dative bond is sometimes denoted by an arrow, showing the direction of donation of electrons. In the structural formula of the tetramminecuprate(II) ion... 

The Re—Cl bonds in the complex may be re rded as dative bonds between the Q l nds and ( / ) ions. The eight J electrons from the two metals will occupy the cr bonding, two v bonding, and one 8 wnding orbitals to form the quadruple bond hence the complex is diamagnetic. The model succ ess(ully accounts fbr the strength of the bond, the short Re—Re distance, and the eclipsed configuration. 

Lithium iodide forms a solid complex with ammonia, Li(NH3)4l, but the related hydrate, alcoholate and amine complexes are less stable. These complexes presumably involve ion-dipole bonds (p. 115), the nitrogen lone pairs surrounding the Li+ some covalent character (dative bonding) is also permissible if s and p orbitals on the Li are invoked. The chloride, bromide and iodide of lithium are much more soluble in alcohol and ether than those of the other alkali metals, but this is not always a reliable indication of covalent character. The property is employed in separating lithium from sodium. 

In the presence of bulky X ligands, a facile methyl halide elimination reaction is observed (Eq. 2) [3]. In this elimination the siliconium ion complex 2, with its two N—>Si dative bonds, is converted into a neutral pentacoordinate complex 3, with only one remaining dative bond (Fig. 1, Table 1). The reaction is probably driven by partial release of steric interaction, caused by the removal of one of the A-methyl groups. This is indicated by a decrease in elimination rate in the presence of less bulky ligands, cyclohexyl and isobutyl, and the failure to observe elimination when X = methyl. The reactivity order of the halide ions follows their nucleophilicities F > Br > CF, while the less nucleophilic ttiflate ion does not react at all. 

Coordination complexes comprise a central metal atom or ion coordinated by dative bonds to a number of ligands in a symmetrical arrangement. Linear,... 

From the valence bond point of view, formation of a complex involves reaction between Lewis bases (ligands) and a Lewis acid (metal or metal ion) with the formation of coordinate covalent (or dative) bonds between them. The model utilizes hybridization of metal s, p, and d valence orbitals to account for the observed structures and magnetic properties of complexes. For example, complexes of Pd(ll) and Pt(Il) are usually four-coordinate, square planar, and diamagnetic, and this arrangement is often found for Ni(II) complexes as well. Inasmuch as the free ion in the ground state in each case is paramagnetic (d, F), the bonding picture has to... 

Figure 7.1 Complex formed between hexadentate EDTA anion and an octahedral six-coordinate metal ion". The six dative covalent bonds point towards the comers of a regular octahedron. The complex ion formed will, in all probability, be water soluble because of the residual negative charge left on the complex, i.e. (4-n) , becoming aquated...

While nonbonded electron pairs in molecules do not enter into covalent bonding in the usual sense, they may exhibit a secondary kind of valency by being transferred into vacant molecular orbitals in suitable acceptor molecules. This results in the transformation of a coordination complex in which the bond formed between the electron-pair donor and the acceptor is said to be a coordinate covalent or dative bond. Brpnsted basicity is the simplest example of coordinate covalent bond formation. A Brpnsted base donates a pair of nonbonded electrons to a vacant Is orbital of a hydrogen ion to form the conjugate acid. The o-bond formed between the base and the hydrogen ion results in the loss of identity of the nonbonded pair previously localized on the base. The formation of coordination complexes has significance in the interpretation of spectra of compounds having nonbonded electron pairs. 

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