Ligand bonding density

Even if chain length is the key parameter, the ligand bonding density (usually above 2.5 umol/m ) may be very influential in determining overall stationary phase hydrophobicity. When the monolayer capacity, theoretically estimated by [L]j, increases as a result of increased bonding density, adsorption competitions are less operative and enhanced retention is expected. It should be noted that ligand bonding density can be calculated on the basis of the column carbon load and the total surface area of the column. 

Surface chemistry (type of bonded ligands bonding density etc.)... 

Bonded alkyl ligands are flexible, although their flexibility is significantly restricted by immobilization on the surface also, the higher the ligand bonding density, the less flexibility these chains have. 

All these effects are strongly dependent on the surface concentration of bonded ligands (bonding density), and despite the presence of a significant amount of unreacted silanols on the sihca surface, only the silanols accessible for analytes should be considered. 

There is an interesting paradox in transition-metal chemistry which we have mentioned earlier - namely, that low and high oxidation state complexes both tend towards a covalency in the metal-ligand bonding. Low oxidation state complexes are stabilized by r-acceptor ligands which remove electron density from the electron rich metal center. High oxidation state complexes are stabilized by r-donor ligands which donate additional electron density towards the electron deficient metal centre. 

Hertwig, R.H., Hrusak, J., Schroder, D., Koch, W. and Schwarz, H. (1995) The metal-ligand bond strengths in cationic gold(l) complexes. Application of approximate density functional theory. Chemical Physics Letters, 236, 194-200. 

Silica-based stationary phases with a chemically bonded ligand on the surface can be characterized by the carbon content (grams of carbon per 100 g of packing) and by the bonding density (micromols of ligand bonded/square meter of initial silica surface area). 

High bonding density is preferable for the majority of separations. The number of silanol groups not covered by the ligand is lower, exerting less influence on the separation, moreover, the hydrophobic ligand enhances the stability of the stationary phase at alkaline pH. 

There is some uncertainty whether this complex should be described as [Vm(bipy- )3] or as [V°(bipy)3], In fact, given that 2,2 -bipyridine can act either as a cr-donor or a n-acceptor, the metal-ligand bond in these complexes is constituted by a cr-bond between the lone pair of electrons of the nitrogen atom and an unoccupied s-orbital of the metal. Such electron donation, increasing the electron density on the metal, can in turn favour a back-bonding from the d-orbitals of the metal and the unoccupied rt -orbitals of the aromatic pyridine ring. In short, if the metal ion is in a high oxidation state pyridine will act as a a donor, whereas if the metal is in a low oxidation state pyridine will act as a n acceptor. 

The two eflPects above constitute what is called central field covalency since they aflFect both the a and the tt orbitals on the metal to the same extent. There is also, of course, symmetry restricted covalency which acts difiFerently on metal orbitals of diflFerent symmetries. This type of covalency shows up in optical absorption spectra as differences in the values of Ps and p -, as compared with 35. The first two s refer to transitions within a given symmetry subshell while 635 refers to transitions between the two subshells. This evidence of covalency almost of necessity forces one to admit the existence of chemical bonds since it is difficult to explain on a solely electrostatic model. The expansion of the metal orbitals can be caused either by backbonding to vacant ligand orbitals, or it may be a result of more or less extensive overlap of ligand electron density in the bond region. Whether or not this overlap density can properly be assigned metal 3d character is what we questioned above. At any... 

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