Stability: the adhesive clamp provides stability and aligns the "active zones" and postsynpatic elements in relation to one another. Certain adhesion molecules, such as N-cadherin, may also change conformation and, therefore, adhesivity in response to synaptic activity and modulate further synaptic responses.
Join LiveJournal Thermal factors affecting stability and durability of cemented metal 3. Some titles may also be available free of charge in our Open Access Theses and Dissertations Series, so please check there first. Access to abstracts is unrestricted. Jul 29, This type of bonding occurs between atoms in a compound in which one atom has lone pair of electron and the other has vacant orbital placed adjacent to each other. A compound with back bonding has pi-bonding character since it results after format.
However, the peak wavelengths and linewidths of their emission as well as their stability are still factors that can be further improved. Importance of microbiology in pharmacy. Overview of Transition Metal Complexes. The coordinate covalent or dative bond applies 2. Specific coordination number and geometries depend on metal and number of d-electrons 4.
HSAB theory useful.
The factors that affect the stability of complexes Metal carbonyl - Wikipedia Toppr makes learning effective for you. Start your free trial today!(Inorganic)Factor effecting stability of complex in hindi
What are the factors which affect the stability of chelates Ecology. Human EcologyAmos H. Cultural EcologyJulian H. Pharm4th year7th semester 40 CFR Ch. I Edition Environmental Protection Agency Factors Affecting Enzyme Activity Knowledge of basic enzyme kinetic theory is important in enzyme analysis in order both to understand the basic enzymatic mechanism and to select a method for enzyme analysis. The conditions selected to measure the activity of an enzyme would not be the same as those The greater the interaction, the stronger the affinity.
Avidity is perhaps a more informative measure of the overall stability or strength of the antibody-antigen complex. It is controlled by three major factors: antibody epitope affinity, the valence of both the antigen and antibody, and the structural arrangement of the interacting parts.
Variety of biological and physical factors. There are significant variations in response to irradiation associated with differences in species, age, and other biological factors, as well as the physical factors of dose, dose rate, or nature of the radiation.
However, the biological responses to radiation are not unique. They fall. Nomenclature m, hcoordination number and electron counting. Lecture 2: Why complexes form. Recap of molecular orbital theory. Metal carbonyls Structure and bonding - Chemistry. The increase in RBC mass does not begin until approximately 20 weeks, but then increases more rapidly than the PV until 28 weeks see Fig.
From 28 weeks to term, the RBC mass rises only slightly, but the slope of erythrocyte increase begins to exceed that of the PV a situation opposite that found earlier in pregnancy. Cleaning and Sanitizing 3 The factors that affect cleaning efficiency are: 1. Selecting the right cleaner for the job. Increasing the temperature of the cleaning solution so that the strength of the bond between the soil and surface is decreased, the viscosity is decreased, and.One of the most striking characteristics of transition-metal complexes is the wide range of colors they exhibit.
In this section, we describe crystal field theory CFTa bonding model that explains many important properties of transition-metal complexes, including their colors, magnetism, structures, stability, and reactivity.
The central assumption of CFT is that metal—ligand interactions are purely electrostatic in nature. Even though this assumption is clearly not valid for many complexes, such as those that contain neutral ligands like CO, CFT enables chemists to explain many of the properties of transition-metal complexes with a reasonable degree of accuracy.
The Learning Objective of this Module is to understand how crystal field theory explains the electronic structures and colors of metal complexes. We will focus on the application of CFT to octahedral complexes, which are by far the most common and the easiest to visualize. Other common structures, such as square planar complexes, can be treated as a distortion of the octahedral model. According to CFT, an octahedral metal complex forms because of the electrostatic interaction of a positively charged metal ion with six negatively charged ligands or with the negative ends of dipoles associated with the six ligands.
In addition, the ligands interact with one other electrostatically. As you learned in our discussion of the valence-shell electron-pair repulsion VSEPR model, the lowest-energy arrangement of six identical negative charges is an octahedron, which minimizes repulsive interactions between the ligands.
We begin by considering how the energies of the d orbitals of a transition-metal ion are affected by an octahedral arrangement of six negative charges. Recall that the five d orbitals are initially degenerate have the same energy. Placing the six negative charges at the vertices of an octahedron does not change the average energy of the d orbitals, but it does remove their degeneracy: the five d orbitals split into two groups whose energies depend on their orientations.
Consequently, the energy of an electron in these two orbitals collectively labeled the e g orbitals will be greater than it will be for a spherical distribution of negative charge because of increased electrostatic repulsions. The energy of an electron in any of these three orbitals is lower than the energy for a spherical distribution of negative charge.
As we shall see, the magnitude of the splitting depends on the charge on the metal ion, the position of the metal in the periodic table, and the nature of the ligands. It is important to note that the splitting of the d orbitals in a crystal field does not change the total energy of the five d orbitals: the two e g orbitals increase in energy by 0.
Thus the total change in energy is. Thus far, we have considered only the effect of repulsive electrostatic interactions between electrons in the d orbitals and the six negatively charged ligands, which increases the total energy of the system and splits the d orbitals. As shown in Figure When we reach the d 4 configuration, there are two possible choices for the fourth electron: it can occupy either one of the empty e g orbitals or one of the singly occupied t 2g orbitals.
Recall that placing an electron in an already occupied orbital results in electrostatic repulsions that increase the energy of the system; this increase in energy is called the spin-pairing energy P. In contrast, only one arrangement of d electrons is possible for metal ions with d 8 —d 10 electron configurations. Source of data: Duward F.Copy embed code:.
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Introduction to Crystal Field Theory
By: rajkiran. Anupama M. Pharm [Q. Objective of stability studies in Preformulation: Objective of stability studies in Preformulation The objective of stability study is to determine the shelf life, namely the time period of storage at a specified condition within which the drug product still meets its established specifications. Stability is an essential factor of quality, safety and efficacy of a drug product. A drug product, which is not of sufficient stability, can result in changes in physical features like hardness, dissolution rate, phase separation etc as well as chemical characteristics formation of high risk decomposition substances.
The Chemical stability of drug is of great importance since it becomes less effective as it undergoes degradation. Also drug decomposition may yield toxic by products that are harmful to the patient. Microbiological instability of a sterile drug product could also be hazardous. PowerPoint Presentation: Stability evaluation of drug substance or drug product is the key to drug quality as it determines the efficacy of any drug or dosage form.
Stability assessment of drug products and drug substances are mandated by regulatory agencies across the globe. In fact, stability-testing issues are responsible for a number of audit findings by regulatory agencies.
Stability testing problems are regularly cited in warning letters and sometimes results in costly product recall. Importance of Stability Study: Importance of Stability Study Stability testing provides evidence that the quality of drug substance or drug product changes with time under the influence of various environmental conditions such as temperature, relative humidity etc. The stability study consists of a series of tests in order to obtain an assurance of stability of a drug product, namely maintenance of the drug product packed in it specified packaging material and stored in the established storage condition within the determined time period.
PowerPoint Presentation: Wherever possible the commercial pharmaceutical product should have a shelf life of 3 years. Drug Degradation : Drug Degradation Drug degradation occurs by four main processes : Hydrolysis Oxidation Photolysis Trace metal catalysis Various factors affecting drug stability : Various factors affecting drug stability The various factors affecting drug stability are as follows; Temperature Order of reaction Hydrolysis The influence of pH Solvolysis Oxidation Chelating agents Photolysis Solid state stability Hygroscopicity 1.
Temperature : 1. Temperature Thermal effects are superimposed on all 4 degradation chemical processes. Often the increase in reaction rate with temperature follows an Arrhenius type relationship: a plot of the log of the rate of reaction against the reciprocal of absolute temperature yields a straight line.
This forms the basis of many accelerated stability tests. This is not easily detected and would invariably lead to erroneous conclusions based on elevated temperature data to predict shelf lives at room temperature or under refrigeration.Instability Constants of Complex Compounds pp Cite as. The stability of a complex particle ion or molecule in solution is determined by the nature of the central atom and the ligands. The most important characteristics of the central atom, determining the stability of the complex compound, are the degree of oxidation charge on the central ion in the case of ionic complexesthe dimensions, and the electronic structure.
In the case of complexes with monatomic ligands, stability is dependent on the same characteristics in the ligand charge, radius and electronic structure. The strength of binding for ligand molecules and polyatomic ions depends, in addition, on the nature of the atoms directly linked to the central atom, and on the particular features of the structure of the ligand molecule orion.
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Cite chapter How to cite? ENW EndNote. Buy options.Correspondingly, a polydentate ligand is a chelating agent, and complexes that contain polydentate ligands are called chelate complexes. Experimentally, it is observed that metal complexes of polydentate ligands are significantly more stable than the corresponding complexes of chemically similar monodentate ligands; this increase in stability is called the chelate effect.
Chelate complexes are more stable than the analogous complexes with monodentate ligands. The stability of a chelate complex depends on the size of the chelate rings.
For ligands with a flexible organic backbone like ethylenediamine, complexes that contain five-membered chelate rings, which have almost no strain, are significantly more stable than complexes with six-membered chelate rings, which are in turn much more stable than complexes with four- or seven-membered rings. A Determine the relative basicity of the ligands to identify the most stable complexes. B Decide whether any complexes are further stabilized by a chelate effect and arrange the complexes in order of increasing stability.
Consequently, we must focus on the properties of the ligands to determine the stabilities of the complexes. Because the stability of a metal complex increases as the basicity of the ligands increases, we need to determine the relative basicity of the four ligands. Our earlier discussion of acid—base properties suggests that ammonia and ethylenediamine, with nitrogen donor atoms, are the most basic ligands.
The fluoride ion is a stronger base it has a higher charge-to-radius ratio than chloride, so the order of stability expected due to ligand basicity is. Consequently, the likely order of increasing stability is.
The chelate effect can be seen by comparing the reaction of a chelating ligand and a metal ion with the corresponding reaction involving comparable monodentate ligands. It has been known for many years that a comparison of this type always shows that the complex resulting from coordination with the chelating ligand is much more thermodynamically stable.
This can be seen by looking at the values for adding two monodentates compared with adding one bidentate, or adding four monodentates compared to two bidentates, or adding six monodentates compared to three bidentates. The Chelate Effect is that complexes resulting from coordination with the chelating ligand is much more thermodynamically stable than complexes with non-chelating ligands.
A number of points should be highlighted from the formation constants in Table E4. For many years, these numbers have been incorrectly recorded in textbooks. For example, the third edition of "Basic Inorganic Chemistry" by F. Cotton, G.These complexes contain a central atom or ion, often a transition metal, and a cluster of ions or neutral molecules surrounding it.
Many complexes are relatively unreactive species remaining unchanged throughout a sequence of chemical or physical operations and can often be isolated as stable solids or liquid compounds.
Other complexes have a much more transient existence and may exist only in solution or be highly reactive and easily converted to other species. All metals form complexes, although the extent of formation and nature of these depend very largely on the electronic structure of the metal.
The concept of a metal complex originated in the work of Alfred Wernerwho in was awarded the first Nobel Prize in Inorganic chemistry. A description of his life and the influence his work played in the development of coordination chemistry is given by G. Complexes may be non-ionic neutral or cationic or anionic, depending on the charges carried by the central metal ion and the coordinated groups. The total number of points of attachment to the central element is termed the coordination number and this can vary from 2 to greater than 12, but is usually 6.
The term ligand ligare [Latin], to bind was first used by Alfred Stock in in relation to silicon chemistry. The first use of the term in a British journal was by H. Irving and R. Williams in Nature,in their paper describing what is now called the Irving-Williams series. For a fascinating review of the origin and dissemination of the term 'ligand' in chemistry see: W.
Brock, K. A Jensen, C. Jorgensen and G. Kauffman, Polyhedron2, Ligands can be further characterised as monodentate, bidentate, tridentate etc. The term chelate was first applied in by Sir Gilbert T. Morgan and H. Drew [ J. Metal complexation is of widespread interest. It is studied not only by inorganic chemists, but by physical and organic chemists and by biochemists, pharmacologists, molecular biologists and environmentalists.
The addition of the four ammine groups to copper shows a pattern found for most formation constants, in that the successive stability constants decrease.
In this case, the four constants are:. It is usual to represent the metal-binding process by a series of stepwise equilibria. This can lead to stability constants that may vary numerically from hundreds to enormous values such as 10 35 and more. That is ,,, For this reason it is useful to use logarithms, since log K is directly proportional to the free energy of the reaction. It has been known for many years that a comparison of this type always shows that the complex resulting from coordination with the chelating ligand is much more thermodynamically stable.
This can be seen by looking at the values for adding two monodentates compared with adding one bidentate, or adding four monodentates compared to two bidentates, or adding six monodentates compared to three bidentates. The entropy term found is still much larger than for ordinary reactions involving substitution of a non-chelating ligand at a metal ion.The antigen-antibody reaction is widely used in laboratory diagnostics, including immunohaematology.
It is a reversible chemical reaction:. According to quantum mechanics, all chemical bonds are based on electrostatic forces. A list of types of weak bonds is shown in table Itogether with their strength energy. Van der Waals forces are the weakest, but they are able to attract all kinds of molecules. Hydrogen or ion-dipole bonds require oppositely charged atoms. Hydrogen bonds are very important in aqueous solutions because water easily forms strong hydrogen bonds.
Moreover, the kinetic energy is not distributed uniformly. Therefore, even the strongest single weak interaction has an ephemeral life of a fraction of a second at physiological temperatures.
For a stable binding, several weak bonds must be present contemporaneously and this requires steric complementarity between the molecules 1.
Types of weak interactions. Their strength is equal to the change in free energy that takes place during binding. The specific binding between the antigenic determinant on the red cell epitope and the antigen-combining site on the immunoglobulin molecule paratope involves very small portions of the molecules 2comprising just a few amino acids and a surface area between 0.
Specific binding must overcome an overall repulsion between the two molecules. As presently understood, events at a molecular level occur as follows 2 : when the epitope and the paratope casually come to a distance of several nanometres, they are attracted by long-range forces, such as ionic and hydrophobic bonds.
These attractive forces locally overcome the hydration energies of the two molecules, water molecules are expelled and the epitope and the paratope approach each other more closely. At this distance, van der Waals forces prevail, but ionic groups still play a role. At that point, the overall strength of the binding depends on the goodness of fit between the two surfaces van der Waals forces are very short-range and decay with the seventh power of the interatomic distance 3 and their total contact area.
Stability constants of complexes
At the beginning, a chemical reaction proceeds predominantly in one direction, but the reverse rate progressively increases until the forward and reverse speeds are equal. At this point, the reaction is said to have reached its equilibrium.
K eq is called the equilibrium constant and is equal to the ratio between the association k a and the dissociation k d rate constants. In order to improve antibody detection, the ratio between bound and free antigen [ c o m p l e x ] [ a n t i g e n ] should be increased as much as possible. Rearranging 1 :. The greater the strength of the bond, the higher its equilibrium constant.
Table II shows a list of values of equilibrium constants and the corresponding free energy changes. In order for a reaction to proceed spontaneously, the free energy change must be negative. Hydrogen bonds are exothermic 3 and the heat derives from the energy released with the formation of the chemical bonds of the product. Thus, hydrogen bonds predominantly form because of the enthalpic factor. In contrast, hydrophobic bonds are endothermic and probably driven by the entropic factor 3.