CHS 4102 Inorganic Chemistry: Structure and Reactivity Course Contents • Structure and Reactivity of Inorganic Molecules: 8-10 L (TKP) (Chemical bonding, MO) • Bonding and Electronic Structure of Coordination Complexes: 5 L (TKP) • Kinetics and Mechanism in Coordination Chemistry: 4 L (RM) • Bioinorganic Chemistry: 6 (TKP) 11/4/2020 Structure and Reactivity of Inorganic Compounds Atoms Molecules Structure Symmetry Chemical bond Molecular Properties Bulk Properties Molecular Spectroscopy Interactions Reactivity What is a chemical bond? “SOMETIMES IT SEEMS to me that a bond between two atoms has become so real, so tangible, so friendly, that I can almost see it. Then I awake with a little shock, for a chemical bond is not a real thing. It does not exist. No one has ever seen one. No one ever can. It is a figment of our own imagination.” - C. A. Coulson (1910-1974), an English theoretical chemist who played a central role in the development of quantum theories of chemical bonding. It is more useful to regard a chemical bond as an effect that causes certain atoms to join together to form enduring structures that have unique physical and chemical properties. So although the "chemical bond" is no more than a convenient fiction, chemical bonding, which leads to the near-infinity of substances (31 million in mid-2007), lies at the very core of chemistry. Chemical bonding occurs when one or more electrons are simultaneously attracted to two nuclei This is the most important fact about chemical bonding, but it is not of itself a workable theory of bonding because it does not describe the conditions under which bonding occurs, nor does it make useful predictions about the properties of the bonded atoms ….views of what constitutes chemical bonding is still evolving "buckyball-and-mitt" synthesized in 2007 by Andrzej Sygula The buckyball C60 resides in the C60H28"buckybowl". What is a molecule? A molecule is an aggregate of atoms that possesses distinctive observable properties The definition written above is an operational one; that is, it depends on our ability to observe and measure the molecule's properties like bond energies, bond parameters, bond stretching and infrared absorption etc…. Structure ….the structure of a molecule is specified by the identity of its constituent atoms and the sequence in which they are joined together, that is, by the bonding connectivity. This, in turn, defines the bonding geometry— the spatial relationship between the bonded atoms. If a particular arrangement of atoms is too unstable to reveal its properties at any achievable temperature, then it does not qualify to be called a molecule. Depicting chemical structures Ordinary structural formula, showing connectivity only. Space-filling model, Ball-and-stick model, showing relative sizes of showing the "chemical the atoms and general bonds" and bonding shape of the molecule, geometry, but with the but not all atoms visible. individual atoms No obvious "chemical unrealisticly separated. bonds" here! Finally, we get to see one! In 2009, IBM scientists in Switzerland succeeded in imaging a real molecule, using a technique known as Atomic Force Microscopy in which an atoms-thin metallic probe is drawn ever-soslightly above the surface of an immobilized pentacene molecule cooled to nearly absolute zero. In order to improve the image quality, a molecule of carbon monoxide was placed on the end of the probe. What is actually being imaged is the surface of the electron clouds of the molecule, which consists of five fused hexagonal rings of carbon atoms with hydrogens on its periphery. The tiny bumps that correspond to these hydrogen atom attest to the remarkable resolution of this experiment Concept Map Models of Chemical Bonding Why do atoms bind together— sometimes? The answer to this question would ideally be a simple, easily understood theory that would not only explain why atoms bind together to form molecules, but would also predict the three-dimensional structures of the resulting compounds as well as the energies and other properties of the bonds themselves. No one theory exists. ….better to start by developing a model. A scientific model is something like a theory in that it should be able to explain observations and to make useful predictions Models of Chemical Bonding Why: • Not all pairs of atoms can form stable species? • Different elements can form different numbers of ‘bonds’ (this is expressed as "combining power" or "valence".)? • The geometric arrangement of the bonds ("bonding geometry") around a given kind of atom is a property of the element? Some early views of chemical bonding Models of Chemical Bonding Classical Models of Bonding Quantum Mechanical Models (i) (ii) (iii) (iv) (v) (i) The Hybrid orbital Models (ii) The Molecular Orbital Model (iii) The Quantum Tunneling Model The ionic Model Coulombic Model Shared Electron Model Polar Covalent Model VSEPR Model Coordination complexes In Chemistry, we can call the various explanations "models" and get away with it even if they are demonstrably wrong, as long as we find them useful……. Molecular Structure The geometric structure of a molecule is described by atom location. Only atomic nuclei, not electrons, can be located. Lone (unshared) electrons and electron pairs affect structure of the molecule. Molecular Geometry Valence Shell Electron Pair Repulsion (VSEPR) model focuses on the bonding and nonbonding electron pairs present in the outermost (“valence”) shell of an atom that connects with two or more other atoms. Ronald Gillespie and Ronald Sydney Nyholm The shape of the molecule is defined by the coordination geometry Digonal and trigonal coordination Linear molecules (AX2 type) Trigonal molecules (AX3 type) Tetrahedral coordination It is interesting to note that the tetrahedral coordination of carbon in most of its organic compounds was worked out in the nineteenth century on purely geometrical grounds and chemical evidence, long before direct methods of determining molecular shapes were developed Tetrahedrally-coordinated carbon chains Tetrahedral coordination with lone pairs The bonding geometry will not be tetrahedral when the valence shell of the central atom contains nonbonding electrons, however. The reason is that the nonbonding electrons are also in orbitals that occupy space and repel the other orbitals The water molecule: AX2E2 A nonbonding orbital has no atomic nucleus at its far end to draw the electron cloud toward it, the charge in such an orbital will be concentrated closer to the central atom. As a consequence, nonbonding orbitals exert more repulsion on other orbitals than do bonding orbitals Although the water molecule is electrically neutral, it is not electrically uniform; the non-bonding electrons create a higher concentration of negative charge (blue color) at the oxygen end, making the hydrogen side relatively positive (red). This charge unbalance gives rise to many of the so-called anomalous properties of water Ammonia: AX3E Central atoms with five bonds The shape of AX5 type molecules is a trigonal bipyramid. Equatorial and axial atoms have different geometrical relationships to their neighbors, and thus differ slightly in their chemical behavior. In 5-coordinated molecules containing lone pairs, these non-bonding orbitals which are closer to the central atom and thus more likely to be repelled by other orbitals, will preferentially reside in the equatorial plane. This will place them at 90° angles with respect to no more than two axially-oriented bonding orbitals AX4E type molecules Trigonal Bipyramidal Lone Electron Pair Position 90º 120º 180º : equatorial 2 2 0 axial 3 0 1 Maximizes the angular separation between lone pairs and other electron pairs. : The lone pairs always occur in the equatorial plane. Octahedral coordination Just as four electron pairs experience the minimum repulsion when they are directed toward the corners of a tetrahedron, six electron pairs will try to point toward the corners of an octahedron Octahedral coordination with lone pairs Number of Lone Electron Pairs Summary of Observed Geometries Atoms Bound to the Central Atom 1 0 linear 2 linear 3 trigonal planar 4 5 trigonal tetrahedral bipyramidal 1 linear bent trigonal (@120º) pyramidal sawhorse, see-saw 2 linear bent T-shaped (@109º) square planar 3 linear linear square pyramidal 6 octahedral Drawback: While VSEPR theory is quite good at predicting the general shapes of most molecules, it cannot yield exact details. For example, it does not explain why the bond angle in H2O is 104.5°, but that in H2S is about 90°. This is not surprising, considering that the emphasis is on electronic repulsions, without regard to the detailed nature of the orbitals containing the electrons, and thus of the bonds themselves Although it post-dates the more complete quantum mechanical models, it is easy to grasp and within a decade had become a staple of every first-year college chemistry course.
