Lecture principles of inorganic chemistry

2P32 Winter Term 2015-16 Dr M Pilkington Principles of Inorganic Chemistry Lecture – Recapping Important Concepts       Inorganic Chemistry and the Periodic Table Bonding Models Shapes of Molecules - Lewis Structures Valence bond theory: cases of NH3 H2O and BF3 Lewis Acids and Bases σ and π bonds in CH2=CH2 The Shapes of Molecules – Relationship between Lewis Structure, VSEPR theory and VBT Assignment – Drawing Lewis structures and predicting the shapes/geometries of molecules due after class Tuesday 12th January   Inorganic Chemistry and the Periodic Table Carbon is only one element and has limited bonding modes, oxidation states and coordination numbers But it does CATENATE well and forms MULTIPLE BONDS with itself and other p-block elements especially N and O For the rest of the elements: Wide range of electronegativity, oxidation states, coordination numbers, ability to form multiple bonds and catenate etc… How can we make sense of such wide ranging behaviors? We have a system called the Periodic Table The ‘Periodic Law’ 1860-1870 (Mendeleev and Meyer): A periodic repetition of physical and chemical properties occurs when the elements are arranged in order of increasing atomic weight [number]’ With the development of atomic theory and spectroscopic techniques the modern Periodic Table has evolved: 2P32 Course Outline: Lectures 1-16 Coordination Chemistry of transition Metal ions Lectures 17 – 34 Descriptive Inorganic Chemistry – Main Group Elements Bonding Models: In covalent species, electrons are shared between atoms In an ionic species, one or more electrons are transferred between atoms to form bonds Modern views of molecular structure are, based on applying wave mechanics to molecules; such studies provide answers as to how and why atoms combine Two such methods are: Valence Bond (VB) approach- overlap of valence orbitals on atoms to form bonds Molecular Orbital theory (MO) of bond formation – allocates electrons to molecular orbitals formed by the overlap (interaction) of atomic orbitals Familiarity with both VB and MO concepts is necessary as it is often the case that a given situation can be approached using one or the other of these models  Shapes of Molecules Understanding the shapes of molecules is an important step in being able to discuss and predict chemical properties Although here we discuss the shapes of “simple” molecules, this topic has also important applications in the understanding of the behavior of much larger molecules, e.g the shape of macromolecules in biology is often important with respect to their biochemical function Lewis structures – you need to be able to draw these    Lewis presented a simple but useful method of describing the arrangement of valence electrons in molecules Lewis structures give the connectivity of an atom in a molecule, the bond order and the number of lone pairs and these maybe used to derive structures Revise your first year notes Test Question  Draw the Lewis Structure of the Nitrato ion NO3-  How many  bonds, how many  bonds?  What is the nitrogen-oxygen bond order?  Are there possible resonance structures, can you draw them? Bond Order    Single bond - first order Double bond = second order Triple bond  third order Bond order is a measure of the number of bonding electron pairs between atoms Single bonds have a bond order of 1, double bonds have a bond order of and triple bonds (the maximum number) have a bond order of A fractional bond order is possible in molecules and ions that have resonance structures In the example of ozone, the bond order would be the average of a double bond and a single bond or 1.5 (3 divided by 2) As the bond order becomes larger, the bond length becomes smaller Remember atoms in the 3rd period or below e.g P, I not always obey the Octet rule! Valence Bond Theory The Shape of Ammonia (NH3) – VSEPR is important here Lewis Structure Lone Pair H N But why isnt the NHN angle 900? H H We have to consider repulsions between the lone pair and valence electrons actual structure: N H H H-N-H angle is just slightly smaller than 109.50 The Nitrogen atom is Pyramidal H Ammonia is a polar molecule with N carrying a partial negative charge Molecular shape is important with respect to determining if a molecule is polar or not Look at Valence Bond Theory (VBT) 2p 2s N [He] 2s2 2p3 Hybridization mix the orbitals -" like mixing together a red and white plant" H HH H 1s1 Hybridization of N = sp3 N [He] 2s 2p We know that sp3 hybrids have a 109.50 angle N H H H Molecular Structure of NH3 - cannot see the lone pair on N but there is a flattened lone pair N H H H The actual shape of NH3 is trigonal pyramidal (approximately tetrahedral minus one atom) Compared to H20   The O in H2O has bond pairs and lone pairs Two corners of the tetrahedron are missing because they are occupied by lone pairs, not atoms The shape is called bent The H-O-H angle is less than NH3, due to the greater repulsions felt with two lone pairs Other molecules with bond plus lone pairs include OF2, H2S and SF2 Bond angles vary, but all are significantly less than 109.50 The Shape of BF3  Treat this as an exception to the octet rule (An atom obeys the octet rule when it gains, looses or shares electrons to give an outer shell containing eight electrons with the configuration ns2np6) Many molecules such as neutral compounds of Boron simply not contain enough valence electrons for each atom to be associated with eight electrons B 2s 2p F 2s2 2p5 sp2 2s 2p this leaves an empty 2p orbital F B F F Six electrons around the Boron This leaves an unused "p orbital" perpendicular to the plane of BF3 F F B F But if we want B to have an octet how can we achieve this? A hybrid of resonance structures is the best Lewis representation for the real strucure of BF3 F F B B F F F F F F B F F B F F However In this structure with a double bond the fluorine atom is sharing extra electrons with the boron The fluorine would have a '+' partial charge, and the boron a '-' partial charge, this is inconsistent with the electronegativities of fluorine and boron Conclusion - the Octet Rule breaks down here  Evidence for a resonance structure comes from the B-F distances measured in the solid state They are shorter by ~15 pm’s compared to the B-F distances in BF4- Generally as we move from a single bond towards a double bond our bond lengths shorten by approximately 15 ppm’s F F B F F C-C Distances CH3CH2 155 ppm CH=CH 140 ppm BF3 Resonance  Rehybridize the F’s to sp2 F F B empty 'p' on B F filled 'p' on F The MO diagram is complex but the result for BF3 is one πbond spread over B-F links F B F empty 'p' on B F filled 'p' on F To Summarize: BF3 The B atom has three bond pairs in its outer shell Minimizing the repulsion causes this molecule to have a trigonal planar shape, with the F atoms forming an equilateral triangle about the B atom The F-B-F bond angles are all 120°, and all the atoms are in the same plane    Lewis Acids and Bases BF3 reacts strongly with compounds which have an unshared pair of electrons which can be used to form a bond with the boron: BF3 – Lewis Acid – electron pair acceptor NH3 – Lewis Base – electron pair donor  σ versus Π-bonding Ethene, C2H4, sp2 H H H H Two lobes one with a positive sign the other with a negative sign go though a node Nodal Plane fn = (wave function) i.e no electron density p orbital not used in hybridization The three sp2 hybrid orbitals arrange themselves as far apart as possible - which is at 120° to each other in a plane The remaining p orbital is at right angles to them C-H overlap to give sigma bonds  The two carbon atoms and four hydrogen atoms would look like this before they joined together: The various atomic orbitals which are pointing towards each other now merge to give molecular orbitals, each containing a bonding pair of electrons σ orbital – no nodal planes Π orbital one nodal plane containing the nuclei 10 Tetrahedral and octahedral Holes Three A-type Lattices „ A second type of lattice is called body-centered cubic (bcc) and, as the name implies, differs from the simple cubic lattice in that a second sphere is placed in the centre of the cubic cell „ In this case the eight spheres at the corners are only one-eighth in the unit cell, the center sphere is completely incorporated into the body of the cell and therefore has a coordination number of „ The third arrangement is face centered cubic (fcc) where the coordination number of a given sphere is 12 „ Cubic packing is not the most efficient way to pack spheres in a layer „ To increase the efficiency of the packing we fit a given sphere in the crevice or depression between two others „ „ „ „ „ „ We can consider this to be layer A of the cubic close packed and hexagonal close packed structures The second layer is laid down such that a given sphere fits in the hole left by three spheres in layer A There are now two types of holes in layer B: those which have no spheres below them, holes a and those which have layer spheres directly below them, holes b If the third layer is placed in the a depressions, they create a new layer C The resulting packing scheme is known as the face centred cubic closepacked (ccp) structure ABCABCABC (c) If the spheres of the third layer are placed in the b depressions, they generate another layer A, the same as the first and the ABABAB hexagonal close packing scheme results (d) The ABABAB and ABCABC closed packed structures Examples of common packing arrangements in ionic structures with AB type lattices 10 2P32 – Principles of Inorganic Chemistry Dr M Pilkington Lecture 24 – The Solid State A-type lattices Ionic crystals – ABn crystal lattices Predicting structure types of ionic compounds: radius ratios Examples of common structure types of ionic solids Surveying the four main classes of crystalline solids: ƒ metallic, covalent network, molecular and ionic solids Three A-type Lattices „ A second type of lattice is called body-centered cubic (bcc) and, as the name implies, differs from the simple cubic lattice in that a second sphere is placed in the centre of the cubic cell „ In this case the eight spheres at the corners are only one-eighth in the unit cell, the center sphere is completely incorporated into the body of the cell and therefore has a coordination number of „ The third arrangement is face centered cubic (fcc) where the coordination number of a given sphere is 12 „ Cubic packing is not the most efficient way to pack spheres in a layer „ To increase the efficiency of the packing we fit a given sphere in the crevice or depression between two others „ „ „ „ „ „ We can consider this to be layer A of the cubic close packed and hexagonal close packed structures The second layer is laid down such that a given sphere fits in the hole left by three spheres in layer A There are now two types of holes in layer B: those which have no spheres below them, holes a and those which have layer spheres directly below them, holes b If the third layer is placed in the a depressions, they create a new layer C The resulting packing scheme is known as the face centred cubic closepacked (ccp) structure ABCABCABC (c) If the spheres of the third layer are placed in the b depressions, they generate another layer A, the same as the first and the ABABAB hexagonal close packing scheme results (d) The ABABAB and ABCABC closed packed structures – remember here that in the A, B and C layers the spheres have all the same size or are of the same type 2 Ionic Crystals „ Most ionic crystals can be described as layers of anions containing cations within holes The holes can be tetrahedral or octahedral „ There are a number of packing arrangements that are very commonly found in ionic crystals ABn-Type Crystal Lattices The spheres representing the atoms, ions or molecules are two different sizes The most common example of these lattices are ionic crystals in which the anion is larger than the cation It is best to picture the anions forming an A-type lattice and the cations fitting into “holes” in that lattice „ The crystal in this case is ionic and the holes in the anionic lattice must be of proper size to adequately accommodate the cations „ We have to then be able to identify the number and type of holes present in the A-type lattice „ A face centered cubic unit cell showing the positions and numbers of octahedral and tetrahedral holes per unit cell There are octahedral holes in the centre and in the middle of the 12 edges of the unit cell There is a tetrahedral hole associated with each corner of the unit cell ABn structures - 1:1 compounds are the easiest to visualise (NaCl, BaO etc… they contain cation and anion) There are three main structural types: NaCl – Na+ is coordinated by Cl- - OCTAHEDRAL COORDINATION CsCl – Cs+ is coordinated by Cl- - CUBIC COORDINATION ZnS – Zn2+ is coordinated by S2- - TETRAHEDRAL COORDINATION Radius Ratios A tetrahedral hole must be quite small, but an octahedral hole is a little larger The ratio of the radius of the cation to the radius of the anion gives us values which enable us to determine the hole size and coordination number and thus structural type of the ionic structure: ƒ If the cation is very small r cation/r anion = 0.225- 0.414 – Coordination No e.g ZnS type structure ƒ If r cation/r anion = 0.414 - 0.732 – coordination No e.g NaCl type structure ƒ If the cation and anion are large r cation/r anion = > 0.732- Coordination No e.g CsCl type structure 4 Examples of ABn Structures AB structures Sodium Chloride “rock salt” „ Both ions have the same packing pattern, the stoichiometry is 1:1 „ The sodium cations occupy octahedral holes (in both the centre and at the cell edges of the unit cell) in the cubic closed packed lattice of chloride ions „ The ABCABC layer structure of chlorides which is consistent with a fcc unit cell Zinc Blende „ 1:1 stoichiometry, four sulfide ions to match four zinc cations found completely within the unit cell „ Zinc cations occupy tetrahedral holes (not octahedral) in the sulfide lattice „ The sulfides form a cubic close packed array – an ABCABC arrangement of the fcc unit cell of anions „ The zinc cations occupy only four of the eight tetrahedral holes (four tetrahedral holes are empty) „ Has a diamond like structure and there is some covalent nature to this structure, not purely ionic Caesium Chloride „ Cs cations form a simple cubic lattice and the chloride ions occupy the holes or, Cl- anions can be pictured as forming the A-type lattice with Cs cations in the cubic holes „ The coordination number for both the anion and the cation is AB2 Structures - In AB compounds the coordination numbers and stoichiometries of anions and cations are equal This is not the case for AB2 compounds - Consider CaF2 (Flourite) Due to the stoichiometry, a larger unit cell of fcc calcium ions with flourides filling tetrahedral holes is consistent with the 1:2 stoichiometry The coordination number of the flourides is The calcium ions occupy cubic holes formed by flouride ions Rutile structure (TiO2) is not close packed This is not a cubic unit cell, but rather a tetragonal The coordination number of the oxides is Types of Crystals „ The realm of inorganic chemistry was considerably expanded in the early 20th Century when X-ray diffraction revealed information on the structure of solids „ Solids are composed of atoms, molecules, or ions arranged in a rigid, repeating geometric pattern of particles known as the crystal lattice „ Crystals are usually categorized by the type of interactions operating among the atoms, molecules or ions of the substance „ These interactions include ionic, metallic, and covalent bonds as well as intermolecular forces such as hydrogen bonds, dipole-dipole forces and London dispersion forces „ In the classification of crystals as well as categorizing them by their lattice types, i.e monoclinic, cubic, tetragonal etc… we can also classify them according to their chemical and physical properties In this respect, we have four types of crystals, metallic, covalent, (covalent) molecular, and ionic Metallic Crystals „ „ „ „ Metals – elements from the left side of the periodic table form crystals in which each atom has been ionized to form a cation and a corresponding number of electrons The cations are pictured to form a crystal lattice that is held together by a “sea of electrons” – sometimes called a Fermi sea The electrons of the sea are no longer associated with any particular cation but are free to wander about the lattice of cations We can therefore define a metallic crystal as a lattice of cations held together by a sea of free electrons The sea analogy allows us to picture electrons flowing from one place in the lattice to another „ If we shape the metal (copper is a good example) into a wire If we put electrons in one end of the wire, electrons will be bumped along the lattice of cations until some electrons will be pushed out of the other end „ The mobility of the delocalized electrons accounts for electrical conductivity „ Metals are characterized by their tensile strength and the ability to conduct electricity „ Both properties are the result of the special nature of the metallic bond Bonding electrons in metals are highly delocalized over the entire crystal „ The great cohesive force resulting from the delocalization is responsible for the great strength noted in metals „ The bonding strength in metals varies with the number of electrons available as well as with the size of the atoms i.e Na – valence electron m.p 980C Mg – valence electrons m.p 6490C W – valence electrons m.p 60000C „ Metal crystals all have a high density which means that they usually have the hcp or fcc structure „ Magnesium, scandium, titanium, cobalt, zinc and cadmium have the hcp structure „ Aluminium, calcium, nickel, copper, palladium, silver, platinum, gold and lead have the fcc structure „ Alkali metals, iron, chromium, barium, and tungsten have the bcc structure Covalent Network Crystals „ A covalent network crystal is composed of atoms or groups of atoms arranged into a crystal lattice that is held together by an interlocking network of covalent bonds „ Covalent bonds (the result of the sharing of one or more pairs of electrons in a region of overlap between two or perhaps more atoms) are directional interactions as opposed to ionic and metallic bonds, which are non directional „ For example diamond – each carbon is best thought of as being sp3-hybridized and that to maximize the overlap of these hybrid orbitals, a C-C-C bond angle of 109.50 is necessary „ Hence the interactions are directional in nature „ Other examples of compounds that form covalent network crystals are silicon dioxide (quartz), graphite, elemental silicon, boron nitride (BN) and black phosphorous ƒ The structure of diamond is based on a fcc lattice There are C atoms at the centre of the cube, C atoms in the face centre, and more within the unit cell Each C is tetrahedrally bonded to four others This tightly bound lattice contributes to diamond's unusual hardness In graphite each C is bonded to three others and the layers are held together only weakly ƒ Covalent crystals are hard solids that possess very high melting points They are poor conductors of electricity Molecular crystals „ Very soft solids that possess low melting points „ They are poor conductors of electricity „ Molecular crystals consist of such substances as N2, CCI4, I2 and benzene „ Generally, the molecules are packed together as closely as their size and shape will allow „ The attractive forces are mainly van der Waals (dipole-dipole) interactions „ Water molecules are held together by directional H-bonds „ Intermolecular forces in this case can either be nondirectional as is the case of crystals of argon, or directional, as in the case of ice „ In the latter case, the H-O···H angle is 109.50, an angle determined by the geometry of the individual water molecules 10 Examples of molecular crystals: „ Ionic crystals „ Hard and brittle solids „ They possess high melting points „ „ „ They are poor conductors of electricity, but their ability to conduct increases drastically in melt The packing of spheres in ionic crystals is complicated by two factors : ‰ charged species are present ‰ anions and cations are generally quite different in size Some general conclusions can be drawn from ionic radii : ‰ ‰ „ within the same period the anions always have larger radii than the cations the radius of the trivalent cation is smaller than that of the divalent cation, which is smaller than that of the monovalent cation It should be realized that the value of any ionic radius only serves as a useful but approximate size of the ion The fact that the ionic radius of Na+ is 0.98Å does not mean that the electron cloud of the ion never extends beyond this value It is significant because when it is added together with the radius of an anion, e.g Cl-, the sum is approximately equal to the observed interionic distance 11 Formation of an Ionic Crystal – when two elements, one a metal with a low ionization energy and the other a nonmetal with a highly exothermic electron affinity are combined, electrons are transferred to produce cations and anions „ These forces are held together by non-directional, electrostatic forces known as ionic bonds A hypothetical view of the formation of sodium chloride, the constituent elements are combined, electrons are transferred and ionic bonds among the sodium and chloride ions are formed „ Examples of compounds that form ionic crystals are CsCl , CaF2, KNO3, alumina, Al2O3, zirconia, ZrO2 (very hard) „ Cubic zirconia is an imitation diamond, when crystallized it can be used as a jewel or it can be used as an industrial product for abrasion „ „ Melting Points of a Class of Ionic Compounds LiF – 8450 BeO - 25300 NaF – 9930 MgO - 28520 KF – 8580 CaO - 26140 RbF – 7950 SrO - 24300 CsF- 6820 BaO – 19180 Next lecture - examine the factors that influence the melting points of these solids? 12 ... the father of coordination chemistry „ Studied in Switzerland at the University of Zurich „ He lectured in both organic and inorganic chemistry „ He developed the theory of coordination chemistry. .. in thiocluster compounds of Fe 2P32 – Principles of Inorganic Chemistry Dr M Pilkington Lecture - Classification and Nomenclature Ligand Classification:  Coordination Chemistry and Ligands ... optically active forms of 6-coordinate octahedral complexes „ His coordination chemistry extended through a whole range of systematic inorganic chemistry and into organic chemistry and he was awarded