Bond angles between participating atoms in a molecule shows specific orientation with respect to each other and their immediate space around them. The overall effect brings in an angle in between the atoms and that becomes the effective orientation till this is ready for next set of chemical changes. According to the simple hybrid orbitals where only the angular dependence of the orbitals is considered, the strongest s, p or d single bonds are formed at angles specific to that molecule. The favoured angles between a double bond and a single bond or even in between a pair of double bonds can be found from these angles. These are assumed on the basis that the axis of a double bond would lie midway between the axis of two single bonds which help in forming the constituent double bond. These are the qualities that have been used for all kinds of calculations and comparison for experimental bond angle values.
Many of the compounds in transition group are found to have a central metal with three carbonyl groups and form various forms of ligands. These bond numbers and angles so formed are again calculated on the basis of same mentioned qualities. The existence of a large number of experimental values of tri carbonyl bond angles helps significantly to understand the hybrid s, p and d bond orbitals. The maximum separation occurs when the angle between any two regions of electron density is 109.5o and hence, we can well predict the H – C – H bond angles. These predictions are made by VSEPR model. The predictions of shape and geometry are all carried out the same manner. The Lewis structure NH3 shows nitrogen surrounded by specific four region of electron density. Similarly the electron density for methane, water etc are also carried out by checking out the electron density of all the participating atoms of a molecule. The four regions of NH3 which are arranged in tetrahedral manner (H – N –H) shows bond angles of 109.5o but the observed angle is 107.5o.
Geometry of Molecules from VSEPR TheoryBack to Top
The application of VSEPR model to the specific geometry of transition metal group compounds is almost similar to what we get to see in main group elements and their compounds. As there are no lone pairs in valence shell of any transition metal, only the basic geometric idea is applied. The deviation from such normal geometry which are produced by various bonding pairs, generally due to differences in ligand formation and their electronegativity.
The main difference between valence shell and d sub shell interaction is also considered, although it might provide a basic geometry of AXn. The VSEPR model is less predictive for any transition metal compounds compared to main group elements.
Usually we find that molecular orbital and ligand field treatments of geometry of transition metal compounds, are assumed with the distortions of geometry by d orbitals taken into consideration. The deviations do not take basic geometry of ligands with different electronegativity or for that matter multiple bond as the basic structure outline.
The best approach to the shape of molecule in VSEPR theory suggests that electron pairs around central atom always repel each other. Bonding as well as lone pairs repel each other due to same charge and tend to distort the shape of the molecule. The pairs take this specific arrangement mainly due to minimum electrostatic force working with each other.
In case of molecule which has only bonding pair and nothing to work on lone pairs around the central atom, the total number electron pairs become equal to number of bonding pairs. In order to understand these better, the VSEPR has some specific rules which helps in predicting the shapes of molecules.
- The molecules Lewis structure is written down
- The counting of electron pairs around the central atom is done next
- The arrangement of electron pairs usually end up in specific numbers and shape.
- Two pairs will show linear shape, three pairs will give trigonal planar, four pairs will end up giving a tetrahedral shape, five pairs will show trigonal bi pyramidal, and six pairs will give an octahedral shape.
- The symbol of central atom is necessary, followed by arrangement of other atoms around the central atom.
While we determine the geometry of molecules with the help of VSEPR, there are multiple bonds which get single region electron density status. This is possible mainly due to the fact that greatest density of electron for multiple bonds always lie in between the two bonding atoms.
Water Molecule StructureBack to Top
The behavior of this unique compound is unusual and complex although it looks very simple H2O formula. The structure of water molecule is not simple as it looks like where all atoms lie in a straight line.
The two bonds between H and O form a right angle approximately where the two H atoms are found on the same side. The reason for this structure is that the outer shell of eight electrons in O atom of H2O is composed of four pairs of electrons.
Two of these are covalent in nature while rest two are not in bonding. The structure of water are very specific and is responsible for waters characteristics.
Water has two unpaired electrons at the end of H atoms giving water polar character and next is the unshared electrons of O atom can also make bond with neighbouring H atom of another water molecule. This results in hydrogen bonding which makes the water molecules attracted to each other.
The Lewis structure for water molecule H2O shows two bonding pairs and two lone pairs around O atom. This results in four electron pairs. The VSEPR indicates four electrons shows tetrahedral arrangement. The lone pairs take up two tetrahedral positions along with another two positions taken up by bonding pairs. The bond angle of H â€“ O â€“ H is 104.5 instead of 109.5.
Octahedral Bond AnglesBack to Top
When a molecule has two electron groups arranged around the central atom, they definitely arrange themselves opposite each other. This is done to minimise the effect of repulsion between the electron groups.This results in a linear arrangement of 180 degree. When three electron groups surround the central atom then the shape that comes out is triangular having a bond angle of 120 degree. This is known as trigonal planar.
If four electron groups surround the central atom then the shape is given out as tetrahedron and is stable as well. This will have 109.5 degree. In case there are five electron groups surrounding the central atom then we get to see trigonal bi pyramidal. The atoms which lie in horizontal plane of molecule are considered as axial while the ones lie along vertical axis is termed as equatorial.
The octahedral arrangement comes into picture when the central atom is surrounded by six other atoms which results in tremendous repelling giving an octahedral arrangement. Octahedral molecule like SF6 consists six fluorine atoms surrounding central S atom and are bonded with each other in single bonds. This results in six different electron dense zones of high electron density corresponding to six S – F bonding pairs. These six electron bonding repel each other resulting in octahedral set up. The final F – S – F symmetry gives a 90 degree angle.
Trigonal Bi-pyramidal Bond AnglesBack to Top
The central atom getting surrounded with five electron pairs can show a shape which resembles a bi pyramid structure and that is what we are expecting in this specific arrangement of five electron pairs around the central atom. There are atoms like phosphorus which can accommodate more than 4 electron pairs and thus show an extended octet or more than 8 electrons in valence shell.
The phosphorus penta chloride (PCl5) ideally should show a trigonal bi pyramidal geometry as it has around 10 electrons in valence shell. Moreover, the angles in between electron pairs are not same, and electrons directions from electron pairs point are not equivalent as well. The AX5 geometry of trigonal bi pyramidal form have other examples as well. The stannous penta chloride ion or stannous tri chloride have the same AX5 geometry.
The five separate electron density regions results asymmetrical arrangement and the bond angles within the plane show 120 degree while the bond angles above and below the plane are having 90 degree.
Trigonal Pyramidal Bond Angles
The AX3E geometry is very interesting. The bond angle of ammonia is 107.2 degree is considered to be small relative to the ideal tetrahedral angle. This is due to presence of lone pair. The bond angle of tri fluoro amine (NF3) is again smaller 102.3 degrees and this is due to the high electronegative value of fluorine.
If we consider the NCl3, the bond angle is 107.1 which is exactly as that of ammonia. This though is expected to be an intermediate value between NF3 and NH3. The bond angle in NCl3 is basically hindered by the repulsion force between chlorine atoms.
Molecular Geometry Bond AnglesBack to Top
The effect of lone pairs and bonded pairs is definitely a major point to decide the molecular geometry. This is mainly because the geometry depends upon the electron pairs present in valence shell and thus predicts an electron pair geometry for any molecule or ion.
The molecular geometry helps in giving an exact description of molecular shape the overall arrangement of atoms around the central atom. The electron pair geometry is decided by the electron pairs present in valence shell around the central atom, which can have both bonded as well as lone pairs. This is quite different than the basic molecular geometry which helps us to understand only the central atom’s geometry.
Electrons of lone pairs around the central atom always arrange themselves in spatial form but the exact location is never described for the final shape of molecule or for that matter an ion. The use of VSEPR model to predict the molecular geometry and bond angles in molecules like ammonia can be carried out by getting the Lewis structure and then locate the electron pairs around the central atom of nitrogen. These are going to show three bonded pairs and one lone pair which finally give the electron pair geometry of tetrahedral arrangement.
The effect of lone pairs on bond angles is again based on the repulsion which exists between electrons which push each other and distort the overall shape the molecule. The relative strength of repulsions that determine the final shape and structure comes out with a specific order.
Lone pair repulsion > lone pair and bonded pair repulsion > bonded pair repulsion with bonded pair.
The decrease in bond angles is obvious and completely depends upon the number of lone pairs around the central atom.