LECTURE 12. VB THEORY: MAKING MOS FROM AOS What is a bond? An overlap of e- density from 2 atomic orbits. Examples of atomic orbits? So, overlap them to make a VB: So what are the types of molecular orbits? There are only 2, sigma and pi. Æ sigma, when e- density is between atoms Æ pi, when e- density is off axis So let’s make sigma and pi bonds by mixing s and p atomic orbits.Note that since s orbit is in all directions, some overlap must be along axis of nuclei, so s orbits are always sigma bonds.Now look at 3 orientations of p atomic orbits:Summarizing: - any bond involving s orbits is sigma - s + s = sigma,σ; s + p = sigma,σ - of 3 p orbits overlapping, 2 will be pi, π and one will be sigma, σ - And that is almost it. Except that we need to consider the case where hybrid atomic orbits: sp, sp2, sp3, dsp3, d2sp3 are mixed. Note, happily, that every hybrid orbit has s character which means that they have some electron density along the axis and consequently: Every hybrid orbit that forms a valence bond with another atomic orbit is making a sigma, σ, bond. Let’s apply to 3 examples: C2H2, C2H4, C2H6. - sigma: bonds along the axis between atoms. It is the first bond formed and is from either atomic orbits with s character or p orbits, or hybrid orbits. Example: - pi: bond above and below nuclei axis; are formed by off-axis overlap of p-p orbits. π bonding is always present when there are 1) double bonds 2) triple bonds 3) resonance. Example: Now let’s identify sigma and pi bonds in C2H6, C2H4, and C2H2. Example C2H6:- note that there are 7 sigma bonds. 6 are C-H and 1 is C-C. - C-H bond is H 1s overlap with Csp3 orbit Æ σ 1s-sp3 - C-C bond is C sp3 overlap with Csp3 Æ σ sp3-sp3 Example C2H4: - first bond type is seen 4 times: C-H Æ σ sp2-1s - second bond type is seen 1 time: C-C Æ σ sp2-sp2 - third bond type is pi bond seen once: C-C Æ π p-p Example C2H2:- 2 sigma C-H bonds Æ σ 1s-2p from H and C - 1 sigma C-C is Æ σ sp-sp from C and C - 2 pi C-C is C-C Æ π p-p from over lap of p orbits above and below, from and back The bad news is that VB (valence bond) theory is needed to explain hybrids. The good news is that all hybrid orbits look pretty much the same. The reason they look the same is that all have s orbit in the mix (sp, sp2, sp3, dsp3, d2sp3). And the best news is that therefore all hybrids for sigma bonds.So while we haven’t learned why hybrids are needed, at least we know that they must form sigma bonds. Now let’s see why Valence Bond (VB) Theory explains hybrids. Imagine what CH4 molecular orbits came from: So spectroscopy should show 2 peaks, right? This means that we should have 4 bonds as shown to the right. They appear to be of two different types: sigma 1s-2s and sigma 1s-2p.But the single σ 1s-2s is different from: the three σ 1s-2pNO!!! Spectroscopy only shows ONE peak: - so the 4 bonds must be the same. They must have mixed or hybridized orbitals - The 4 sp3 hybrids combine to form the one peak shown on the left.So how many kinds of hybrids? They break down by # e- rich regions = # hybrid orbits # e- rich regions Atomic orbits before mixing Mixed Hybrid 2 s,p 3 s,p,p 4 s,p,p,p 5 d,s,p,p 6 d,d,s,p,p Example: What is hybridization of BeCl2? Create Lewis structure Æ Cl-Be-Cl 2 e- rich regions so hybrid of two atomic orbits = sp hybrid Example: What is hybridization for SF6 ? 6 e- rich regions means hybrid of six atomic orbits = d2sp3 hybridSummarizing what you know: # e- rich regions Electronic geometry Angle Hybrid Molecular geometry 2 Linear 180 Sp AB2 3 Trigonal planar 120 Sp2 AB2 or AB3 4 Tetrahedral 109.5 Sp3 AB4, AB3U, AB3U2 5 Trigonal bipyramidal 120, 90, 180 Dsp3 AB5, AB4U, AB3U2 6 Octahedral 180, 90 D2sp3 AB6, AB5U, AB4U2 What starts it all, from 3D Lewis structure 3 things you know right away from # e- rich regions Knowing bonding and unbonded pairs lets you determine molecular geometry Summarizing what you know: the 13 molecular
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