Overlap of bonding orbitals

What of the factors that involve quantum mechanics They concern the electron overlap of bond orbitals and the effect of tunneling through a barrier (Chapter 9). 

The effectiveness of overlap of bonding orbitals of ihe same symmetry appears to decrease as the principal quantum number increases and as the difference between the principal quantum numbers increases. This is reflected in the bond strengths shown in Table 10, The covalent radius of hydrogen is especially subject to effects of this kind, and has the values 0.3707, 0.362, 0.306. 0.284 and 0.293 A respectively in H2. HF. HCI. HBr and HI. The apparent anomaly of the P-P, S-S. and Cl-Cl bonds being stronger than the N—N. O-O. and F—F bonds has been considered in paragraph (I). 

The shape of a molecule deteraiines many of its physical and chemical properties. Molecular shape, in turn, is determined by the overlap of orbitals that share electrons. Theories have been developed to explain the overlap of bonding orbitals and are used to predict the shape of the molecule. 

When an electron pair is shared by the direct overlap of bonding orbitals, a sigma bond results. The overlap of parallel orbitals forms a pi bond. Single bonds are sigma bonds. Multiple bonds involve both sigma and pi bonds. 

An alternative approach is to first master the VSEPR theory and the related topic of molecular polarity for different structures, and then learn how the VB theory describes the overlap of bonding orbitals in these structures. If your instructor takes this approach, you should study this chapter in the following order ... 

TETRAHEDRAL STRUCTURES INTERPRETED BY AN OVERLAP OF BONDING ORBITALS (COVALENT BONDING STATE)... 

FIGURE 2 17 The carbon-carbon double bond in ethylene has a cr component and a tt compo nent The cr component arises from overlap of sp hybridized orbitals along the internuclear axis The tt component results from a side by side overlap of 2p orbitals... 

The TT bond in ethylene generated by overlap of p orbitals of adjacent carbons... 

Although both stereoisomers yield 4 tert butylcyclohexene as the only alkene they do so at quite different rates The cis isomer reacts over 500 times faster than the trans The difference in reaction rate results from different degrees of rr bond develop ment in the E2 transition state Since rr overlap of p orbitals requires their axes to be parallel rr bond formation is best achieved when the four atoms of the H—C—C—X unit he in the same plane at the transition state The two conformations that permit this are termed syn coplanar and anti coplanar... 

Bonding m alkenes is described according to an sp orbital hybridization model The double bond unites two sp hybridized carbon atoms and is made of a ct component and a rr component The ct bond arises by over lap of an sp hybrid orbital on each carbon The rr bond is weaker than the CT bond and results from a side by side overlap of p orbitals... 

The degree of overlap of these orbitals is smaller than in the triple bond of an alkyne... 

TT bond (Section 2 20) In alkenes a bond formed by overlap of p orbitals in a side by side manner A tt bond is weaker than a u bond The carbon-carbon double bond in alkenes con sists of two sp hybridized carbons joined by a a bond and a TT bond... 

The bonding between carbon monoxide and transition-metal atoms is particularly important because transition metals, whether deposited on soHd supports or present as discrete complexes, are required as catalysts for the reaction between carbon monoxide and most organic molecules. A metal—carbon ( -bond forms by overlapping of metal orbitals with orbitals on carbon. Multiple-bond character between the metal and carbon occurs through formation of a metal-to-CO TT-bond by overlap of metal-i -TT orbitals with empty antibonding orbitals of carbon monoxide (Fig. 1). 

Here, the bonding between carbon atoms is briefly reviewed fuller accounts can be found in many standard chemistry textbooks, e.g., [1]. The carbon atom [ground state electronic configuration (ls )(2s 2px2py)] can form sp sp and sp hybrid bonds as a result of promotion and hybridisation. There are four equivalent 2sp hybrid orbitals that are tetrahedrally oriented about the carbon atom and can form four equivalent tetrahedral a bonds by overlap with orbitals of other atoms. An example is the molecule ethane, CjH, where a Csp -Csp (or C-C) a bond is formed between two C atoms by overlap of sp orbitals, and three Csp -Hls a bonds are formed on each C atom. Fig. 1, Al. 

FIGURE 10.5 (a) Isolated double bonds are separated from one another by one or more sp -hybridized carbons and cannot overlap to give an extended orbital, (b) In a conjugated diene, overlap of two orbitals gives an extended system encompassing four carbon atoms. 

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