Hypervalent 3c-4e bonds

Hypervalent 3c/4e bonding becomes a ubiquitous feature of transition-metal bonding (to the extent that normal-valent coordination complexes may appear the exceptional case ). 

Aryl-A5-iodanes ArIL4 have a square pyramid structure with an aryl group in an apical position and four heteroatom ligands in basal positions. Two orthogonal hypervalent 3c -4e bonds accommodate all of the heteroatom ligands and the apical aryl group has a character of a normal covalent bond using hybridized 5sp orbital [3]. 

This model implies that the TBP M atom could use its nsp2 orbitals for bonding with the equatorial ligands to form two-center bonds, and its npz orbital could be involved in the interaction with an appropriate orbital of the axial substituent X and a lone electron pair of the donor atom to form a hypervalent, 3c-4e bond in the axial moiety D — M—X. The simplest MO diagram of a 3c-4e bond may be represented by three molecular orbitals bonding (b), nonbonding (nb) and antibonding (ab) (Scheme 1). 

Simultaneous Participation of Multiple Neighboring Groups. The strength of a hypervalent 3c-4e bond increases with electronegativity of the apical ligands. In a sulfuranyl radical such as... 

It is difficult to give a localized orbital description of the bonding in a period 3 hypervalent molecule that is based only on the central atom 3s and 3p orbitals and the ligand orbitals, that is, a description that is consistent with the octet rule. One attempt to do this postulated a new type of bond called a three-center, four-electron (3c,4e) bond. We discuss this type of bond in Box 9.2, where we show that it is not a particularly useful concept. Pauling introduced another way to describe the bonding in these molecules, namely, in terms of resonance structures such as 3 and 4 in which there are only four covalent bonds. The implication of this description is that since there are only four cova-... 

What is the region of bond distances for hypervalent bonds of chalcogens Bond orders of hypervalent 3c-4e are typically around 0.5. A bond distance with a bond order of 0.5 is predicted to be 0.18 A longer than that with a bond order of 1.0.16... 

TBP of RR E X2 (and/or R2EX2) will be in equilibrium with molecular complexes (MC), together with the components, in solutions (Scheme 2).20 22 Both X-E-X and E-X-X bonds are hypervalent 3c-4e.15e 20 According to the N-X-L notation, proposed by Martin,23 the hypervalent atoms in X-E-X (TBP) and E-X-X (MC) are represented by 10 t 4 and 10-X-2, respectively. The notation will be employed in this chapter. 

Indeed, the general tendency toward 3c/4e bonding in transition-metal complexes is so pronounced that such hypervalency should be considered the rule, rather than the exception, in transition-metal chemistry. 

However, we have shown how the 18-electron rule is commonly satisfied in the absence of any significant p-orbital participation, on the basis of hypervalent 3c/4e cu-bonding interactions wholly within the framework of normal-valent sd" hybridization. Results of NBO and Mulliken analyses of high-level wavefunctions for transition-metal complexes commonly exhibit only paltry occupation of the outer p orbitals (comparable in this respect to the weak contributions of d-type polarization functions in main-group bonding). 

The Pt - -Pt—H anti-H-bond linkage can also be viewed as a hypervalent 3c, 4e (co-bond) resonance hybrid of the form... 

Hypervalent bonds in hexacoordinate compounds are formed in a similar fashion the four equatorial M—X bonds (two 3c-4e bonding) in Oh are longer and more polar than the two axial bonds. 

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