M-E bonds

Table 15. Selected M—E bond distances (pm) and bond angles (°) of base-stabilized monomeric group 15-aIanes dmap—AI(R2)E(Tms)2...

If a 7r-bond is present, it should result in a barrier to rotation around the M—E bond, which may in some cases be observed by variable-temperature (VT) NMR. However, an observed barrier to rotation may be due to steric factors associated with the presence of very bulky substituents rather than to the presence of a 7r-bond, or may relate to rotation of the substituent groups on M or E rather than around the M—E bond itself. 

Another complication arises from the fact that in main group 3-6 species the dynamic process involving the M-E bond may occur by two different mechanisms as illustrated by transition states (i) and (ii) ... 

Table 1 M-E bond lengths (A) and multinuclear NMR chemical shifts20 for compounds having M E single bonds...

Table 2 M- E bond lengths (A), energies (kcal mol-1), and stretching frequencies (cmT1) for H2M=E calculated at the B3LYP/TZ(d, p) level...

BC13, BBr3 and BI3. B-N distances obtained from single crystal X-ray diffraction studies as well as gas phase measurements yielded only small variations (X-ray 158.5-161.Opm gas phase 165.2-167.4pm),38 again demonstrating that the thermodynamic stability of group 13/15 adducts not necessarily correlates with their M-E bond length. 

It should also be stated that the M-E bond distances within the Et3M <— E(SiMe3)3 and t-Bu3M <- E(/-Pr)3 adduct families increase by about 40 pm according to the increase of the covalent radii of the group 15 element (rCOv(P) 110 pm, rcov(Bi) 150 pm). Within the sterically overcrowded t-Bu3M <— E(z-Pr)3 adduct families the arsine and bismuthine adducts show... 

Fig. 12. M-E bond lengths [pm] of identically substituted adduct families.

The heterocubanes each contain four-coordinate group 13/15 atoms and consequently show longer M-E bond distances than the monomeric and (base-free) heterocyclic derivatives with three-coordinate central atoms. Structure 677 is somewhat special since it contains five-coordinate In atoms due to the presence of thf donors. Consequently, the In-N bond distances differ more (213.6(5)—227.9(5) pm) than those of the base-free pentafluoro-phenyl analog 676501 (216.7( 1)—222.7( 1) pm). 

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