M—O bond lengths

A survey of crystal structures of 29 compounds (Table 8), in which the alkyl hydroperoxide anions serves as ligand to metal ions, transition metal ions or group 13-17 elements, provides a mean 0—0 bond length of 1.46 0.03 A, an O—O—C angle of 109 2.1° and a M—O—O angle of 112 6.9°. More specialized aspects that deserve to be addressed separately refer to the nature of the M—O bond, the magnitude of the dihedral angle M—O—O—C and the tetrahedral distortion of the peroxide bound C atom. 

Covalent radii versus experimental M-0 bond lengths 

The experimentally observed (M—0)exp distances fall into three categories, if compared to the sum of covalent radii (M—0)cov based on values from a CSD recommendation (Table 9) (i) (M—0)exp values that fall short of (M—0)cov, (ii) (M—0)exp distances that are longer than (M—0)cov and (iii) (M—0)exp data that fit (M—0)cov 

FIGURE 16. The formation of intramolecular hydrogen bonds nitrogen functionalities as acceptor for the hydroperoxide proton  

2 (tert-Butylperoxy)chloro[bis()) -cyclopentadienyl]titanium(lV) (35) (MUPEAE) 1.467 1.458 156.6 123 

5 [Dimethyl(phenyl)methylperoxy] hydrogen[tris(3-lert-butyl-5-isopropylpyrazol-l-yl]borato manganese(II) (HIZDTD) Group 8 1.410 1.463 176.9 126 

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Fig. 20. The variation in strain energy, V, for various conformations of [M(18-crown-6)]n+ complex, as a function of strain-free M-0 bond length. The M—O bond lengths of various metal ions are indicated on the M-0 bond length axis. The curves are for the planar D3d (+-+-+-), half-buckled (+-+--), and buckled (++-+ +-) conformers shown in Fig. 21, and for the complex of the open chain complex of pentaethylene glycol. The calculations were carried as described in the text, and in Refs. (4 and 60). Redrawn after Ref. (60). 

TABLE 9. Experimental M—O distances versus calculated covalent M—O bond lengths (Aov = 0.68 A for... 

A parameter, which is of special importance for the description of M(OR)n in addition to the coordination polyhedron of the central atom and the M-O bond lengths, is the M-O-C bond angle. Bradley [198] was the first to emphasize the role of unshared electron pairs at the oxygen atoms that can be involved into interaction with the vacant t/-orbitals of the transition metal. Thus in the molecules containing maingroup metals such as Bi, Sn, Ge, Te, and Al. the values of these angles usually do not exceed 120 to 130°, while in those of... 

The cubic AMO3 perovskite structure consists of an MO3 array of comer-shared MO6/2 octahedra with a large A cation at the body-center position. As is illustrated in tig. 1, this structure allows formation of the Ruddlesden-Popper (1957,1958) rock-salt/perovskite intergrowth structures MO (AMO3 ) . In all these structures, the mismatch between the equilibrium (A-O) and (M-O) bond lengths is given by the deviation from unity of the geometric tolerance factor... 

An increase in the tolerance factor with pressure (dt/dP > 0) signals an unusually compressible M-O bond length it has only been found at a crossover where there is a doublewell potential for two equilibrium (M-O) bond lengths. 

Jaccarino (1976) and by Smith (1969) have given a bca r n with n 2.5-3.0 for an equilibrium M-O bond length r. If U and A of eq. (23) are pressure-independent, it follows that 7 n r 10 V-3-3. If the compressibility K = — V ldV/dP remains constant, the pressure dependence of 7n should conform to Bloch s rule. Therefore, any deviation from Bloch s rule would be an indication that either the superexchange perturbation approach breaks down or the assumption that U and A are pressure-independent is not valid. 

It should be noted that generally, the tungsten complexes are more efficient catalysts than their analogous molybdenum complexes. One reason for the difference may be due to the slight differences in the p-oxo (M-O) bond lengths in the two complex types. 

Fig. 3. Comparison of structures and M—C and M—O bond lengths of CpjM-(CH3XTHF)" (n = 0,l) complexes. M—O n bonding is possible for 3, but not for (C5H5)Yb(CHjKTHF), 4 or 5.

For a closed transition structure, shorter M-O bond lengths amplify the van der Waals interactions between Ri, R2, and X relative to enolates with longer bond lengths, resulting in higher stereoselectivities [16]. With boron enolates for example, Z((9)-enolates are highly syn selective [52]. 

Compound Metal coordination M-O Bond lengths (A) Terminal Bridging Bond angles (°) MOC Reference... 

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