Schomaker-Stevenson rule

Pauling s revised Schomaker-Stevenson rule Rather, it is clear that steric repulsion alone is sufficient to explain why no methyl-bridged dimer forms the boron atom is too small to accommodate four methyl groups. In a hypothetical dimer (Fig. 3d) the eight Cb Q distances would be only about 2.70 A. In dimeric trimethylaluminum the corresponding distance is 3.23... 

A modified Schomaker-Stevenson rule for the prediction of single bond distances between p-block elements... 

Blom R, Haaland A (1985) A modification of the Schomaker-Stevenson rule for prediction of single bond distances. J Mol Struct 128 21-27... 

The molecular structures of the fluorinated methanes have been accurately determined by rotational spectroscopy (Table 9). The C-H bmid distances all fall in the narrow range from 108.4 to 109.1 pm. The C-F bond distance in CH3F is indistinguishable from the value predicted by the modified Schomaker-Stevenson rule, 138.2 pm, but decreases monotonically with increasing number of F atoms to 131.5 pm in 03 4. 

The dimeric cyclophosphazene molecule (F3PNMe)2 contains distorted trigonal bipyramids around each of the phosphorus atoms, with an axial-equatorial orientation of the planar four-membered cycle with respect to the bipyramids. Thus, in this case the rule does not hold that fluorine atoms ould be in axial positions in trigonal bipyramids of substituted fluorophosphoranes In such a structure the PN bonds are not equivalent, with axial bond distances approximating the Schomaker-Stevenson estimate of 1.75 A. The equatorial bond distances, 1.595 A, are considerably smaller than all the other PN bond distances we are aware of. 

The dative N Al bond in (CH3)3NA1(CH3)3, 210 pm, is 20 pm longer than the Al-N bond in crystalline AIN and 28 pm longer than the normal covalent N-Al bond distance predicted by the modified Schomaker-Stevenson (MSS) rule see Sect. 6. The dative P—>A1 bond in (CH3)3PA1(CH3)3 is about 23 pm longer than the covalent bond distance predicted by the MSS rule. 

Schomaker, V. and Stevenson, D.P. (1941). Some Revisions of the Covalent Radii and the Additivity Rule for the Lengths of Partially Ionic Single Covalent Bonds. J.Am.Chem.Soc., 63, 37-40. 

L. B. Kier, Quant. Struct.-Act. Relat., 5, 1, 7 (1985), Distinguishing Atom Differences in a Molecular Graph Shape index. [Covalent radii are from V, Schomaker and D. P. Stevenson,/. Am. Chem. Soc., 63, 37 (1941). Some Revisions of the Covalent Radii and the Additivity Rule for the Lengths of Partially ionic Single Covalent Bonds.]... 

Schomaker V, Stevenson DP (1941) Some revisions of the covalent radii and the additivity rule for the lengths of partially ionic single covalent bonds. J Am Chem Soc 63 37 0... 

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