Covalent radius values

When a specific value above is not known, the rule of Pauling (the van der Waals radius is approximately 0.76 Alarger than the covalent radius) is used. 

The strain energies of these five-membered heterocycles are relatively small with values of 23.5, 24.8 and S.SkJmoF estimated for tetrahydrofuran, pyrrolidine and tetrahy-drothiophene respectively (74PMH(6)199). The closeness of the values for the two former compounds reflects the almost identical covalent radii of oxygen (0.66 A) and nitrogen (0.70 A) atoms. The sulfur atom with a much larger covalent radius of 1.04 A causes a... 

The single-bond covalent radius of C can be taken as half the interatomic distance in diamond, i.e. r(C) = 77.2pm. The corresponding values for doubly-bonded and triply-bonded carbon atoms are usually taken to be 66.7 and 60.3 pm respectively though variations occur, depending on details of the bonding and the nature of the attached atom (see also p. 292). Despite these smaller perturbations the underlying trend is clear the covalent radius of the carbon atom becomes smaller the lower the coordination number and the higher the formal bond order. 

Using the carbon atom covalent radius 0.77 A and the covalent radii given in Figure 19-3, predict the C—X bond length in each of the following molecules CF<, CBr4, CI4. Compare your calculated bond lengths with the experimental values C—F in CF4 = 1.32 A, C—Br in CBr = 1.94 A, C—I in CI4 = 2.15 A. 

Other Covalent Radii. In Cu20 and Ag20 each metal atom is equidistant from two nearest oxygen atoms, the interatomic distances corresponding to the radius values 1.18 and 1.39 A for Cu1 and Agl with coordination number two. In KAg(GN)2, in which each silver atom is similarly attached to two cyanide groups1), the effective radius of Agl is 1.36 A. It has been pointed out to us by Dr. Hoard that the work of Braekken2) indicates the presence of strings —Ag—G=N—Ag—G... 

Values of the reciprocal of the covalent radius for ligancy 12 are shown in Fig. 3. It is seen that the points for each sequence can be represented by three curves. The first curve for each sequence represents the effect of the increase in valence from 2 to 6 and the corresponding increase in binding energy for C to Cr, Sr to Mo, and Ba to W. Each of these curves is extrapolated to a maximum at v — 8.3, corresponding to nine spd orbitals with 0.7 metallic electron. It is seen that for all three sequences the values deviate from the extrapolated curves. From Cr to Ni they are represented by a straight line, interpreted as corresponding to the constant value 6 for the metallic valence. 

In the course of the work it was found that the value assumed five years ago for the carbon double-bond covalent radius (obtained by linear interpolation between the single-bond and the triple-bond radius) is 0.02 A. too large in consequence of this we have been led to revise the double-bond radii of other atoms also. 

G for gallium. These values (see also in the next section) are consistent with the unpaired electron residing in a Ti-orbital. The stability of these compounds was attributed to the large size and electronic properties of the Si(f-Bu)3 substituents [26-28]. Computational data for the aluminum compound indicate an Al—Al distance of 2.537 A and a wide Al—Al-Si angle of 174.90° [26]. The longer distance for the aluminum species is a result of the larger covalent radius for this metal [18]. 

According to Gordy (1946), electronegativity is represented by the value of the potential resulting from the effect of the nuclear charge of an unshielded atom on a valence electron located at a distance corresponding to the covalent radius of the atom. 

A simple model for this compound with both the five-rings planar gives an O — O distance oi about bid A, while BbSlEGOBBS [3] value for the covalent radius of oxygen is 1 23 A. Consequently there must be... 

Octahedral Radii.—In pyrite (Fig. 7-8) each iron atom is surrounded by six sulfur atoms, which are at the corners of a nearly regular octahedron, corresponding to the formation by iron of 3d 24 4p bonds. The iron-sulfur distance is 2.27 A. from which, by subtraction of the tetrahedral radius of sulfur, 1.04 A, the value 1.23 A for the cPsp9 octahedral covalent radius of bivalent iron is obtained (Table 7-15). 

The value of the effective van der Waals radius of an atom in a crystal depends on the strength of the attractive forces holding the molecules together, and also on the orientation of the contact relative to the covalent bond or bonds formed by the atom (as discussed below) it is accordingly much more variable than the corresponding covalent radius. In Table 7-20 there are given the ionic radii of nonmet llic elements for use as van der Waals radii. They have been rounded off... 

It is noteworthy that the Be—O distance (1.60 A) is of the same order as that found for other coordination compounds of beryllium and leads to the value of 1.0—1.1 A for the covalent radius of beryllium in this type of coordination.26 Clearly arguments based on relative ionic radii are invalid. Thus the dihydrate of zinc oxinate has been shown to form a distorted tetrahedron with two long Zn—H20 bonds while the lengths Zn—O and Zn—N to the ligand are 2.05 and 2.06 A respectively, whence the zinc radius is 1.38 A. Clearly the use of an ionic radius (Zn2+ = 0.74 A) would be misleading. Similarly the Cu—N bond in compounds of Cu11 with ammonia and ethylenediamine (1.99,2.05,2.01 A) implies a radius of 1.3-1.4 A in these coordination compounds, a value considerably larger than the ionic radius of 0.7 A.23... 

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). 

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