Covalent radii of the elements

Fig. 4.—Covalent radii of the elements of the very long period.

What general trends are noticeable across the Periodic Table in the values of (a) the first ionization energies, (b) the first electron attachment energies, and (c) the covalent radii of the elements ... 

The dependence of the covalent radii of the elements on atomic number is shown in Figure 7-1. The relation is a simple one for... 

TABLE 11. COVALENT RADII OF THE ELEMENTS (IN ANGSTROM UNITS) ... 

Of the monomeric metal carbonyl hydrides [HMn(CO)s,H2Fe(CO)4,HCo(CO)4], only HMn(CO)s has been analyzed crystallographically 10 4S). The lack of results for H2Fe(CO)4 and HCo(CO)4 is probably related to the fact that the compounds are thermally unstable. The neutron diffraction analysis of HMn(CO)s by La Placa and co-workers45) produced a Mn—H distance of 1.60(2) A. This result, together with the earlier measurement of the Re—H distance of 1.68(1)A in K2ReH9, provided conclusive proof that M—H distances in metal hydride complexes are normal (i.e., are consistent with the known covalent radii of the elements), and not anomalously short as suggested by some researchers (this controversy is reviewed in Refs. 1—3). 

Thilo (25) classified oxo acids of various elements into three groups on the basis of the stability of the oxo acid in aqueous solutions, as shown in Table I. Although Thilo placed the elements in order of their ionic radii, covalent radii of the elements are used in Table I. The most stable phosphate in aqueous solutions is the monomer, orthophosphate. However, isopoly acid anions of phosphorus, i.e., condensed phosphates, are generally stable in an approximately neutral aqueous solution at room temperature. The rate of hydrolysis of the condensed phosphates is very low under these conditions. On the other hand, polysulfates and polyarsenates are very rapidly hydrolyzed into their monomers in aqueous solutions. In an alkaline solution, vanadate anions are present as monomer, i.e., orthovanadate. When the pH of the solution is de- creased, the orthovanadate anions are successively polymerized to form polymers with medium degrees of polymerization. Although silicate anions behave similarly, a highly polymerized form of silicate anions,... 

FIGURE 2.21 Cov.ilent radii ot hydrogen and the p-block elements (in picometers). Where more than one value is given, the values refer to single, double, and triple bonds. Covalent radii tend to become smaller toward fluorine. A bond length is approximately the sum of the covalent radii of the two participating atoms. 

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... 

This disilaborane was an unexpected co-product in the synthesis of decaborane-alkylamine polymers. The Si2Bio cluster core consists of a distorted icosahedron in which the two silicon atoms occupy adjacent positions. The Si-Si interatomic distance is 2.308(2) A, which is slightly less than the Si-Si distance in organodisilanes (2.35 A) and the Si-B distances [2.017(3) to 2.116(3) A] are very close to the sum of the covalent radii of the two atoms (2.07 A). Further derivatives with disilaborane cluster geometry are known for the phenyl substituted compounds l,2-Ph2-doso-l,2-Si2B1oH1o and l-Me-2-Ph-doso-l,2-Si2BioHi0 [6, 7]. In addition to these disila-doso-dodecaborane clusters one example with two different group 14 elements as a part of the cluster core is known. In Scheme 3.3-2 the synthesis of this sila-stanna-doso-dodecaborate is shown. The structure of this heteroborate was determined in the solid state and the Si-Sn distance is 2.608(4) A (Scheme 3.3-2) [8]. 

Figure 1.6 The covalent radii of the main group elements of the 2nd, 3rd, 4th and 5th periods...

Values of the single-bond covalent radii of the nonmetallic elements are given in Table 7-2. These values, which were originally obtained... 

The cfleet of bond order is exemplified by the familiar shortening of Ihe carbon-carbon bond in ethane (1.543 A), graphite (1.4210 A), benzene 11.397 A), ethylene (1.353 A), and acetylene (1.207 A) as the bond order goes from I to 1 i to I j to 2 to 3. Bond orders affect the covalent radii of other elements similarly. 

A. This parallels the trend in covalent radii of these elements. 

We shall now consider some properties of M—X bonds (M = Ge, Sn, Pb) in comparison with Si—X and C—X. As the atomic number of M increases, these bond distances d (Table 2) become longer. It is caused by the increase in the covalent radius of the group 14 element as its atomic number rises. The d values of the Me3M—Me and Me3M—MMe3 bonds coincide to within 0.05 A with the sum of covalent radii of the atoms forming this bond. The Si—Cl bond distances in SiCLt are 0.15 A shorter than the sum of covalent radii of Si and Cl atoms. As the atomic number of M increases, the difference between the experimental d values in MCLj molecules and the expected ones (based on the sum of the... 

The Ge atom in germocanes is shifted from the equatorial plane toward the X substituent. Both axial bonds are elongated as predicted by the concept of the 3c-4e hypervalent bond. The D — Ge distance is larger than the sum of covalent radii of these elements but is still significantly shorter than the sum of their van der Waals radii (Table 1). 

The covalent radii of transition elements are subject to two additional effects that influence the values of ionic radii also. A large covalent radius for a given atom is favored by both a low oxidation number and a high coordination number. These two effects are independent neither of each other nor of bond order effects however, an adequate unified treatment of the interrelationships between bond number, coordination number, oxidation number, and bond distances for compounds of the transition metals is best postponed to a more advanced text. 

Except for helium, neon, and argon, all the elements in the Periodic Table form halides, often in several oxidation states, and halides generally are among the most important and common compounds. The ionic and covalent radii of the halogens are shown in Table 13-1. 

VSEPR concept, valence bond description and hybridization, molecular orbital description, bond energies, covalent and van der Waals radii of the elements, intermolecular forces... 

E2.10 Table 2.7 lists selected covalent radii of the main group elements. As we can see from the table, the trends in covalent radii follow closely the trends in atomic and ionic radii covered in Chapter 1 (Section L7(a)). Considering first the horizontal periodic trends we can see that the single bond covalent radii decrease if we move horizontally from left to right in the periodic table. Recall that the atomic radii decrease while the increases in... 

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