Group 17 elements diatomic compounds

Molecular orbital theory explains the formation of multiple bonds both in the case of transition metal complexes as well as diatomic molecules of main group elements and compounds of these elements. There is an analogy between E2 molecules of the elements of the second period of the Periodic Chart (and their compounds containing multiple bonds) and M2 molecules as well as dimeric transition metal complexes which possess metal-metal bonds. The molecules C2, N2, and O2 have ttV , and elec-... 

Ditellurides, also in the fifth period, seem quite analogous to distibines. Like tetraphenyldistibine (1) the red diphenylditelluride (56) does not associate in the solid state. The closest intermolecular Te---Te contact is 4.255 A, near the van der Waals separation of 4.40 A 61). On the other hand, di(p-methoxyphenyl)ditelluride (57), which has a brown-green metallic luster in the solid, has close intermolecular Te---Te contacts of 3.57 and 3.98 A (62). The ratio Te---Te/Te—Te is 1.32. Just as in the distibines the intermolecular bonding in ditellurides is sensitive to substitution. It is also interesting to note that the intermolecular interaction in ditellurides and dihalogens occurs normal to the metal-metal axis, as well as colinear as in distibines (63). Thus, it is clear that the intermolecular association shown by distibines is a general property of many of the diatomic like compounds of the heavier main group elements. 

The relationships between bond enthalpy, bond length and bond order which appear relatively simple in the case of a main group element such as carbon and its compounds, are more difficult to establish when the d-transition metal elements and their compounds are considered. Progress in establishing these relationships for metals is severely hindered by a lack of relevant thermochemical data. This paper reviews some of the more useful information that is available for diatomic molecules, for polynuclear binary carbonyls and for binuclear complexes of the d-transition elements. 

Because MINDO/3 did not meet Dewar s aims, Dewar and Thiel developed the MNDO (modified neglect of diatomic overlap) method. The MNDO method has been parametrized for nearly all the main-group elements and for Zn, Cd, and Hg. MNDO gives substantially improved results as compared with MINDO/3. For the same sample of C, H, O, N compounds used above for MINDO/3 errors, average absolute MNDO errors are 6.3 kcal/mol in heats of formation, 0.014 A in bond lengths, 2.8° in bond angles, 0.30 D in dipole moments, and 0.5 eV in ionization energies [M. J. S. Dewar and W. Thiel, J. Am. Chem. Soc., 99, 4907 (1977)]. 

All six of the possible diatomic compounds between F, Cl, Br, and I are known (Table VI) and, except for BrF and IF, which are too unstable with respect to disproportionation to permit isolation at room temperature, they can be prepared by direct combination of the elements X2 and Y2. The properties of the compounds tend to be intermediate between those of the pure, parent halogens. Most add to carbon-carbon double bonds (Section IX. C), and some are useful as nonaqueous solvents. Liquid ICl, in particular, dissolves the chlorides of Group lA to give highly conducting solutions. 

The Xe-Xe distance in this compound is 3.087(1) A, which holds the record for the longest recorded bonded distance between main-group elements. It is consistent with the simple considerations we can obtain from a molecular orbital energy diagram for a diatomic molecule, from which we can deduce a bond order of V2 for [Xe2]. Similar long bonds have only previously been encountered in the chemistry of heavy transition elements, such as the Re-Re bond [3.041(1) A] in [Re2(CO)io]. 

There is little evidence for 1 1 compounds between elements in this group under normal conditions. The diatomic van der Waals molecules, CaMg, SrMg and SrCa, however, have been synthesized by codepositing the atoms from separate sources with argon or krypton into solid matrices at 12 K. These low-T species are identified from their laser-induced fluorescence spectra. The ground-state spectroscopic data for these alkaline-earth dimers form a sensible series between the parent molecules Mg2, Caj and Sr2. ... 

The first column of the periodic table, Group 1, contains elements that are soft, shiny solids. These alkali metals include lithium, sodium, potassium, mbidium, and cesium. At the other end of the table, fluorine, chlorine, bromine, iodine, and astatine appear in the next-to-last column. These are the halogens, or Group 17 elements. These four elements exist as diatomic molecules, so their formulas have the form X2 A sample of chlorine appears in Figure EV. Each alkali metal combines with any of the halogens in a 1 1 ratio to form a white crystalline solid. The general formula of these compounds s, AX, where A represents the alkali metal and X represents the halogen A X = N a C 1, LiBr, CsBr, KI, etc.). 

Figure 5.2 Correlation of the hardnesses of the Group IV elements, and the associated isoelectronic III-V compounds, with their bond moduli. Room temperature data. For the elements, the molecular volumes refer to the diatoms C-C, Si-Si, Ge-Ge, and Sn-Sn.

---