Thermochemistry of organic and elementoorganic species. Part III. The enthalpies of formation of the hydrides of main group elements

For the first time, the thermochemistry of main group elements and their hy drides has been analyzed separately, in the framework of 5 groups - molecul es EHn (n = 1-4) and their fragments: free radicals.EHn-1 (n = 1-4), carben e-(:CH2)-like :EHn-2 (n = 2-4), carbin-(:CH.)-like :EHn-3.(n = 3, 4) and ca rbon-(:C:)-like :E: species. The relative thermodynamic stability for each group of molecular fragments was characterized using isodesmic reactions. I t was found that thermodynamic stability of the free radicals EHn-1 (n = 1- 4) increases with i) the growth of polarizability of the core element, ii) the fall of its electronegativity, iii) the appearance of lone electron pai rs, and iv) the increase of their number in 15 --> 16 --> 17 groups. For th e other types of species :EHn-2, :EHn-3 and :E: the polarizability of core element plays an overwhelming role in stabilizing the species. The polariza bility effect increases in the direction.EHn-1 < :EHn-2 < :EHn-3.< :E: for each of the elements. For group 14 elements C and Si, the influence of H -- > Me and H --> F replacements on the thermochemistry of each species was ex amined. As a result some new or corrected values for enthalpies of formatio n of the elements' hydride molecules and their fragments in the gas phase w ere deduced. Analysis of bond dissociation energies (BDEs) HnE-H in each gr oup of species displayed the trend of decreasing stability for lower Valent state for each of the main group elements. It was concluded that those BDE values can not be used as a support for estimates of the stabilization of lower valent states for some elements of groups 13-15. (C) 1999 Elsevier Sc ience B.V. All rights reserved.