The present issue, Volume 2 of "Boron Compounds" 4th Supplement of the
Gmelin Hand- book, updates the previous issues by reporting the
literature on boron-oxygen systems published up to 1988. For some
important recent developments literature is covered through mid-1992;
this concerns, for example, the compounds -Ba3[B306h and U[B305J which
became of interest as materials with nonlinear optical properties. The
volume directly com- plements the earlier "Boron Compounds" 3rd
Supplement Volume 2. In the original literature, alternative
formulations are frequently used for the same com- pound. This is
especially true for many borates. Often, these species are neither
completely heteropolar nor covalent, and an experimentally based
decision has not been made. Hence, the use of brackets does not
necessarily reflect a truly salt-like character. Volume 1 (systems with
hydrogen and noble gases) of this particular supplement will be
published subsequently, whereas Volume 3a (boron and nitrogen), Volume
3b (boron and nitrogen, boron and fluorine), and Volume 4 (boron
compounds containing Cl, Br, I, S, Se, and Te, as well as a section
containing carboranes) have already been published. All volumes of the
4th supplement will be augmented by a formula index. The IUPAC
nomenclature is generally adhered to; thf means tetrahydrofuran; and
occa- sionally additional abbreviations for compounds are explained in
the text. Positive signs for chemical shifts of the NMR signals
indicates downfield shifts from the references, usually internal
(CH3)4Si for olH and 013C with others being specified.