Bond, homopolar metallic like

The other complicated structures come at the ends of the groups in the periodic table, and as we have said they correspond to something more like homopolar bonds than metallic bonds. We have already commented on germanium and tin (the so-called gray modification of tin), which crystallize in the diamond structure, corresponding to the four homopolar bonds which they could form. They are of course very different from diamond in their properties, though silicon is between a... 

It is, nevertheless, not possible to classify organometallic compounds strictly into different types such as homopolar and heteropolar, since the physical and chemical properties of these compounds alter continuously within a given Period or Group. In a given main Group the polarity of the metal-carbon bond and thus the salt-like character of the compounds increase slowly from top to bottom, and in a given Period from left to right. 

For earlier data, see Boron Compounds 3rd Suppl. Vol. 1, 1987, pp. 20/49, Boron Compounds 2nd Suppl. Vol. 1, 1983, pp. 12/41, and Boron Compounds 1st Suppl. Vol. 1, 1980, pp. 8/70. The current treatment does not deal with free and complexed [BH4] separately, since is becoming clear that the distinction between the two species may not exist. Even in salts of the most electropositive metals and [BH4] , there is always some metal-hydride interaction (see Boron Compounds 3rd Suppl. Vol. 1,1987, Section 2.3.4.5, p. 27). In most cases the homopolar portion is not known, though some investigations have been done. In the original literature, the alternative formulations M BH4, M (BH4), M [BH4] and M"(BH4)2, M"[BH4]2, etc. are often used for the same compound. For systematic and practical reasons, and both to conform most closely with the real bonding situation and to not be too far from the common practice in the current literature, we have chosen to write M [BHJ (e.g., [(n-C4H9)4N]-[BH4], Na[BH4], and Li[BH4]), but M"(BH4)2, M" (BH4)3, etc. (e.g., Ca(BH4)2 and Al(BH4)3). Thus, whether brackets or parentheses, the formula does not indicate the degree of salt-like or homopolar character for a particular compound. 

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