Fractional charge Polar covalent bonds

Figure 4.6 The spectrum of bonding from ionic to covalent via polar covalent. The delta symbols shown for polar covalent bonding represent fractional charges on the two atoms... 

We can estimate the numerical trends more directly from the statement of Bent s rule based on (4.58). Noting that the Os—H bond of OsH4 is almost perfectly covalent (i.e., 49.6% polarization to Os, Qw — 0 in OsH4), we can expect for a substituted OsH3X compound that the charge Qx of a general monovalent X will vary from Qx = — 1 in the pure ionic case to Qx = 0 in the pure covalent case. As a crude measure of the fractional weightings /ion and /cov of ionic and covalent forms in (4.58), we can therefore write... 

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