agreement with the qualitative trend in electronegativities reported earlier.26*27 A final point of interest relates to the sensitivity of the various series of mercurials, RHgR', to alkyl substitution. In Figure 5, the ionization potentials of a series of dialkylmercury compounds R-Hg-R' have been plotted against the oxidation potentials of the corresponding Grignard reagent (representing a common scale). The slopes decrease in the order R'HgMe > R'HgEt > R'Hg-i-Pr > R'Hg-t-Ru. We attribute this result to the fact that partial delocalization of charge by the first R group in the molecular ion diminishes the effect of the second group R'; or, stated alternatively, there is less charge remaining to be delocalized. The apparent intercept at about 7.2 eV (disregarding the experimental scatter in the highly substituted compounds) represents a limiting example, in which one group, extremely effective at delocalizing charge, completely nullifies the effect of the second group. Acknowledgment.X-Band ESR spectra of nine reduced heteropoly anions (heteropoly blues) have been recorded at temperatures between 4.2 K and room temperature. The spectrum of PM0~Mo11040~-is isotropic (g = 1.945) at all temperatures and exhibits hyperfine structure (six lines) below 64 K, with ( u ) = 55 X cm-'. The intensity of the hyperfine spectrum relative to the central (nonmagnetic) line increases as the temperature is lowered. The spectra of five reduced 1:12 tungstates are rhombic at low temperatures, with gl = 1.848-1.852, g2 = 1.805-1.827, and g3 = 1.759-1.79. Above 50-80 K, depending on the anion, the spectra appear isotropic' due to line broadening. The spectra of reduced ~~P 2 W 1 8 0 6 2~ and a-As2W180& remain rhombic at higher temperatures, with gl = 1.905, g2 = 1.85, and g3 = 1.81, The spectrum of the reduced / 3 isomer of P2w180626-is isotropic (g = 1.852) at all temperatures down to 13 K. All of the ESR spectra are discussed in terms of weakly trapped Mo(V) or W(V) valences. The rate of intraionic electron delocalization in PMoVMo110404-at 6 K must be greater than ca. lo8 s-l implying a much lower activation energy than predicted by the Hush model for mixed valence compounds. The source of the rhombic crystal field for tungsten atoms in the 1:12 (Keggin) structure is discussed and the major differences between the spectra of 1:12 and 2:18 anions are rationalized. Two-electron heteropoly blues appear diamagnetic, probably through antiferromagnetic coupling of electrons localized on adjacent metal atoms.A Cobalt-59, Indium-1 15, Gallium-69, Gallium-71, Bromine-79, Bromine-81, and