You might be able to find some relevant information in
Kielland, J. (1937). Individual Activity Coefficients of Ions in Aqueous Solutions. J. Am. Chem. Soc., 59(9), 1675–1678. doi:10.1021/ja01288a032
However, Co++ has very specific interactions with water that are not well-suited to implicit solvent treatment so you should proceed with caution.
Pacific Northwest National Laboratory
From: AlbertG [mailto:firstname.lastname@example.org]
Sent: Tuesday, June 26, 2012 10:23 PM
Subject: [apbs-users] effective Born Radius for cobalt ion
Dear APBS gurus,
I want to perform ion solvation calculations with cobalt2+ ion, but I can't find the cavity radius (or effective Born radius) for this cation. So far I used mainly Rashin&Honig (1985) as a reference for other calculations, but there is nothing about cobalt unfortunately. Could someone please provide with any references for cobalt Born radius?
With best regards,