  Experiments of Electron Spin Resonance (ESR) were performed on Co$% ^{2+}$
substituting Zn$^{2+}$ or Mg$^{2+}$ in powder samples of Zn$_2$(OH)PO$_4$ and
Mg$_2$(OH)AsO$_4$. The observed resonances are described with a theoretical
model that considers the departures from the two perfect structures. It is
shown that the resonance in the penta-coordinated complex is allowed, and the
crystal fields that would describe the resonance of the Co$^{2+}$ in the two
environments are calculated. The small intensity of the resonance in the
penta-coordinated complex is explained assuming that this site is much less
populated than the octahedral one; this assumption was verified by a molecular
calculation of the energies of the two environments, with both Co and Zn as
central ions in Zn$_2$(OH)PO$_4$.
