Give the oxidation number for the central atom or ion in each of the following complexes.
(a) $\left[\mathrm{Al}(\mathrm{OH})\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\right](\mathrm{OH})_{2}$
(b) $\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]$
(c) $\mathrm{Na}_{3}\left[\mathrm{CoCl}_{6}\right]$
(d) $\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$
(e) $\mathrm{K}\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{2}(\mathrm{OH})_{2} \mathrm{Cl}_{2}\right]$
(f) $\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right] \mathrm{I}$