And thus #"mercurous halide"# is #Hg_2X_2#; and #"mercuric halide"# is #HgX_2#. As is common with #"ous"# versus #"ic"# endings, the #"ic"# denotes the higher oxidation state.
In #HgX_2# the metal displays a formal #+II# oxidation state; in #Hg_2X_2# the metal displays a formal #+I# oxidation state. Where there is an #"element-element bond"#, as here in #Hg-Hg#, or in a #C-C# linkage, the 2 electrons of the bond are assumed to be distributed EQUALLY to the bound atoms. When we assign oxidation states for #"element-heteroelement bonds"#, the most electronegative atom gets the 2 electrons from the bond.
And thus for #HO-CR_3#, we get formally #HO^(-)# and #""^(+)CR_3#. The ipso carbon in #""^(+)CR_3# has a FORMAL #C(+I)# oxidation state.
I write #"formal"# guardedly. Of course with such designations, the assignment of oxidation number is very much a formalism, a practice with little fundamental significance, but there is utility in assigning oxidation numbers for balancing redox reactions.
