How many outer-shell d electrons are there in each of the following transition-metal ions?
re(iii) sc(iii) ru(iv) hg(ii)

1) The electron configuration of Re is [Xe] 4f14 5d5 6s2

The ion Re (3+) has lost 3 electrons, so its configuration is  [Xe] 4f14 5d4

Then, the number outer-shell d electrons is 4. 

2) Sc: [Ar] 3d1 4s2

Sc (3+) has lost 3 electrons -> [Ar] which has not d electrons.

Then, the number of d electrons is 0

3) Ru: [Kr] 4d7 5s1

Ru (4+) has lost 4 electrons -> [Kr] 4d4

Then, the number of outer-shell d electrons is 4

4) Hg: [Xe] 4f14 5d10 6s2

Hg (2+) has lost 2 electrons -> [Xe] 4f14 5d10

Then, the number of outer-shell d electrons is 10

"among the transition metal elements, manganese and zinc have a slight lowering of electronegativities. the reason behind this is that the nuclear charges of these elements are slightly weaker than the other transition metal elements due to the way their valence electrons are filled up. both manganese and zinc have their valence electrons filled up to the 4th shell with 1 pair of electrons (4s2) completely occupying it. although the electrons in the 3rd shell (3d) are yet to be paired since the outer most shell is filled up, the ability of the nucleus to attract electrons is lessened. hence,   the electronegativity (or the ability to attract electrons) is lessened."