Textbook Problem 2E. 19

[Alvin Lai 3J]

Predict the shapes and estimate the bond angles of (a) the thiosulfate ion, S2O32−; (b) (CH3)2Be; (c) BH2−; (d) SnCl2.

For (b), do you predict the shapes with the carbons as the central atoms?

[Simrah_Ahmed1J]

Actually in this problem part b you have to take Be as the central atom because you're not seeing the whole molecule written as C₂H₆Be, but it is written (CH₃)₂Be, so you are taking CH₃ as methyl group not as separate Carbons and Hydrogens.
So you put aside the usual rule that you choose the central atom based on electronegativity of one atom, you are evaluating it as the electronegativity of the whole CH₃ methyl group vs the Be. So in this case you will find that Be has to be the central atom in this molecule.

[Lisa Wang 3B]

(a) the thiosulfate ion, S2O32−; (b) (CH3)2Be; (c) BH2−; (d) SnCl2

for part b I like to think of it almost like 2 CH3's sandwiching the Be. So the central atom would be Be because of its low ionization energy. For part c Boron would be the central atom, and for part d Sn would be the central atom.

[Bailey Giovanoli 1L]

I like to have in my mind that whenever I see parenthesis, I cannot separate the molecule. This will help when you begin to see more complicated formulas.