Single Bonds of Coordination Compounds

[Clarissa Cabil 1I]

When drawing Lewis structures of coordination compounds, at a review session I attended, the UA mentioned how the transition metal (central atom) will only have single bonds around it.

In the example, [HgF2(OH2)2]+, the oxygens only had a single bond with Hg and a lone pair. Why couldn't the oxygens have a double bond with Hg? Is it because Hg is a Lewis acid so it accepts electrons?

[ian_haliburton_1f]

When coordinating, the oxygen in the water molecule is making three single bonds, making its formal charge already positive. It will not trade its remaining lone pair for another bond as its formalcharge would increase even more.