Oxymercuration and demercuration mechanism There are two proposed mechanisms for Oxymercuration and demercuration.

The first mechanism is concerted (i.e. single transition state connecting reactants and products) characterized by an attack of the $\pi$-bond on the $\ce{Hg(OAc)2}$.

Why does mercuric acetate act as an electrophile in the first mechanism? It is in its most common 2+ oxidation state. Does this happen because mercury has empty $6d$ orbitals.

The second mechanism is a stepwise mechanism (i.e. contains at least one intermediate species along the reaction pathway between reactants and products) that is characterized by incorporating the first ionization of $\ce{Hg(OAc)2}$ followed by an attack of the nucleophilic $\pi$-bond.

$\ce{Hg(OAc)2}$ dissociates in water as follows:
$$\ce{Hg(OAc)2 + H2O&lt;=&gt;Hg+OAc + OAc-}$$

Which one is the actual mechanism?
The mechanism you are looking for is probably not known and might well be never known. According to Brown et. al. there is a rapid and reversible equilibrium between associated and dissociated reactants.[1] They tested this for cyclohexene, norbonene and apobornylene, but I believe it is fair to generalise in this matter.
$$\ce{R2C=CR2 + Hg(OOCCF3)2 &lt;=&gt; (F3CCOO)R2C=CR2Hg(OOCCF3)}$$

Due to this, even the formation of a mercurinium ion seems to be sometimes not observed. For more information please have a look at the mechanistic studies of Pasto and Gontarz,[2] where they focussed on the stereo selectivity of substituted cyclohexenes, and references within.

Whatever molecule you might assume acting as the electrophile, it is quite obvious, that the bond mercury oxygen bond is highly ionic, with most of the electron density located in the acetyl moieties, leaving a rather substantial positive charge at the mercury itself. The following structures were optimised at DF-BP86/def2-SVP and the charges displayed are natural charges from the Natural Bond Orbital analysis to illustrate the above point.





The lower structure of $\ce{Hg(OAc)2}$ is slightly more stable.




Herbert Charles Brown, Min-Hon Rei, Kwang Ting Liu, J. Am. Chem. Soc., 1970, 92 (6), 1760–1761.
Daniel J. Pasto, John A. Gontarz, J. Am. Chem. Soc., 1971, 93 (25), 6902–6908.