Contradictory Properties of Mercuric Cyanide
Mercury cyanide behaves like Mercury chloride $\ce{HgCl_2}$. Both are soluble in water, and both do not dissociate in water. These compounds are not salts. The bonds $\ce{Hg-Cl}$ or $\ce{Hg-CN}$ are more covalent than ionic. When the atom Hg is included in compounds it often behaves as a non-metal.  

This is due to a relativistic effect. In all elements with atomic number higher than $70$, the outer electrons move at a speed that is not far from c, the velocity of light in vacuum. Of course everybody knows that electrons do not move like planets in the atom. But the relativistic corrections are significant "as if" the electrons were moving. And one of the effect due to relativity is that the length decreases at velocities going near to c. Pekka Pyykko has shown that this entails a contraction of the relativistic electron orbitals. And this contraction is proportional to the sum of the two quantum number n and l. 

Mercury has the configuration {$\ce{Xe}$} $\ce{4f^{14} 5d^7 6s^2}$. The three external orbital have a sum n+l equal to $8$ for $4$f, $8$ for $5$d, and $7$ for $6$s. So the electron $4$f and $5$d (with n+l = $8$) are so much contracted that they nearly disappear in the internal core of electrons. It must be nearly the same for the $6$s electrons. And the mercury atom looks a bit like Xenon when seen from outside. That is why it is "nearly" a noble gas : its boiling point is rather low with respect to its neighboring atoms. 

All the chemical properties of Mercury are due to its two $6$s electrons which are rather difficult to extract, as they are partly included in the inner core. These electrons "prefer" making covalent bonds rather than ionic bonds, when included in molecules with $\ce{CN}$ group or $\ce{Cl}$ atoms. 

Pekka Pyykko, Relativity and the Periodic System of Elements, Accounts of Chemical Research, Vol. 12, No. 8, (1979), p. 276 - 281.