Does the magnesium 2+ ion prefer an octet or more?

In organic chemistry I was told that the magnesium ion which is part of Grignards is coordinated to several ether molecules. The overall structure around the magnesium is *tetrahedral*, which implies that *two* ether molecules have coordinated to the magnesium center. The professor said something about fulfilling magnesium ion's octet with this configuration. 


In inorganic chemistry and biochemistry however I learned about hexacoordinated magnesium - i.e. $\ce{Mg(H2O)6}$. Magnesium ion tends to form octahedral complexes; this implies a coordination number of 6.


So - 


1) Does magnesium ion adopt both these configurations? Can it be found as part of tetrahedral and octahedral complexes? 


2) Which is preferred? I suspect this answer depends on a variety of factors. 


