Re: ? does nitrogen form NCl3 but not NCl5, whats the difference in behavior

Simply put, the answer is "yes, it is because of the d-orbitals" that 
phosphorus has available and nitrogen doesn't. That probably doesn't go
all the way to explaining the difference, though.

In essence, the orbitals available to atoms are grouped in increasing
order of energy. The (1s) orbital is at a lower or more negative energy
than the (2s,2p) grouping, which is in turn at a lower energy than the
(3s,3p,3d) grouping. This difference in energy between these orbitals is
sizeable and means that electrons in the (2s,2p) group do not occupy the
(3s,3p,3d) group. Without electrons in the orbital and the possibility
of using those atomic orbitals to form molecular orbitals, there is no
bonding.

Thus, nitrogen, with electrons only occupying the (2s,2p) group is limited
to four molecular orbitals and three bonds (the remaining one is a "lone
pair" of electrons). Phosphorus, on the other hand, has the possibility
of forming up to 9 molecular orbitals and can have a much richer 
coordination chemistry. 

Hope this answers your question.