Controller design using pole placement

hi every one can any one help me if i have my system which is unstable and have following poles(eigenvalues) 0 -8.3+ 0*i 8.3+ 0*i -10 +180*j -10 - 180*j -6

and also my system's controllability rank is 5 (which is less than 6) can i implement controller using pole placement (and if not then i using lqr) it solves the system but the question is why pole placement is not working on ,,,because of less controllability ?