  The crystal structure of boron doped superconducting
MgC_{1-x}{11}^B_{x}Ni_{3}, studied by powder neutron diffraction, is reported.
The solubility limit of boron is determined to be approximately x=0.16. The
unit cell expands from a = 3.81089(2) to 3.81966(2) Angstroms as x increases
from x=0 to x=0.155. Boron ({11}^B) doping decreases Tc with increasing x: from
7.09K (x=0) to 6.44K (x=0.155).
