A systematic study of the neutron single-particle states in N=51 isotones has been performed. Final states in 89Sr, 91Zr and 93Mo have been populated using the (d,p) and (alpha,3He) single-neutron adding reactions. The Yale tandem provided beams of 15-MeV deuterons and 50-MeV alpha particles with the outgoing ejectiles momentum analysed using an Enge split-pole spectrometer. A supplementary measurement of the d(86Kr,p)87Kr reaction, at a beam energy of 10 MeV/u, was made in inverse kinematics using the HELIOS spectrometer. Outgoing protons execute a helical orbit in a uniform field before detection, before they return to axis, using a position-sensitive silicon array. Absolute cross sections were measured for states up to 4 MeV in excitation. The transferred angular momentum was identified through a comparison of angular distributions and the ratio of cross section between reactions. Relative spectroscopic factors were extracted through a DWBA analysis.The measured centroids for the single-particle energies of the s1/2, d3/2, d5/2, g7/2 and h11/2 orbitals were compared to calculations using a Gaussian central potential with and without the addition of a tensor interaction. Through this comparison it was deduced that the inclusion of a tensor interaction is required to explain the observed shifts in the single-particle energies of the d3/2 and g7/2 orbitals. This system provided an observation of a switch in behaviour, from repulsive to attractive and vice versa, of the effect of the tensor interaction. This occurs due to a change from j> to j< proton occupancy at Z=40.