Steric properties of crystallographically and computationally determined structures of linear palladium(0) and square planar palladium(II) complexes of di(tert-butyl)neopentylphosphine (P(t-Bu)(2)Np), tert-butyldineopentylphosphine (P(t-Bu)Np-2), and trineopen-tylphosphine (PNR,) have been determined. Structures of linear palladium(0) complexes show that steric demand increases as tertbutyl groups are replaced with neopentyl groups (P(t-Bu)(2)Np < P(tBu)Np-2< PNp3). In square planar palladium(II) complexes, PNp3 gives the smallest steric parameters, whereas P(t-Bu)Np-2 has the largest steric demand. The change in the steric demand of PNp3 compared to P(tBu)(2)Np and P(t-Bu)Np-2 results from a significant conformational change in PNp3 depending on the coordination number of the metal. The steric properties of these ligands were also probed by measuring the equilibrium constant for coordination of free phosphine to dimeric [(R3P)Pd(mu-Cl)Cl](2) complexes. Binding equilibria follow the same trend as the steric parameters for square planar complexes with PNp3 having the highest binding constant. In contrast to the normal trend, the neopentylphosphines show increased pyramidalization at phosphorus with increasing steric demand. We hypothesize that this unusual dependence reflects the low back side strain of the neopentyl group, which allows the ligand to be more pyramidalized while still exerting a significant front side steric demand.