The electron-donating and steric properties of Buchwald-type ligands ([1,1'-biphenyl-2-yl]dialkylphosphine; R-JohnPhos, where R = Me, Et, Pr-i, Cy, Bu-t) were determined. The pi-acidity and sigma-donating properties of the R-JohnPhos ligands were quantified using a Cotton-Kraihanzel analysis of the Cr-0(CO)(5)(R-JohnPhos) complexes. Somewhat surprisingly, the sigma-donating abilities of the R-JohnPhos ligands follow the trend Bu-t-JohnPhos < Et-JohnPhos < Pr-i-JohnPhos < Cy-JohnPhos << Me-JohnPhos. This ordering is proposed to arise from competition between the intrinsic electron-donating ability of the R groups (Me < Et < Pr-i approximate to Cy < Bu-t) and steric interactions (front and back strain) that decrease the electron-donating ability of the phosphine. X-ray crystallographic data of 22 metal complexes (general forms: trans-Cr-0(CO)(4)(PR3)(2), Pd-0(PR3)(2)(eta(2)-dba), and trans-Pd-II(Cl)(2)(PR3)(2)) were also analyzed to help explain the electronic trends measured for the R-JohnPhos ligands. The R-JohnPhos ligands are exceptionally sensitive to back strain in comparison to typical phosphines, and the strong sigma-donating ability of the Methyl-JohnPhos ligand is attributed to its ability to avoid both front strain and back strain. Consequently, the -PMe2 moiety allows for very short phosphorus-metal bond distances. Because of the sterically dominating o-biphenyl and close phosphorus-metal bond distances, MeJPhos maintains a large overall steric profile that is actually larger than that of CyJPhos as measured by percent buried volume (%V-bur). Overall, the -PMe2 moiety is a powerful way to incorporate strong sigma-donation into designer phosphines while retaining other advantageous structural and reactivity properties.