Single crystal X-ray diffraction studies have been carried out on the PF3, P(OEt)(3), and PMe(3) monoadducts of the open titanocene, Ti(2,4-C7H11)2 (C7H11 = dimethylpentadienyl), and on the PF3 adduct of the corresponding vanadium compound. The titanium (vanadium) PF3 adduct crystallizes in the monoclinic space group P2(1)/m (No. 11) with a 8.130(2) (8.051(2)) Angstrom, b = 14.277(5) (14.313(4)) Angstrom, c = 7.426(2) (7.323(1)) Angstrom, beta = 116.17(2)degrees (115.90(2)degrees), and V = 773.59 (758.97) Angstrom(3) for Z = 2. re compound Ti(2,4-C7H11)(2)PMe(3) crystallizes in the monoclinic space group P2(1)/c (No. 14) with a = 7.889(4) Angstrom, b = 15.312(7) Angstrom, c 15.072(7) Angstrom, beta = 101.81(4)degrees, and V = 1782.1 Angstrom(3) for Z = 4, while the P(OEt)(3) adduct crystallizes in the tetragonal space group I (4) over bar (No. 82) with a = b = 23.406(4) Angstrom, c = 8.406(2) Angstrom, and V = 4605.1 Angstrom(3) for Z = 8. The studies reveal Ti-PX(3) bond lengths of 2.550(2), 2.472(4), and 2.324(1) Angstrom for X = Me, Oft, and F, respectively, and a V-PF3 bond length of 2.246(1) Angstrom. A comparison with previously reported Ti-PX(3) binding energies demonstrates that while the Ti-PMe(3) bond is longer than the Ti-P(OEt)(3) bond, it is nonetheless significantly stronger, and the Ti-PX(3) bond lengths in these species seem to be determined primarily by the electronegativity of the phosphine substituents X. It can be expected that such situations may be fairly common in other classes of compounds.