The binding of pyridine by V(II) in aqueous solution shows evidence for the late onset of cooperativity. The K, governing formation of [V(py)](2+) (lambda(max) = 404 nm, epsilon(max) = 1.43+/-0.3 M-1 cm(-1)) was determined spectrophotometrically to be 11.0+/-0.3 M(-)1, while K-1 for isonicotinamide was found to be 5.0+/-0.1 M-1. These values are in the low range for 3d M2+ ions and indicate that V(II).py back-bonding is not significant in the formation of the 1:1 complex. Titration of 10.5 mM V(II) with pyridine in aqueous solution showed an absorption plateau at about 1 M added pyridine, indicating a reaction terminus. Vanadium K-edge EXAFS analysis of 63 mM V(II) in 2 M pyridine solution revealed six first-shell N/O ligands at 2.14 Angstrom and 4+/-1 pyridine ligands per V(II). UV/vis absorption spectroscopy indicated that the same terminal V(II) species was present in both experiments. Model calculations showed that in the absence of back-bonding only 2.0+/-0.2 and 2.4+/-0.2 pyridine ligands would be present, respectively. Cooperativity in multistage binding of pyridine by [V(aq)](2+) is thus indicated. XAS K-edge spectroscopy of crystalline [V(O3SCF3)(2)(PY)(4)] and of V(II) in 2 M pyridine solution each exhibited the analogous 1s --> 5E,(g)and 1s --> T-5(2g) transitions, at 5465.5 and 5467.5 eV, and 5465.2 and 5467.4 eV, respectively, consistent with the EXAFS analysis. In contrast, [V(py)(6)](PF6)(2) and [V(H2O)(6)]SO4 show four 1s --> 3d XAS transitions suggestive of a Jahn-Teller distorted excited state. Comparison of the M(II)-N-py bond lengths in V(II) and Fe(II) tetrapyridines shows that the V(II)-N-py distances are about 0.06 Angstrom shorter than predicted from ionic radii. For [VX2(R-py)(4)] (X = Cl-, CF3SO3-; R = 4-Et, H, 3-EtOOC), the E-1/2 values of the V(II)/V(III) couples correlate linearly with the Hammett sigma values of the R group. These findings indicate that pi back-bonding is important in [V(py)(4)](2+) even though absent in [V(py)](2+). The paramagnetism of [V(O3SCF3)(2)(py)(4)] in CHCl3, 3.8+/-0.2 mu(B), revealed that the onset of back-bonding is not accompanied by a spin change. Analysis of the geometries of V(II) and Fe(II) tetrapyridines indicates that the ubiquitous propeller motif accompanying tetrapyridine ligation may be due to eight dipole interactions arising from the juxtaposed C-H edges and pi clouds of adjoining ligands, worth about -6 kJ each. However, this is not the source of the cooperativity in the binding of multiple pyridines by V(II) because the same interactions are present in the Fe(II)-tetrapyridines, which do not show cooperative ligand binding. Cooperativity in the binding of pyridine by V(II) is then assigned by default to V(II)-pyridine back-bonding, which emerges only after the first pyridine is bound.