The syntheses, spectroscopic properties, crystal and molecular structures, and bonding of several cyclopentadienyl or pentamethylcyclopentadienyl niobium and tantalum calixarene complexes are described. Included are the mononuclear complexes [Cp*Ta(calix[4]arene)] (1), [Cp*Ta(p-tert-butylcalix[4]arene)] (2a), and [CpNb(p-tert-butylcalix[4]arene)] (3), in which the electron-deficient metal centers are protected in the exo-calix position by sterically bulky ligands. These complexes are well suited to bind small-molecule guests within their macrocyclic pockets, as demonstrated in the solid state and in solution for the complexes [Cp*Ta(p-tert-butylcalix[4]arene)]. toluene (2a . toluene), [Cp*Ta(OH2)(p-tert-butylcalix[4]arene)] (2b), and [Cp*Ta(NCCH3) (p-tert-butylcalix[4]arene)] (2c). In 1, the calixarene cavity is elliptically shaped as a result of two sets of trans-Ta-O-Ph interbond angles, one wide and one narrow, a geometry that nicely accommodates the toluene guest molecule in 2a . toluene. In 2b and 2c, however, the cavity is cylindrical. Insights into these differences and bonding in these compounds are provided by Fenske-Hall molecular orbital calculations on [Cp*Ta](4+)[calix[4]arene](4-), (1), a model of 1 with C-4V symmetry, [Cp*Ta(OH2)(calix[4]arene)], and [Cp*Ta(NCCH3)(calix[4]arene)]. Crystal data : 1, monoclinic, P2(1)/c, a = 11.270(3) Angstrom, b = 18.168(4) Angstrom, c = 15.655(2) Angstrom, beta = 110.86(1)degrees, Z = 4, R = 0.028, R(w) = 0.034; 2a . 2toluene . pentane, monoclinic, P2(1)/c, a = 13.678(2) Angstrom, b = 19.404(2) Angstrom, c = 23.230(2) Angstrom, beta = 94.790(9)degrees, Z = 4, R = 0.030, R(w) = 0.038; 2b, monoclinic, P2(1)/n, a = 11.168(3) A, b = 22.211(6) Angstrom, 20.957(5) A, beta = 91.17 (1)degrees, Z = 4, R = 0.031, R(w) = 0.037; 2 . 2.5CH(2)Cl(2), triclinic, P ($) over bar 1 a = 20.21(1) Angstrom, b = 22.70(1) Angstrom, c = 13.346(4) Angstrom, alpha = 95.10(2)degrees, beta = 97.62(3)degrees, gamma = 108.67(4)degrees, Z = 4, R = 0.036, R(w) = 0.040.