The syntheses, properties, and characterization of several new complexes, Re(CNtBu)(3)(PCy(3))(2)H (1a), Re(CNtBu)(3)(PR(3))(2)Cl [R = Cy, Ph (2a, 2b)], the 17-electron [Re(CNtBu)(3)(PCy(3))(2)Cl](+) (3a), the dihydrogen complex, [Re(CNtBu)(3)(PR(3))(2)(H-2)](+) [R = Cy, Ph (4a, 4b)], the coordinatively unsaturated [Re(CNtBu)(3)(PCy(3))(2)](+) (5a), and [Re(CNtBu)(4)(PCy(3))(2)](+) (6a) are reported. In addition, spectroscopic evidence for the dinitrogen complex [Re(CNtBu)(3)-(PCy(3))(2)(N-2)](+), and the dihydrogen complex [Re(CNtBu)(5)(H-2)](+) is presented. Thermodynamic parameters have been obtained for the equilibrium system, Re(CNtBu)(3)(PCy(3))(2)Cl (2a) + H-2 reversible arrow [Re(CNtBu)(3)(PCy(3))(2)(H-2)]Cl (4a). H-1 and P-31{H-1} NMR studies (CD2Cl2) over the temperature range 286-316 K afford values of Delta H degrees = -18.0 +/- 0.7 kcal/mol, Delta S degrees = -44 +/- 2 eu, and Delta G degrees(298) = -4.8 +/- 1.3 kcal/mol for this equilibrium. The complexes 4a and 4b are characterized as dihydrogen complexes. Their J(HD) (R = Cy, 30.3 Hz; R = Ph, 30.9 Hz) and T-l(min) values of 8 ms (300 MHz) are consistent with H-H distances of ca. 0.80 Angstrom. [Re(CNtBu)(5)(HD)](+) is also characterized as a dihydrogen complex based on a J(HD) of 33.4 Hz. This complex could not be isolated at room temperature, even with noncoordinating counteranions, due to the lability of the H-2 ligand. [Re(PMe(3))(5)HD](+) shows no H-D coupling and is characterized as a dihydride complex. The unsaturated complex [Re(CNtBu)(3)(PCy(3))(2)](+) (5a) is found to undergo a dynamic isonitrile rearrangement on the NMR time scale. H-1 NMR spin saturation transfer studies over a temperature range of 254-297 K afford the activation parameters Delta H double dagger = 11.4 +/- 0.8 kcal/mol, Delta S double dagger = -17.6 +/- 1.6 eu, and Delta G double dagger(298) = 16.6 +/- 1.2 kcal/mol. The molecular structures of 2a and 3a have been determined by X-ray crystallography. 2a shows an unusually long Re-Cl bond distance of 2.596(2) Angstrom.