Detailed herein are synthetic, spectroscopic and reactivity studies for two isolable four-coordinate iridium(I) monohydride complexes of the simple formulation HIrL(3). Such complexes have been postulated as reactive species in several transformations, but definite evidence for their existence has remained elusive. To stabilize these complexes, the methyleneadamantyl substituted phosphine ligand P(CH(2)(1)Ad)(i-Pr)(2) (abbreviated L(mAd)) was employed because of the resistance of the adamantane cage toward cyclometalation reactions. Treatment of the dihydride-chloride complex, H(2)IrCl(L(mAd))(2) with PhMgBr under N(2) afforded the square planar complex HIr(N(2))(L(mAd))(2). Contrastingly, treatment of H(2)IrCl(L(mAd))(2) with Li[HBEt(3)] under N(2) generates the trihydride complex H(3)Ir(L(mAd))(2), which possesses an agostic interaction between the L(mAd) ligand and the Ir center. Dissolution of HIr(N(2))(L(mAd))(2) in Et(2)O or C(6)D(12) rapidly establishes an equilibrium mixture with the cyclometalated complex H(2)Ir(kappa(2)-P,C-L(mAd))(L(mAd)). Despite the equilibrium between HIr(N(2))(L(mAd))(2) and H(2)Ir(kappa(2)-P,C-L(mAd))(L(mAd)), addition of 2 equiv of H(2) or 1 equiv of H(2)O to the mixture cleanly generates the pentahydride complex H(5)Ir(L(mAd))(2) or the dihydride hydroxide complex H(2)Ir(OH)(L(mAd))(2), respectively. Sequential addition (n)BuLi and 12-crown-4 (12-c-4) to a HIr(N(2))(L(mAd))(2)/H(2)Ir(kappa(2)-P,C-L(mAd))(L(mAd)), mixture provides the salt [Li(12-c-4)(2)][HIr(kappa(2)-P,C-L(mAd))(L(mAd))], which contains another four-coordinate Ir(I) monohydride. (31)P{(1)H} NMR studies provide evidence that four-coordinate HIr(N(2))(L(mAd))(2) is deprotonated en route to [Li(12-c-4)(2)][HIr(kappa(2)-P,C-L(mAd))(L(mAd))]. [Li(12-c-4)(2)][HIr(kappa(2)-P,C-L(mAd))(L(mAd))] deprotonates both H(2)N(2,6-(i-Pr)(2)C(6)H(3)) and HOC(6)F(5) under an N(2) atmosphere to regenerate HIr(N(2))(L(mAd))(2)/H(2)Ir(kappa(2)-P,C-L(mAd))(L(mAd)) equilibrium mixtures.