In contrast to the benzene and naphthalene compounds (eta(6)-PhH)Mo(PMe3)(3) and (eta(6)-NpH)Mo(PMe3)(3), the anthracene complex (eta(6)-AnH)Mo(PMe3)(3) reacts with H-2 to undergo a haptotropic shift and give the eta(4)-anthracene compound (eta(4)-AnH)Mo(PMe3)(3)H-2. Density functional theory calculations indicate that the increased facility of naphthalene and anthracene to adopt eta(4)-coordination modes compared to that of benzene is a consequence of the fact that the Mo-(eta(4)-ArH) bonding interaction increases in the sequence benzene < naphthalene < anthracene, while the MO-(eta(6)-ArH) bonding interaction follows the sequence benzene > naphthalene approximate to anthracene.