Excited states in the proton-unbound nuclide Au-177 were populated in the Mo-92(Sr-88, p2n) reaction and identified using the Jurogam-II and GREAT spectrometers in conjunction with the RITU gas-filled separator at the University of Jyvaskyla Accelerator Laboratory. A strongly coupled band and its decay path to the 11/2 alpha-decaying isomer have been identified using recoil-decay tagging. Comparisons with cranked Hartree-Fock-Bogoliubov (HFB) calculations based on Skyrme energy functionals suggest that the band has a prolate deformation and is based upon coupling the odd 1h(11/2) proton hole to the excited 0(2)(+) configuration in the Hg-178 core. Although these configurations might be expected to follow the parabolic trend of core Hg(0(2)(+)) states as a function of neutron number, the electromagnetic decay paths from the strongly coupled band in Au-177 are markedly different from those observed in the heavier isotopes above the midshell. This indicates that a significant change in the structure of the underlying (A+) Hg-1 core occurs below the neutron midshell.