Two sets of molecular isomers, the iron monoisocyanide (FeNC) / iron monocyanide (FeCN) pair, and the hydroboron monoxide (HBO) / boron hydroxide (BOH) pair, are investigated with a variety of high-level ab initio techniques. The electronic structure of the FeNC and FeCN pair does not easily succumb to the coupled cluster ansatz, even when newly developed correlation-consistent polarized valence iron basis sets and inclusion of perturbative or partial iterative triple excitations are included in the wave function. Due to disparities between the one-electron properties obtained with multireference configuration interaction (MRCI) and coupled cluster methods, the coupled cluster wave function could be considered inapplicable, at least at the currently tractable coupled cluster excitation level. On the contrary, the accuracy of linear and bent ground state BOH MRCI geometries, dipole moments, and harmonic frequencies are spectacularly poor with a full valence active space, while coupled cluster methods perform well. Using a new variant of the equation-of-motion coupled cluster (EOM-CC) method to include partial triple excitations within excited state wave functions, seven linear and nine bent excited states of HBO and BOH are characterized. Adiabatic and vertical transition energies of low-lying BOH states will guide an experimental hypothesis that BOH, which has not yet been synthesized in any form, may be a suitable high energy-density material.