In the new class of N-heterocyclic carbene (NHC) chelated ruthenium catalysts for Z-selective olefin metathesis, the nitrato-supported complex 3cat appears distinct from all the other carboxylato-supported analogues. We have performed DFT calculations (B3LYP and M06) to elucidate the mechanism of 3cat-catalyzed metathesis homodimerization of 3-phenyl-1-propene. The six-coordinate 3cat transforms via initial dissociation and isomerization into a trigonal-bipyramidal intermediate (5), from which two consecutive metathesis reactions via the side-bound mechanism lead to (Z)-PhCH2CHCHCH2Ph (major) and (E)-PhCH2CHCHCH2Ph (minor). In the overall mechanism, 3cat functions similarly to the pivalate-supported analogue 1cat. The substitution of a smaller nitrato group does not change the side-bound olefin attack mechanism for either the initiation or homocoupling metathesis. The chelation of the NHC ligand causes this class of Ru catalysts to favor the side-bound over the bottom-bound mechanism. The calculated energetics corroborate the experimental observation that 3cat is somewhat more active than 1cat in catalyzing the homodimerization of 3-phenyl-1-propene.