  Nitrogen is the dominant impurity in the majority of natural and synthetic
diamonds, and the family of nitrogen vacancy-type ($\mathrm{N_{n}V}$) defects
are crucial in our understanding of defect dynamics in these diamonds. A
significant gap is the lack of positive identification of
$\mathrm{N_{2}V}^{-}$, the dominant charge state of $\mathrm{N_{2}V}$, in
diamond that contains a significant concentration of electron donors. In this
paper we employ isotopically-enriched diamond to identify the EPR spectrum
associated with $^{15}\mathrm{N_{2}V}^{-}$ and use the derived spin Hamiltonian
parameters to identify $^{14}\mathrm{N_{2}V}^{-}$ in a natural isotopic
abundance sample. The electronic wavefunction of the $\mathrm{N_{2}V^{-}}$
ground state and previous lack of identification is discussed. The
$\mathrm{N_{2}V}^{-}$ EPR spectrum intensity is shown to correlate with H2
optical absorption over an order of magnitude in concentration.
