  We report the clear detection of C$_2$ and of abundant NH$_2$ in the first
prominently active interstellar comet, 2I/Borisov. We observed 2I on three
nights in November 2019 at optical wavelengths 4800--9300 \AA with the
Multi-Unit Spectroscopic Explorer (MUSE) integral-field spectrograph on the
ESO/Very Large Telescope. These data, together with observations close in time
from both 0.6-m TRAPPIST telescopes, provide constraints on the production
rates of species of gas in 2I's coma. From the MUSE detection on all epochs of
several bands of the optical emission of the C$_2$ Swan system, a rich emission
spectrum of NH$_2$ with many highly visible bands, and the red (1-0) bandhead
of CN, together with violet CN detections by TRAPPIST, we infer production
rates of $Q$(C$_2$) = $1.1\times10^{24}$ mol s$^{-1}$, $Q$(NH$_2$) =
$4.8\times10^{24}$ mol s$^{-1}$ and $Q$(CN) = $(1.8\pm0.2)\times 10^{24}$ mol
s$^{-1}$. In late November at 2.03~au, 2I had a production ratio of
C$_2$/CN$=0.61$, only barely carbon-chain depleted, in contrast to earlier
reports measured further from the Sun of strong carbon-chain depletion. Thus,
2I has shown evolution in its C$_2$ production rate: a parent molecule
reservoir has started sublimating. At $Q$(NH$_2$)/$Q$(CN) = 2.7, this second
interstellar object is enriched in NH$_2$, relative to the known Solar System
sample.
