In the title paramagnetic 19-electron neutral complex, W(BH(4))(C(21)H(24)N(2))(2)(NO)(2)], the W(0) atom is coordinated by two 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) carbene ligands, two NO groups and two H atoms of an eta(2)-tetrahydroborate ligand. Depending on the number of coordination sites (n) assigned to the BH4(-) ligand, the coordination geometry of the W atom may either be described as approximately trigonal-bipyramidal (n = 1) or as very distorted octahedral with the bridging H atoms filling two coordination positions (n = 2). In the latter case, the coplanar NO groups and bridging H atoms (r.m.s. deviation = 0.032 angstrom) form one octahedral plane, with mutually trans-oriented carbene ligands. In the crystal, molecules are connected via C-H center dot center dot center dot O interactions.