Series of two-coordinate and quasi-two-coordinate transition-metal complexes: synthesis, structural, and spectroscopic studies of sterically demanding borylamide ligands -NRBR'2 (R = Ph, R' = Mes, Xyl; R = R' = Mes), their lithium salts, Li(Et2O)2NRBR'2, and their transition-metal derivatives, M(NPhBMes2)2 (M = Cr, Co, Ni), Co(NPhBXyl2)2 and M(NMesBMes2)2 (M = Cr .fwdarw. Ni)