SYNTHESIS OF D2 COMPLEXES THAT CONTAIN [W(NAR)2] AND [RE(NAR)2] CORES, SCF-X-ALPHA-SW CALCULATIONS, AND A DISCUSSION OF THE MCP2/M'(NR)2 ISOLOBAL RELATIONSHIP

W(NAr)2Cl2(dme) is reduced by sodium amalgam in the presence of a phosphine in THF to give W(NAr(2(PMe2Ph)2 (1a) or W(NAr)2(PMePh2)2 (1b). An X-ray study of 1b showed that the geometry about tungsten is nearly tetrahedral with P-W-N bond angles of 107-degrees (average), a N-W-N angle of 128.8(2)-degrees, and a P-W-P angle of 94.60 (6)-degrees; the W=N bond lengths are identical and consistent with their being pseudotriple bonds, and the W=N-Ar bond angles are almost linear (166 and 172-degrees). W(NAr)2(PMe2Ph)2 reacts readily with ethylene to form orange W(NAr)2(PMe2Ph)2(eta2-C2H4), with acetylene to form dark red W(NAr)2(PMe2Ph)2(eta2-C2H2), and with (trimethylsilyl)acetylene to form W(NAr)2(PMe2Ph)(eta2-HC-CSiMe3). Addition of norbornene to la gives W(NAr)2(PMe2Ph)(eta2-norbornene), while addition of acetone, propionaldehyde, or cyclopentanone gives orange-yellow W(NAr)2(PMe2Ph)(eta2-OCMe2) (6a), W(NAr)2(PMe2Ph)(eta2-OCHEt), or W(NAr)2(PMe2Ph)(eta2-cyclopentanone). An X-ray study showed 6a to have the proposed pseudotetrahedral structure in which W, P, O, and C all lie in the plane that bisects the N-W-N angle (mean deviation of 0.034 angstrom) and in which the C-O distance is 1.39(1) angstrom. Reduction of Re(NAr)2Cl3(PY) (Ar = 2,6-C6H3-i-Pr2) with zinc or sodium amalgam in the presence of excess pyridine in tetrahydrofuran yields brown, microcrystalline Re(NAr)2(PY)2Cl (7). Compound 7 reacts with 2-butyne to yield Re(NAr)2(eta2-MeC=CMe)Cl and with phosphines to yield Re(NAr)2(L)(py)Cl (L = PMePh2 (9a) or PPh3 (9b)). Addition of thallium tetrafluoroborate to 9a in the presence of PMePh2 yields [Re(NAr)2(PMePh2)2]-[BF4]. Reduction of 7 in the presence of PMePh2 gave Re(NAr)2(PMePh2)2H in low yield. Reduction of Re(NAr)2(CH2-t-Bu)Cl2 in THF in the presence of pyridine yields green Re(NAr)2-(CH2-t-Bu)(py)2 (12). Compound 12 reacts rapidly with 2-butyne to give Re(NAr)2(CH2-t-Bu)(eta2-MeC=CMe) and with acetone, pivaldehyde, or norbornene to give Re(NAr)2(CH2-t-Bu)(Bu)(eta2-OCMe2), Re(NAr)2(CH2-t-Bu)(eta2-OCH-t-Bu), or Re(NAr)2(CH2-t-Bu)(eta-2-norbornene), respectively. Reduction of Re(NAr)2(CH2-t-Bu)Cl2 in the presence of PMe2Ph gave highly crystalline, diamagnetic Re(NAr)2(CH2-t-Bu)(PMe2Ph)(14) in 80% yield. Compound 14 reacts with ethylene, acetylene, and carbon monoxide to give Re(NAr)2(CH2-t-Bu)(PMe2Ph)(eta2-C2H4) (15a), Re(NAr)2(CH2-t-Bu)(PMe2Ph)(eta2-C2H2) (15b), and Re(NAr)2(CH2-t-Bu)(CO)(PMe2Ph) (15c), respectively. Compound 15b rearranges readily to the ''alkylidene'' complex, Re(NAr)2-(CH2-t-Bu)(CHCHPMe2Ph). An analysis of the bonding in hypothetical W(NH)2(PHS)2 by the SCF-Xalpha-SW method shows that the HOMO is a metal=centered nonbonding orbital (7a1). As the N-M-N angle increases, the four nondegenerate pi levels evolve into two sets of doubly-degenerate pi orbitals, in agreement with calculated changes for MCp2 complexes. Calculations on hypothetical [Re(NH)2(PH3)2]+ yielded similar results. The frontier orbitals of M(NR)2 complexes and their isolobal relationship with M'Cp2 complexes, supported by SCF-Xalpha-SW calculations, are discussed.