The reduction of water by CO to form CO2 and H-2 (Water-Gas Shift Reaction (WGSR)) is an important process catalyzed in homogeneous phase by metal carbonyls. Here we report a theoretical study on four mechanisms of the gas-phase mononuclear carbonyl complexes M(CO)(6) (M = Mo, W) catalyzed WGSR. The energetic span model (ESM) proposed by Shaik et al. has been applied to reveal the kinetic behavior of the four catalytic cycles. Our calculation results indicate that W(CO)(6) is a promising candidate for an improved WGSR catalyst. An accessible reaction mechanism for the binuclear species M-2(CO)(10) is also proposed. Mechanistic studies on the formation of and elimination from M-2(CO)(9)COOH- suggest a stepwise process with addition of water to the binuclear hydride species as the key step prompting elimination. Both bimetallic catalysts W-2(CO)(10) and Mo-2(CO)(10) show higher activity than mononuclear metal-based catalysts with W-2(CO)(10) considering as the most efficient catalyst for WGSR. To confirm whether the activity trends in WGSR can be understood in terms of the Hammer-Norskov d-band model in combination with the linear energy relations, we studied correlations between the turnover frequency (TOF) and the position of the d-band center (epsilon(d)) relative to the Fermi energy (E-F) of various metal complex catalyst. The results indicate that the epsilon(d) - E-F value is useful as a qualitative activity descriptor in catalysis of WGSR. The conclusions will be useful to design of the WGSR catalyst with high performance. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.