The tax bill emerging from Congress not only would cut corporate taxation nearly in half by 1986, but would bless many companies with far more tax cuts than they can use, creating a new market for corporate swapping of tax credits and deductions.

Rather than cutting corporate rates directly, the House and Senate have agreed to sweeping changes in taxation on business investment that are likely to wipe out a major area of corporate taxation for most companies. The value of the tax deduction would be eroded if inflation continues at a high rate, but if the inflation rate continues to drop, the change would permit virtually tax-free purchase of new machinery, equipment and facilities.

A second major change, in the tax rules on the leasing of equipment and other assets, is expected to benefit hard-hit firms in the auto, metals and airline industries, as well as many thriving firms. By opening up the leasing rules, Congress intends to permit widespread trading of tax credits and deductions between money-losing firms that can't use them and money-making firms that can.

Treasury officials say that without this change, unused tax deductions and credits would pile up on the books of unprofitable companies and make them prime candiates for corporate takeovers. "Revolutionary" is how some experts characterize the change in corporate tax credits, which may cost the Treasury $9.6 billion a year by 1986.

The magnitude of the overall reduction in corporate taxes, now estimated at more than $179 billion over the next six years, has been overshadowed by the debate over President Reaganhs personal tax cut plan. But the business-supported Tax Foundation organization calls the corporate tax changes "astounding" and says they would have been inconceivable only a few years ago.

The bill would cut business taxes nearly in half by five years from now, reducing the share of all federal taxes paid by corporations from 12.5 percent today to 7 percent.

Joseph Pechman, a tax expert with the Brookings Institution, says, "There won't be much left of the corporate tax, except for financial firms and some consulting firms and others that use labor more intensively. So long as a firm keeps growing, there would be no corporate tax on new investments."

This profound change, enacted after a little more than two years of concentrated lobbying and campaigning by a coalition of business groups and academic allies, is their answer to inflation and erratic economic growth.

Philip M. Caldwell, chairman of Ford Motor Co., which will be a major beneficiary of the reductions in taxes on new investment, said the passage of President Reagan's tax and budget programs provides a "remarkable opportunity" for recovery.

The vast majority of the tax savings for business arise from the fundamental changes in depreciation, the system businesses use to deduct money spent on capital assets that have long-term value, such as machinery.

In place of complex depreciation rules based on the "useful life" of an asset, Congress would create three main categories of assets. In the legislation, investments in industrial buildings generally would be deductible over the 10 years, machinery and equipment (and some facilities such as oil refineries) would be deductible over five years, and vehicles over three years.

In most cases, the new method would permit companies to deduct investment costs over fewer years, thereby reducing taxes and leaving them with more cash.

The political selling point of the new system was its promise of greater investments in new business plants and equipment, which supporters say will improve business productivity and profitability.

Charls Walker, tax lobbyist and economic adviser in President Reagan's presidential campaign, agreed that the accelerated depreciation rules are biased in favor of capital investment, and thus are likely to shift a greater portion of business wealth toward the area. "I call it a redressing of a bias that ran the other way for 20 years," he said.

"I think about capital investment the way Mark Twain did about good bourbon whiskey," Walker said. "Too much is barely enough."

But critics contend that the new system is likely to be wasteful, giving tax breaks to companies that aren't short of investment capital, and favoring some industries over others.

"Most economists would say the depreciation changes Congress made were not the right ones," contends Pechman, who favored a simpler, more even-handed approach proposed by House Democratic leaders in a bid to block the president's program.

Robert McIntyre of the labor-supported organization Citizens for Tax Justice, said the depreciation change "is in fact a targeted tax reduction directing huge tax subsidies to the oil industry, the petrochemical industry and others who have no need of such subsidies to meet their investment needs."

The change in the leasing rules was prompted by concern over imbalances in the distribution of tax savings from the new depreciation system.

Ford, for instance, already has $341 million in available tax credits from its huge financial losses in 1979 and 1980. Companies are permitted to "carry" losses backward or forward for a limited number of years.

Ford, facing another unprofitable year, is expected to pile up another $80 million to $100 million in tax credits in 1981. But its backlog of tax-deductible losses will simply accumulate for another year, losing value because of inflation.

The leasing provision would permit Ford to trade its tax credits to a profitable company that could use them to reduce its own tax burden -- a manufacturer of new robot auto-making machinery, for instance, which would lease its equipment to Ford in return.

Richard Bentley, Ford assistant comptroller, said his understanding of the new measure is that Ford can't use the $341 million in prior year credits for such a leasing swap. In can only use the $80 million-plus that is expected this year.

The value of the lease arrangement would disappear rapidly once Ford begins making profits once again, he said.

In addition to the changes in depreciation and leasing rules, the new tax bill would:

Allow companies to take a 25 percent tax credit on certain research and development spending.

Lower the maximum capital gains tax rate from 28 percent to 20 percent, retroactive to June 9.

Give small businesses a substantial tax break by permitting a limited amount of capital investment to be deducted immediately in the year of purchase rather than over a longer period through depreciation. Additionally, corporate rates would be reduced in 1982 and 1983 for small businessmen in the lowest two tax brackets.

Restore a more generous tax treatment of stock options for executives that existed before passage of the 1976 tax reform bill.

Provide a special break for trucking companies whose operating licenses lost commercial value because of trucking deregulation.

Cut in half the windfall profits tax on "new" oil first produced in 1979 from 30 percent to 15 percent over a four-year period.

Provide an additional investment tax credit for the rehabilitation of old industrial and commercial buildings -- 15 percent for buildings more than 30 years old and 20 percent for those more than 40 years old.